The “Calories In, Calories Out” Confusion: A Comprehensive Guide to Understanding Energy Balance

In Sigma Statements by Danny Lennon26 Comments

Estimated Reading Time = ~ 40 minutes

What's The Confusion?

Unfortunately nutrition is a subject that gives rise to many emotive debates.

One passionately debated concept is that of “calories-in, calories-out” (CICO).

This is colloquial phrasing for how energy balance relates to bodily energy stores. And this gets translated as shorthand for indicating how energy balance influences gain/loss of body mass.

Although one may suspect that CICO is something that seems simple, it is in fact a concept with a lot of nuance buried within it. Unfortunately this nuance is often overlooked, leading to misunderstandings and misleading characterizations being commonplace. At best, this results in people talking past each other when “debating” the idea, whilst at worst, it serves as fertile ground for misinformation or “evidence” to support fringe pseudoscientific ideas that centre on the rhetoric of calories being irrelevant.

When looking at some of the commentary related to CICO, there are two opposing positions that are both incorrect. On one hand you have people claiming that “CICO is wrong” and that looking at energy balance as the main driver of changes in body mass is misguided. As we'll discuss later, this is often based on a caricatured respresentation of the concept.

On another hand, there is a clear error in putting too much focus solely on energy balance. You don’t have to look far to some corners of the fitness industry where people just shout “calorie deficit” as the answer to every problem, without acknowledging the pragmatic reality of what drives food intake and energy expenditure.

Both of these positions illustrate a fundamental misunderstanding about what the energy balance equation is, how that relates to human metabolism and body composition or the interacting variables within human diet.

The hope is that this Sigma Statement will provide sufficient context and detail to allow for more accurate understanding. And at least to prevent people talking past each other when the topic arises.

Understanding Calories-in Calories-out (CICO)

Energy intake (calories in) is simply the caloric value of the food we consume. Energy expenditure (calories out) has four main components:

  1. Resting metabolic rate (RMR)
  2. Thermic effect of feeding (TEF), or diet-induced thermogenesis (DIT)
  3. Physical activity (PA) thermogenesis
  4. Non-exercise activity thermogenesis (NEAT)

Of these sub-components, those driven by activity (i.e. PA and NEAT) are the most variable. And unless there are conscious changes in planned exercise, most of that variation is that of NEAT. Resting metabolic rate is primarily determined by body size and lean body mass, whilst TEF stays relatively stable unless there are drastic changes to calorie intake and/or macronutrient composition. 

As already stated CICO is really just shorthand for how energy balance influences energy stores in the body.

The energy balance equation simply states that the difference between the energy coming into the body and leaving the body is equal to the energy stored/lost in/from the body:

Energy in - energy out = energy stored/lost

Calories (kcal) are simply a unit of energy. So as an example:

If we take in 2,000 kcal in the form of food but expend 2,500 kcal, then there is 500 kcal of energy “lost” from the system (i.e. the body). This shortfall in energy will then translate to a loss of some stored form of energy, for example energy stored as fat within a fat cell (adipocyte). But energy can also be stored in other forms, for example as glycogen (stored carbohydrate) in muscle and liver, or as protein that makes up muscle tissue. Breaking down any of these stored forms of energy can contribute to making up that energy shortfall.

Conversely, if we take in 3,000 kcal from food but expend 2,500 kcal, then there is 500 kcal stored in the system. Again, this can be stored in different forms, but the storage of fat (in the form of triglycerides) within adipocytes tends to be the primary focus of these conversations.

It is for this reason that we draw the parallel between energy balance and body composition changes; i.e. it is why a calorie deficit is discussed in relation to fat loss or a calorie surplus in relation to fat gain. A calorie deficit or calorie surplus provides conditions suitable for loss or gain of body tissue respectively.

Crucially however, the energy balance equation is technically only telling us about differences in energy, not about amounts of tissue change, and not about body weight change. Energy balance alone doesn’t tell us what the change in body weight will be, what the change in fat mass will be or what the change in muscle tissue will be. To give some examples:

  1. If someone eats at energy balance (calorie intake matches calorie expenditure) but dehydrates themselves, they will lose body weight (mass as measured on a scale).
  2. If someone eats at energy balance and starts taking creatine, they can see their weight increase due to an increase in body water stores.
  3. If someone eats in a 30% calorie deficit for a month, then the amount of fat mass and lean body mass lost will depend on factors other than calories; it will depend on protein intake, physical activity (stimulus on the muscle), etc. 
  4. If someone eats in a calorie deficit, but starts resistance training for the first time, they will likely gain muscle mass and lose fat mass at the same time. So their energy balance isn’t directly a measure of changes in fat, muscle or body weight.

However, let’s not throw the baby out with the bathwater here. With all this said, this certainly does not nullify the legitimacy of the energy balance equation, nor does it mean that considering calorie intake and expenditure is unuseful as a proxy measure to predict body composition changes. Altering energy balance through modifying calories in and/or calories out is still the primary tool at our disposal when it comes to maximizing the rate of fat loss. Which in turn has knock-on effects on health.

As a heuristic, in most practical circumstances it holds true that a calorie deficit predicts weight/fat loss. Insofar as, a sustained caloric deficit over time will lead to a decrease in fat mass. With other factors dicate the amount of change in body tissues or the ease at which it is achieved? Absolutely. But this does nothing to take away from the fact that a sustained caloric deficit is required for fat loss.

And similarly, with properly structured training and diet, a sustained caloric surplus will create the best environment for muscle growth. However, energy balance does not perfectly describe changes in actual body tissues.

Differences in Changing Fat Mass vs. Muscle Mass

The impact of energy balance on fat mass and muscle mass is not the same.

For loss of fat mass to occur, a calorie deficit is both necessary and sufficient:

  • Necessary: Without a calorie deficit, fat mass cannot be lost.
  • Sufficient: Regardless of other aspects of the diet, if there is a sustained caloric deficit, loss of fat mass will result.

