Just How Effective is Low-Carb Dieting?

In All Articles, Blog Posts by Danny Lennon2 Comments

In a past post, “Eat More Fat, Burn More Fat: Myth, Magic or Metabolic Advantage?”,  I talked about why carbohydrates do not cause you to gain fat, unless there is caloric surplus. Similarly, a diet that is low-carb but too high in calories will also lead to weight gain. So it is a calorie issue for sure.

But what most people miss is that calories in affects calories out.

The example I gave in that post was from an examination of overfeeding studies. Time and time again, when subjects consume a calorie intake in excess of their maintenance intake, we see all sorts of feedback loops and adaptive mechanisms kick in. Body heat production, thyroid output, non-exercise