For muscle mass gain to occur, whilst a calorie surplus provides the best conditions for muscle growth, a calorie suplus is neither necessary nor sufficient:

  • Not necessary: With an appropriate training stimulus and protein intake, growth of new muscle tissue is possible for most people (with this possibility decreasing as someone approaches their genetic potential of muscle mass).
  • Not sufficient: Simply having a calorie surplus in place does not necessarily mean muscle mass will be gained. Take the example of someone on a low-protein diet who does no resistance training; in such a case there would likely be no increase in muscle mass despite a period of a caloric surplus.

Additionally, the greater the calorie deficit, the faster the rate of fat loss will be. However, a greater calorie surplus does not necessarliy drive a faster rate of muscle gain. There is a limit to how fast one can build muscle. And as already noted, the main driver is exercise stimulus, not calories. An appropriate amount of calories is permissive to maximal rate of muscle growth and increasing beyond that will not translate to additional muscle gains. Hence why in most cases a slight surplus is beneficial, but aggressive overfeeding is usually counterproductive.

The CICO Strawman

As already discussed, CICO is not an explanation of specifc changes in body tissue. Remember, all it does is explain differences in energy, not mass of different tissues. However, knowing whether there is a calorie deficit or surplus present over a specified period of time will let us know the directionality of likely changes in mass; i.e. whether a loss or gain of stored energy, and thus mass, is going to result. So whilst the energy balance equation alone can’t account for specific changes in body composition,  it still holds true. A calorie deficit is still a requirement for fat mass. 

And this is why it is frustrating to see the pseudoscientific gurus on the internet tell people that CICO is “wrong” based on a false definition of what it means. They give a strawman (or sometimes just flat out false) depiction of what the CICO concept means.

It’s common to hear people dismiss the energy balance equation in favour of narratives that talk about how “it’s hormones, not calories” or similar rhetoric. Such claims usually emanate from the realms of pseudoscience peddlers and diet cults. In dismissing the relevance of energy balance, a common move is to cariacture CICO as a simplistic input-output idea that claims "calories are all that matter" and that “all calories are created equal”. The process such pseudoscientists follow is always the same:

  • Step One: The guru attempts to win over their audience by painting "the opposition" as those who claim a calorie surplus is what leads to fat gain or that calorie deficits are required for fat loss.
  • Step Two: The guru then tells their audience that these people who accept CICO holds true are simply going around telling people to eat less, demanding they count calories, or stating that food quality is irrelevant.
  • Step Three: Now that the audience seems how stupid CICO is, the guru can proceed to weaving a narrative about how it's really hormones that matter, not calories. The audience then views this as a much more elegant, scientific narrative and so it surely must be correct.
  • Step Four: Members of that audience are now equipped with a lovely story to tell others and are able to confidently call true nutrition experts idiots for being brainwashed by outdated notions about calories.

This would be a fine narrative. If only it were true.

So just to be clear, CICO is not synonymous with:

  1. Advice to “eat less, move more” - That advice is largely unactionable and unhelpful, as is discussed in the final section of this statement. CICO is not advice, it's just a descriptor of energy storage/release.
  2. Tracking/counting calories - Again, CICO is just a descriptive concept, not a strategy or intervention. You can track calories if you want or you can use an intervention that doesn't track calories. No matter what the intervention, if fat mass is lost then the mechanism of action is a calorie deficit, regardless of whether the individual is aware of their calorie intake or not. 
  3. Basing diet decisions solely on calorie values of food - Another weak, strawman argument utilised by the 'hormones, not calories' crew is that of ridiculing the notion of diet decisions being based solely on calories. And correctly, such a notion should be ridiculed, it's absurd. The problem is: no one is saying the only factor one should consider is calories. Where are these people who say only calories matter? It's fighting against a position no one is taking. 
  4. The “calorie is a calorie” meme - Similar to the last point, another pushback to CICO is that it dismissed the impact of different foods and macronutrient profiles of the diet. And correct, different foods and different macronutrient breakdowns don’t have the same metabolic effects. But this isn't the claim of CICO. It's another arugment against a caricatured version of the concept. This will be discussed more below.

Other “arguments” you may hear against CICO include variations of: 

  • “The Calorie (kcal) is a unit of heat/energy. And we care about mass!”
  • “Your body isn’t a bomb calorimeter!”
  • “Advising people to “eat less, move more” doesn’t work”
  • “Eating less calories just means you expend less calories”
  • “Your metabolism slows when you eat less calories”
  • “CICO doesn’t account for the role of hormones, which are the real determinants of body fat storage/release”

However, none of these points illustrate that the energy balance equation doesn’t hold true. 

To get to the core of the misunderstandings, there are three primary topics I feel need to be understood:

  1. Drivers of energy intake and energy expenditure
  2. Fat oxidation vs. loss of fat mass
  3. ‘Calories in’ and ‘calories out’ are not independent

Drivers of Energy Intake & Expenditure

There are both homeostatic and non-homeostatic drivers of energy intake and expenditure.

Homeostatic Drivers

Various hormones orchestrate a suite of responses to our intake and expenditure in an attempt to exert homeostatic control over our body composition. In response to both undereating and overeating, hormones will drive us to want to consume more food or to stop eating, or to burn more energy or to rest and use up less energy.

It is for this reason, that when things are working as they should (metabolically speaking), we have evolved to maintain body weight and body fat levels within a range that is neither too low nor too high. There are a number of different models of body weight regulation that won't be discussed here, but are worthy of deep discussion another time. However, this general premise that there are homeostatic attempts by the body to avoid underweight/overweight is part of each model.

It is because of this homeostatic control of intake/expenditure in humans that we see examples of people who, without ever monitoring their food intake or calculating how much to exercise, remain within a relatively stable body weight range for years and decades.

“Energy intake and energy expenditure clearly do not correlate over a short period of time such as a day or two. Equally clearly, however, their correlation over an extended period of time, such as a week to a few months to years, is excellent”

- Gropper, Smith & Groff

But clearly this is not everyone and in modern society it is becoming increasingly difficult for humans to regulate body mass without concious restraint. Why? Because intake and expenditure are not only driven by internal homeostatic controls but also by external non-homeostatic controls.

Non-homeostatic Drivers

However, as has become increasingly central to discussions around nutrition and health, our modern environment and behaviour patterns have led to the ability to override our internal homeostatic hormonal controls of intake and expenditure. Now it is easy to continue to consume calories long after there is any hormonal drive to eat or experience hunger:

  • Hyperpalatable, calorie-dense food allows large calorie consumption beyond the point of nourishment.
  • Such foods are often low in protein and fibre, leading to less satiation and satiety.
  • Many people consume calories in the form of sugar-sweetened beverages, which are known to have a much lower satiety response than an equal amount of calorie and sugar from solid food.
  • Convenience and the low cost of such foods makes them an easy option.
  • Marketing messages nudge our behaviour, even subconsciously, towards being more likely to over consume such products. 

On the energy expenditure side, it is incredibly easy for modern humans in the developed world to spend entire days with close to zero physical activity. One can wake up, drive to work, sit at a desk, order food via apps on their phone, talk with friends over social media, and do many of their leisure activities online or without leaving home.

Peer groups influence our energy intake and expenditure. Despite what our hormones might say about our current hunger levels or what our current drive to move around is, if we are spending time in a social circle that makes inactivity and consumption of calorie-dense meals/snacks at the centre of the time shared together, then our behaviour is likely going to be different than if our social time was centred around physical activity or with people who hold health conscious worldviews.

So one can acknowledge that hormones, environment, food availability, socioeconomics, peer groups, habits and behaviour patterns all influence body composition. But this doesn’t undermine the fundamental claim about energy balance driving changes in body composition. Those factors all exert impacts on our caloric intake and energy expenditure. These are not reasons “against” the CICO concept, they are factors that modify intake (calories in) or expenditure (calories out).

So how does all this talk of energy balance actually relate to our fat mass? How does it influence fat storage or fat usage? What puts fat into a fat cell and what causes it to be released and "burned"?

Fat Oxidation vs. Loss of Body Fat

When thinking about the movement of fat (stored energy) in and out of a fat cell, there are three main processes to consider:

  1. De novo lipogenesis (DNL): conversion of glucose to fat. Accounts for a relatively small part of the equation.
  2. Re-esterification (RE): re-assembling fatty acids on a glycerol backbone inside fat cell depends on how much fat is consumed and the metabolic demands for fatty acids.
  3. Lipolysis (L): the breakdown of stored fat. Involves hydrolysis of triglycerides into glycerol and free fatty acids.

The first two of these processes (DNL & RE) relate to storing energy in a fat cell. Whilst lipolysis is the process of releasing stored fat from the fat cell. So we are in a state of fat balance when the amount of fat going into the cell is equal to the amount of fat leaving the cell.

Consider an instance where you have just consumed a meal. You take in food, it’s broken down into its basic components (e.g. protein into amino acids, etc.). And therefore you now have a large amount of these energy substrates (fatty acids, glucose, amino acids) that can be used for many processes. These substrates are available to go one of three ways:

  1. Immediate energy demands - The substrates are oxidized (or “burned”) to produce energy that the body needs right now.
  2. Metabolic processes - For example, providing amino acids for building/repairing muscle tissue (process of muscle protein synthesis).
  3. Storage for later usage - For substrates that don’t need to be used immediately, the body will store them for later use.

So if one doesn’t use up these substrates immediately (for either option 1 or 2 above), does that mean one is gaining body fat? Well, certainly the excess energy may be stored in fat cells. But just because we store this energy as fat right now, that is not synonymous with what we usually define as gaining body fat (i.e. a practically meaningful amount of fat storage that persists, leading us to have more body fat than previously). But rather, over the course of the rest of the day, as we go through hours of not eating, moving, and expending energy, we will need to use our stores of energy to do this. At these times, we can tap into these reserves of energy that we stored in fat cells after meals earlier in the day.

So the storage of excess energy after a meal in fat cells can be transient. This storing and releasing of fat in/from fat cells is happening every day, regardless of if we are in a calorie deficit, calorie surplus, or at energy balance. What dictates whether one actually gains fat mass that is sustained beyond those few hours between meals relates to net energy balance over 24 hours.

In the above graphic, three scenarios are shown for a hypothetical case where someone is eating three meals across a 24 hour period:

  1. At energy balance over 24 hours (blue) - Net energy storage is equal to net energy release/use
  2. Negative energy balance over 24 hours (green) - Net energy storage is less than net energy release/use
  3. Positive energy balance over 24 hours (red) - Net energy storage is greater than net energy release/use

In all three cases we can see that after consuming a meal, there will be net energy storage at that moment in time. And this should make total sense; if I eat 500 kcal at a meal, there is rarely a case where the body would need to burn all those calories immediately (in such a case, we’d have to eat constantly all day to survive). So if I don’t need to burn all those calories now, I will store some of them for later parts of the day when I won’t have energy coming in, but will still need to expend energy to move, have my organs function, etc.

In between each of these energy storage periods, we can see periods of energy release/use. These are periods where we are not eating food (no calories coming in) but continue to expend calories (we expend energy to maintain normal function to survive, plus more for physical activity). In these periods, this is where we call upon the stored energy in the body. One example would be some energy from our earlier meal which we stored as triglycerides in a fat cell (or as glycogen in muscle and liver cells).

But what dictates whether one is gaining body fat or losing body over time is not whether we ever store fat (as shown here, we do this everyday regardless of what we eat), but rather what is the net balance between this fat storage and fat release/use over an extended period (in this case 24 hours). And what dictates the net amount of fat storage or release (i.e. the size of the curve above or below the line respectively)? The net amount of calories consumed or expended in that time.

And in practice we should consider this extended period beyond 24 hours; i.e. what is the net difference between fat storage and fat release over ]one week, two weeks, a month, etc.? That is what dictates increases/decreases in body fat stores.

And so focusing on whether an acute elevation in insulin promotes fat storage processes (which it does) is irrelevant, because that is looking at a snapshot in time, not the net effect on fat stores over longer periods.

Fat Oxidation

When fat is stored within a fat cell (adipocyte), it is stored as triglyceride (three fatty acids attached to a glycerol molecule). So in order to decrease fat stores, these triglycerides must first be broken down into their constituent parts (free fatty acids and glycerol), which are then released into the bloodstream (via lipolysis).

Once the free fatty acids are circulating in the bloodstream, in order to “burn” that fat for energy, the free fatty acids need to undergo a process called beta-oxidation. This fat oxidation is what is often colloquially called “fat burning”. However, it’s important to note that this “fat burning” is not the same as what most people in the population would assume “fat burning” to be. It is not the same as losing practically meaningful amounts of body fat stores. In other words, there are many ways in which fat oxidation can be increased acutely, without having any impact on body fat stores. 

For example, if you eat a high-fat meal, fat oxidation increases. This is how a “Bulletproof Coffee” (coffee blended with tablespoons of butter and MCT oil) gets spun as a “fat-burning” meal to start your day. The marketing uses the confusion around fat oxidation as a means of painting this as a good choice for weight loss and health. If marketing were more honest, perhaps the concoction would be renamed “Atherosclerosis Accelerant Coffee”?

And as we’ve seen reducing carbohydrate intake and replacing those calories with dietary fat leads to increased fat oxidation. However, it also leads to a reduced carbohydrate oxidation, that leaves net energy balance being the same.

Finally, we need a discussion of how calorie intake and calorie expenditure are not independent, i.e. if you change one, you will cause a change in the other. The energy balance model is not at odds with this fact. And in fact, this is how CICO aligns with issues like metabolic adaptations to dieting, influence of hormones on food intake or activity, and many other of the concepts some people erroneously believe are examples of “it not being about calories”.

'Calories In' and 'Calories Out' Are Not Independent

  1. Calories In Impacts Calories Out

There are several ways in which our food intake influences energy expenditure. There are obvious and direct mechanisms, for example diet-induced thermogenesis (DIT), or as it’s also called: the thermic effect of feeding (TEF), where we expend energy to digest and metabolise food. And different nutrients have different values. So a high-protein meal will lead to greater DIT/TEF than a calorie-matched low-protein meal. So clearly meals and diets of differing macronutrient composition have different metabolic effects, with just one of those effects being the impact on energy expenditure via DIT/TEF.

But this is accounted for within the energy balance model. It’s why the often heard rebuttals by calorie-denilists go along the lines of “oh, so you’re saying 2,000 kcal of broccoli is the same as 2,000 kcal from beef?” (or “a calorie of brownies” and “a calorie of kale” as some quack doctors might say). Again it's attacking a position nobody holds as a means of elevated some other fringe idea about how calories are irrelevant.

There are a suite of metabolic adaptations to both overfeeding and underfeeding, that the body coordinates in an attempt to maintain stasis. In overfeeding situations, the body increases energy expenditure in an attempt to account for this increased energy intake. The primary adaptation is an increase in non-exercise energy thermogenesis (NEAT). NEAT can be thought of as those movements we make throughout the day but that are not actually planned physical activity per se. So with increased NEAT levels, that means increased subconscious movements (e.g. fidgeting) and an increased drive to move about.

To underscore the dynamic interplay between 'calories in' and 'calories out', it is worth exploring the metabolic adaptations to dieting in a bit more detail. In the below graphic, scenario A represents the situation at energy balance; i.e. average energy intake is ~2,500 kcal and expenditure is matching this amount (2,500 kcal).

Now let’s take a situation where this person purposely restricts their calorie intake by 700 kcal, leading their intake to now be 1,800 kcal. Scenario B represents what some people assume is now the case; i.e. there is a 700 kcal deficit created. However, scenario B is what would be the case if there was no adaptation in energy expenditure. But we know that calorie expenditure will decrease in response to decreased calorie intake (with stark differences between individuals). This decrease will also increase the longer the deficit is maintained. So just for illustrative purposes, imagine a situation where the combined effect of all the metabolic adaptations to dieting leads to an individual’s energy expenditure to decrease by ~ 300 kcal. This is scenario C. Because of this decrease, the net calorie deficit is now 400 kcal. So in this hypothetical example, a decrease of 700 kcal in calorie intake does not equate to a 700 kcal deficit, but rather a 400 kcal deficit.

Again, this doesn’t violate the energy balance equation. It fits perfectly within it. So anecdotes of reducing calorie intake but weight loss slowing down aren’t evidence against CICO at all. In fact, it’s exactly what one would predict based on the physiology of dieting and weight loss.

An overfeeding study by Levine illustrates nicely how ‘calories in’ influences ‘calories out’. Healthy participants were overfed by 1,000 kcal/d for 8 weeks. However the resulting gain in fat mass wasn’t what would be “predicted” by a 1,000 kcal surplus. Instead some of that surplus was offset by an increase in energy expenditure, with two-thirds of this coming from increased NEAT. Whilst there was wide variation in how much their NEAT increased (and hence how much fat each person gained), in one of the participants NEAT alone was seen to be able to increase by almost 700 kcal per day (average was 328 kcal/d). So for this person the total increase in energy expenditure almost completely offset the increase in calorie intake, which was reflected in the very small amount of fat this person gained (0.36 kg), despite the 1,000 extra calories per day over the eight week period. (Strangely, at the opposite end, another individual actually saw their NEAT decrease by ~ 100 kcal/d, and had a fat mass gain of over 4 kg across the study).

This inter-individual variation seems to be at least partly explained by genetics, as similar overfeeding studies done on identical twins would suggest. In one such study by Bouchard et al., the researchers had 12 pairs of twins complete a 100-day overfeeding phase, where they ate 1,000 kcal above baseline for six days of the week, every week for the duration of the trial. And whilst there was a large inter-individual variation among all the participants in a similar fashion to that in the Levine study, there was three times more variance among twin pairs than within twin pairs; i.e. a pair of twins showed a more similar response, than compared to others. This suggests that genetics is driving much of the adaptive response to overfeeding. And the same happens with underfeeding.

And so we could take two individuals with the same “maintenance calories”, and increase their intake by the same amount but the net calorie surplus may be different.

By way of example, let’s consider two individuals with very similar baseline characteristics who maintain their current weight on an average intake of 2,500 kcal per day (image below). Now we increase their intake to 3,200 kcal per day (i.e. a 700 kcal increase). But as has been discussed, metabolic adaptations will lead to an increase in energy expenditure. In person A, their expenditure increases by 100 kcal/d (from 2,500 to 2,600 kcal/d). Whilst person B experiences a 300 kcal increase in energy expenditure (up to 2,800 kcal/d). So despite increasing both individuals intake by 700 kcal, we have a situation where person A has a net calorie surplus of 600 kcal, whilst person B has a net calorie surplus of 400 kcal. The important point here is that an increase in calorie intake is not equal to the actual net calorie surplus.

And likewise, a decrease in calorie intake is not equal to the net calorie deficit. Hence why simplistic calculations are erroneous. For example, consider someone trying to calculate expected fat loss over an 8-week dieting period. They decrease their calorie intake by 300 kcal/d, and accurately do this for the full 8 weeks. The amount of weight/fat loss can not be estimated on the basis of presuming this is a 300 kcal deficit running consistently for 8 weeks. There will be metabolic adaptations to decrease energy expenditure, and such a decrease will only increase over the duration of the diet, thus meaning there is no 300 kcal deficit (unless energy intake or expenditure is purposefully modified to re-attain this level of deficit). 

Exercise exerts a role on body composition in both direct and indirect ways. There is of course a contribution to total energy expenditure through the energy burned during exercise bouts. There is also the influence of resistance exercise stimulating muscle growth or retention of muscle mass. And clearly what we eat and how much we eat can have impacts on exercise performance. Taking the resistance training example, if we go into a hard training session having already consumed 1,500 kcal earlier in the day versus if we hadn’t eaten at all, is it plausible that how much volume one could do in that session is different? Or how intense one could work for a set amount of volume? In either case, it’s at least hypothetically possible that we are changing energy expended during the training session. Calories in influencing calories out.

And to further explore some of the ways in which exercise can exert an indirect role on body composition, one can consider how exercise impacts our food intake. Which is an idea that sets the stage for our next section; how ‘calories out’ impacts ‘calories in’.

2. 'Calories Out' Impacts 'Calories In'

Work done by the team at the University of Leeds (which includes Mark Hopkins and John Blundell) has shown how physical activity and appetite control are not independent of one another, but rather are interconnected. At very low levels of physical activity there seems to be an inability to regulate appetite and energy intake appropriately. Whereas at higher levels of physical activity there seems to be an ability for us to appropriately match our calorie intake to our energy demands. They have referred to these as “unregulated” and “regulated” zones, respectively.

So changes in energy expenditure therefore can have knock-on effects on calorie intake. Again highlighting that these are not two independent variables, but rather that they are inextricably tied.

Common Claims “Disproving” CICO

There are a number of justifications that people give as to why ‘calories-in, calories-out’ is nonsense. Much of these are built around personal anecdotes, followed by an interpretation of what said anecdote must mean for CICO. For example:

  1. “I am eating more now and my body composition is better. CICO is nonsense”
  2. “I reduced my intake but I stopped losing weight. CICO is nonsense”
  3. “Tracking calories never worked for me, but when I went low-carb weight just dropped off. CICO is nonsense”
  4. “I developed hypothyroidism and gained weight on the same diet, see it’s hormones. CICO is nonsense”
  5. “Tracking calories isn’t psychologically healthy and isn’t a sustainable way to live. CICO is nonsense”
  6. “There is more to diet and body composition than calories. CICO is nonsense”

Of course, in each of these cases, the initial observations can be correct but the interpretation that “CICO is nonsense” is incorrect. Each of those observations can align perfectly with CICO holding true:

“I am eating more now and my body composition is better.”

  • There are absolutely cases where someone can be eating “more” and see their body composition improve. For example:
    • Ex. 1: When the person says “more” they are referring to more food (i.e. in terms of mass or volume) but because of changes in food choices they are actually eating less calories. As a result, they drop into a calorie deficit and lose body fat.
    • Ex. 2: Someone has increased their calorie intake but has also recently started resistance training, and so lean body mass increases, thus leading to better body composition.
  • So there are cases where either subjectively or objectively someone eats more food and experiences either decreases in fat mass or increases in lean mass. All of this can be explained without any violation of CICO.

“I reduced my intake but I stopped losing weight.”

  • One can reduce their calorie intake but eventually stop losing weight. As discussed already, there are metabolic adaptations to dieting that occur to cause a decrease in energy expenditure. This, in addition to having a lower body mass, means over time the deficit shrinks more and more, even if you stay eating the lower amount of calories.
  • Therefore, even when stalls occur in weight loss (and presuming the person has adhered to a lower calorie intake), this doesn’t violate CICO. In fact, this is to be expected given our previous discussion of ‘calories in’ influencing ‘calories out’. 

“Tracking calories never worked for me, but when I went low-carb weight just dropped off.”

  • Many people don’t find tracking calories a useful strategy. But not succeeding in losing weight with such a strategy isn’t an indication that CICO is wrong, but rather that tracking calories is a strategy that the individual finds difficult to adhere to, is tracking incorrectly, or hasn’t accounted for reductions in energy expenditure over time. CICO and tracking calories are not synonymous. 
  • For some people, going on a low-carbohydrate diet leads to spontaneous reduction in calorie intake due to a combination of factors (reduced food options, reduced processed food intake, increased protein, greater satiety, etc.) that allows that person to reduce body weight. In the early week or so of embarking on such a diet there may also be losses of water and glycogen which would lead to a lower body weight measurement.

“I developed hypothyroidism and gained weight on the same diet.”

  • Hypothyroidism can lead to weight gain, but this is due to a decrease in metabolic rate and therefore decreased energy expenditure. So yes, a hormonal change is resulting in weight gain, but the mechanism is still via the creation of a calorie surplus, thus fitting within the energy balance model. Appropriate drug intervention can offset this decreased energy expenditure.

“Tracking calories isn’t psychologically healthy and isn’t a sustainable way to live.”

  • Correct, tracking calories isn’t psychologically healthy for many people. And a strong case can be made that in the long-term people should aim to move away from it, at least for the majority of the time. But again, tracking calories and CICO are not synonymous!
  • CICO simply is a recognition that energy imbalance can cause alterations in body tissue stores. It says nothing about having to use any specific intervention or strategy, whether that be tracking/counting calories or otherwise.

“There is more to diet and body composition than calories.”

  • Of course there is more to diet and body composition than calories. I know of no reasonable person who says otherwise. This is perhaps the weakest argument that someone can give in arguing “against” CICO.

Problems Calculating Calories

Another point of discussion relates to problems in accurately assessing calorie intake and energy expenditure. And unless we are in a metabolic ward, it is absolutely true that we can’t assess these with precision. For calorie intake we can get relatively close (or at least a decent approximation that works for practical purposes). If someone weighs all the food they consume and logs that into an app that tracks calories, then a calculation of total energy intake for the day can be obtained. But as someone may (correctly) outline, this is based on estimates and averages of the calories in the foods measured. There may also be inconsistencies with the precise amounts of ingredients used. And for processed foods, food labels come with a margin of error. So yes, even meticulously tracking calorie intake doesn’t likely give the exact correct number.

But the more important question is: how much does that matter? Even for calorie estimates that are a bit off, comparisons can still be made between the calculated intake and the average habitual intake for that person. Comparisons can still tell us if someone is likely in a calorie deficit or not. It doesn’t really matter if one’s calculated 350 kcal deficit is in reality a 327 kcal or 386 kcal deficit. 

For energy expenditure, any estimation we make in real-world settings is just a rough approximation. But again, how much does this matter? What any estimation of energy expenditure is doing is giving a starting point to help determine what an appropriate calorie intake for this individual may be. 

In cases where an estimated calorie intake is calculated with the goal of fat loss, but no change in body composition occurs, it does not invalidate CICO. Rather it simply signifies that the calculation was inaccurate and that this individual is not in a calorie deficit, for one or more of the reasons already discussed. So a further reduction in intake (and/or increase in expenditure) may be required.

Influence of Macronutrients & Food Quality

Even between isocaloric diets, impact on body composition will differ based on the macronutrient profile of the diets. This fact is often misconstrued as undermining CICO. But such a conclusion is based on the false premise that CICO is synonymous with diets matched for calories will have the exact same impact. This leads to the whole “a calorie is not a calorie” rhetoric, which attempts to justify the conclusion that CICO is nonsense by comparing 2,000 calories of lean beef to 2,000 calories of sugar and claiming CICO-proponents are suggesting these are equal in their impact on body composition. This is of course ridiculous and a fallacious argument.

Of most importance in this respect is the protein content of the diet. A metabolic ward study out of George Bray’s lab showed that when people were put in a 40% calorie surplus (an extra ~ 950 kcal/d) for eight weeks, whilst fat-free mass increases on moderate and high-protein diets (15% and 25% of calories from protein respectively), fat-free mass actually slightly decreased on a low-protein (5% of kcal) diet.

So differing amounts of protein in the diet have differing impacts on muscle mass. But who is disputing this? The “calorie is not a calorie”, beef vs. sugar comparisons are fighting against a position that doesn’t exist. A tactic that does well to embolden followers of certain ideologies, but nothing to help advance honest discourse.

Similarly, it is well established that the different macronutrients have different thermogenic potential (i.e. different levels of thermic effect of feeding, TEF, which is a measure of how much energy is expended in digesting food). Protein has by far the highest TEF value of the macronutrients. This is yet another example of how calories in affects calories out. As you change the amount of protein in the diet, there will be a change in TEF, and therefore potential in total energy expenditure.

The different macronutrients can also influence satiety differently. Protein exerts a strong influence on satiety, and so low protein intake can mean an increased likelihood of eating more calories, compared to a similar diet with more protein consumed. It is also well understood that dietary fibre increases satiation and therefore isocaloric diets with a high vs low fibre intake can have differing effects on satiation, and therefore calorie intake.

It is known that what foods are selected in the diet can have implications on appetite, satiety and drive to consume more food later in the day. Hall and colleagues showed that when food is available as ultra-processed food (e.g. breakfast cereal, muffins, white bread, sausage, etc.) it leads to greater ab libitum calorie intake compared to unprocessed foods. 

All of this is to say that the macronutrient profile of the diet does absolutely matter. And beyond that, food quality also matters. But this doesn’t undermine CICO, they both co-exist without any conflict.

Influence of Other Factors

There are a number of other factors that influence body composition, that deserve longer commentaries at another time, but are worth highlighting here. Three of particular significance are resistance training, sleep and hormones.

Resistance training

When engaging in resistance training of sufficient stimulus over a sufficient period of time, there will be creation of new muscle and therefore an increase in body mass, even without a caloric surplus. In fact, even in a calorie deficit it is possible to gain lean mass, given the right conditions. The stimulus placed on the muscle is by far the most important variable. And from a nutrition perspective, if an appropriately high supply of protein is provided, ideally distributed across the day in 3+ high-protein meals, then there should be accrual of new muscle tissue. Of course, other factors dictate how likely this is and the extent of the muscle gain (genetics, training experience, proximity to genetic potential, returning from a phase of de-training, etc.). However, to maximize the amount of muscle gained in a certain period of time, a slight calorie surplus provides the best environment, all else being equal. 

But to circle back to what was discussed towards the beginning of this statement, CICO is telling us about the amount of energy to be stored in, or released from, the body. It is not an explanation of amounts of muscle tissue that will result. And in the case of resistance training and gaining muscle, calories are not driving the gain in muscle, training is. A surplus of calories is just the most conducive environment for building new muscle tissue, which is an energy intensive process.

Sleep

Some experimental evidence exists suggesting that body composition changes differ between calorie-matched and macronutrient-matched diets, based on differences in sleep duration. In one particular crossover study, participants spent two 14-day periods (with at least three months washout period between them), having to spend either 8.5 hours or 5.5  hours per night in bed. They consumed the same diet, with calories set to 90% of their resting metabolic rate (so they were in negative energy balance). The study was carried out in the lab, with all food being weighed before and after each meal to determine actual consumption.

As shown in the graphic below, there were significant differences in body composition between the conditions. Although weight change was the same, in the sleep-restricted condition less fat was lost and more lean mass was lost. In the 5.5 hours/night condition, fat oxidation seemed to be impaired.

In more real world situations, sleep restriction can influence body composition through altering food choices. As was discussed in a previous Sigma Statement, sleep curtailment has significant effects on hunger and appetite regulation, therefore promoting increased calorie intake. Sleep curtailment also results in reduced dietary restraint and increased disinhibited eating. And in cases of chronic sleep curtailment, there may be a reduced drive for physical activity and greater feelings of tiredness, resulting in lower energy expenditure. So sleep can exert an influence on both ‘calories in’ and ‘calories out’, and thus changes in mass.

Hormones

Are hormones important when it comes to body composition? Of course. It is well-known and universally accepted that a long list of hormones play vital roles; from ghrelin driving appetite, to leptin impacting both energy intake and expenditure, to cortisol playing a role in fat release from cells. However, just because hormones have important functions, this doesn’t somehow make calories irrelevant. In fact the two are linked.

When someone fasts for 10 hours, and their ghrelin levels rise throughout, their appetite increases. However, we wouldn’t say that this has nothing to do with them not ingesting any calories and it’s simply a hormone issue. But rather it is the lack of nutrient ingestion that is driving the hormonal change.

Stating that CICO holds true does not oppose any of the well-known mechanisms by which hormones influence body composition. But hormones are exerting this influence via driving energy intake, energy expenditure, food choices, stimulating muscle growth/repair, etc. 

Much of the other “hormones, not calories” rhetoric comes from people citing the carbohydrate-insulin model of obesity (CIM), which has been promoted by a number of figures, particularly those within the low-carbohydrate community. In recent times, Ludwig and Ebbeling have been two of the most prominent advocates for the model within academia. Whilst there have been various iterations and interpretations of the model, Ludwig & Ebbeling state that according to the CIM, obesity is a result of the fact that increased “consumption of processed, high-glycemic-load carbohydrates produce hormonal changes that promote calorie deposition in adipose tissue, exacerbate hunger, and lower energy expenditure.” Essentially, the problem is claimed to be elevated insulin, driven by increased intake of carbohydrates. The elevated insulin “traps” substrates in the fat cells and decreases circulating levels of substrates (e.g. glucose, free fatty acids, etc.), which in turn leads to weight gain, increased calorie intake, and decreased energy expenditure. However, this seems to be at odds with much of the extant evidence. For example, it is well documented that individuals with obesity have at least normal, and often increased, levels of free fatty acids and glucose in circulation. Additionally, their fat cells release more, not less, free fatty acids, compared to those without obesity, and this is often in the presence of hyperinsulinemia.

And in metabolic ward conditions, despite a carbohydrate-restricted diet resulting in reduced insulin and increased fat oxidation, this does not result in greater total energy expenditure or greater body fat loss compared to a fat-restricted diet that is matched for calories and protein intake. The increased fat oxidation in the carb-restricted diet is in parallel to an increased intake of fat. So those who restrict carbohydrates will see an increase in fat oxidation. But they also have a greater proportion of the energy in their diet coming from fat. And so in conditions where calories and protein are matched, the net impact on body fat stores is the same, because energy balance is the same. Those oxidising more fat, are also oxidising less carbohydrate. And as shown in the Hall study, the increased fat oxidation in carbohydrate-restricted group was offset by their decreased carbohydrate oxidation.

Overall, yes hormones are important. But this notion that hormones and calories are independent concepts with only one answer being “right” is wholly misleading.

“Eat Less, Move More” is Unhelpful Advice

It is unfortunately common to see the conflation of CICO with the advice to “eat less, move more”. Whilst understanding energy balance leads us to conclude that a sufficient reduction in calorie intake and/or sufficient increase in energy expenditure would lead to a calorie deficit, and thus fat loss, simply advising people to eat less is at least unhelpful, and in some instances harmful.

As has been alluded to earlier, a modest reduction in intake will lead to some decrease in energy expenditure. Additionally, feedback mechanisms lead to an increased drive to consume food, with the drive increasing the greater the duration and extent of the caloric restriction.

After creating a calorie deficit, that net deficit will decrease over time, thus requiring future modifications to calorie intake and expenditure in order to keep that deficit at the same level. This closing of the deficit happens both physiologically and/or behaviourally.

Physiologically, as the diet persists, metabolic adaptations to the calorie deficit and the weight loss will lead to decrease in energy expenditure. So if energy expenditure decreases by 150 kcal over a number of months dieting, but average calorie intake stays the same, then what started out as a 400 kcal deficit is now a 250 kcal deficit, thus resulting in a slowing in the rate of fat loss, despite eating the same amount of calories.

But beyond the physiology, given that most people who diet will not be precisely controlling calorie intake, even when trying to keep the same habits, it seems that there is a gradual increase in calorie intake over time. In one paper by Hall et al., they referred to it as an “exponential decay in adherence”.

For fat loss, the answer is a calorie deficit. But is that actionable or useful information for the individual asking the question or embarking on a specific diet?

In order to actually make change it is much more beneficial to counsel on the factors that in turn influence calorie intake and physical activity:

  • Generally increasing diet quality
  • Learning how to prepare healthy meals
  • Modifying the food environment
  • Building a habit of regular exercise
  • Getting an adequate amount of sleep
  • Creating a consistent structure with diet 

There also needs to be an acknowledgement of the many barriers that exist to someone consistently making good food and lifestyle choices or achieving a certain body composition. Some individuals have a greater genetic predisposition towards obesity. There is a large body of evidence on social determinants (the conditions in which people are born, grow, live, work and age, and the fundamental drivers of these conditions: the distribution of power; money; and resources) on health and obesity.  Social inequalities also mean there is a difference in access to healthy food markets, health centres, safe areas to exercise, and social supports. Within these social determinants of health, socioeconomic conditions specifically (e.g., poverty and the associated stresses) act as a significant barrier to implement what is often thought of as “simple” advice. 

The advice to “eat less, move more” isn’t just ineffective, it can often cause harm. On receipt of such advice, many individuals may restrict food intake in an unsustainable or unplanned manner. The likely result is that any initial weight loss will eventually be regained, after the individual is driven to increase calorie intake in response to the hormonal response to undereating, as was discussed earlier. Without appropriate planning, guidance and/or education, simply trying to restrict food intake will be unlikely to work. But even more problematically, this oscillation between weight loss attempts and weight regain can cause feelings of shame, guilt and negative self-appraisal. People are made to feel like they are at fault or that they simply lack willpower. This is one of core issues at the centre of weight-neutral approaches (with Health At Every Size being perhaps the most well-known movement). Advocates for weight-neutral approaches quite correctly outline how damaging it can be psychologically to feel that one is “failing” at weight loss. And such negative psychological effects are potentially worse than physical effects of a certain degree of adiposity. A full discussion of the potential harm of weight loss interventions is beyond the scope of this particular statement but is acknowledged as an incredibly important issue.

The conflation between “eat less, move more” as advice with simply acknowledging the role of energy balance in body composition is both erroneous and problematic. Energy balance is the primary driver of body fat stores, and CICO is not “wrong”. However, advice to “eat less, move more”, whilst technically the basis for creating a calorie deficit, is incomplete advice at best, and can potentially lead to problems.

Summary of Key Points

  1. The energy balance equation simply states that the difference between the energy coming into the body and leaving the body is equal to the energy stored in, or lost from, the body.

  2. The energy balance equation describes differences in energy, not about specific amounts of tissue change.

  3. A caloric deficit is both necessary and sufficient for a decrease in fat mass.

  4. A calorie surplus provides the best environment for optimal muscle growth, however a calorie suplus is neither necessary nor sufficient for muscle growth to occur.

  5. CICO is not synonymous with:
    • Advice to “eat less, move more”
    • Tracking/counting calories
    • Basing diet decisions solely on calorie values of food
    • The “calorie is a calorie” meme

  6. There are both homeostatic and non-homeostatic drivers of energy intake and expenditure.

  7. What dictates whether one is gaining or losing fat mass is the net balance between fat storage and fat release/use over an extended period.

  8. ‘Calories in’ and ‘calories out’ are not independent: Calorie intake influences energy expenditure, and vice versa.

  9. Even between isocaloric diets, impact on body composition will differ based on the macronutrient profile of the diets.

  10. Other factors such as resistance training, sleep and hormones exert influences on body composition. However this still doesn’t undermine the fundamental influence of energy balance on body mass.

  11. “Eat less, move more” is unhelpful advice and likely ineffective. In order to achieve changes in body mass, advice around drivers of intake and expenditure need to be discussed.

Statement Author: Danny Lennon
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Danny Lennon is the founder of Sigma Nutrition. He has a MSc. in Nutritional Sciences from University College Cork, Ireland, in addition to a BSc. in Biological Sciences and Physics.

He has hosted the popular evidence-based podcast Sigma Nutrition Radio since 2014, where he talks with academic researchers and dietitians about nutrition and health science.

You can follow Danny on Instagram at @dannylennon_sigma or on Twitter at @NutritionDanny.

Comments

  1. Excellent write-up, love all the work you do to counter misinformation

  2. It’s a good read. I think the non homeostatic drivers and sleep are huge issues, and its full effects on society are going to be huge issues that might only be revealed years after it initially started. Diet quality and food environment is also a huge factor.
    More people are noticing these things but unfortunately the demand is still not huge enough at the moment to make a significant impact cost wise. Anecdotally, i am usually made to pay a huge premium for good quality food, especially when eating out. Hopefully, with this current focus on health due to covid, this would change.

    1. Author

      Agreed Sean. I too feel that the greatest gains in population health can come from a focus on some of the environmental and societal factors that you mention.

  3. Thank you for this excellent article. Kevin Hall’s studies demostrated that the cumulative body fat loss was more pronounced with reduced fat diet. His last study has shown that there was less energy intake in the ad libitum low-fat diet when compared with ad libitum low-carb diet.
    The only thing that bothered me was the levels of insulin; since it was significantly lower with lower-carb diets. Having a better control of your blood sugar and lower weight significantly lowers microvascular complications of metabolic syndrome but it is the lower insulin level that decrease macrovascular complications. What is your thought on this particular topic ?

    1. Author

      Correct, that study did so more in the reduced fat condition, however translating that to pragmatic terms it is likely useful to view it as no real difference between isocaloric, protein-matched diets differing in carb/fat ratios, given the magnitude of the difference.

      And yes the ad libitum work is informative, and is supported by the epidemiology of plant-based diets, where it is the norm to see lower average caloric intakes.

      For me, it’s important to distinguish between chronic hyperinsulinemia versus elevations in insulin in response to feeding. On a higher carb diet, there will be increases in insulin postprandially, but this is just a normal response; we want blood glucose to be brought back down. So in healthy people, having greater insulin secretion over the day due to consuming more carbohydrates isn’t really a problem. This needs to be differentiated from situations where there is chronic hyperinsulinemia and hyperglycemia. The role of low-carb diets in managing diseases like type 2 diabetes has of course got a lot of attention, and it may be a useful management strategy. The extent of the benefits do seem to relate most to 1) overall energy, and 2) protein intake.

  4. WoW!!One of the best and lucid write ups i have ever come across.Extremely worth the time spent reading this.
    In the resistance training section where you mention , new muscle growth, are you suggesting to hyperplasia for newbie lifters or is it just a generic reference to hypertrophy.

    1. Author

      Thanks Abhishek! New muscle growth can occur in people beyond newbie lifters. With an appropriate stimulus, programmed with progressive overload, and supported by adequate protein intake, then muscle can be gained without a caloric surplus. But the more advanced one becomes the more difficult this is. So for newbies, then muslce gain is easy, even if in a calorie deficit. For very advanced lifters, then it’s likely a surplus will be needed in order for practically significant amouts of tissue to be gained.

  5. A wonderfully put together summary. If only more people would read it! So much misinformation fed out using cherry-picked or improperly interpreted science it is refreshing to know that you provide well-researched accurate information. Thank you

  6. Thank you Danny for a really great read to devote a morning to. I am glad you touched on the misunderstandings and real scientific meaning behind fat oxidation. And too, all the examples of how the physiology plays out in real life – alongside definitive metabolic ward studies and the like, providing a thorough and clear background of understanding. Appreciated.

  7. A brilliant article guys. Thank you so much for taking the time to put this out there.

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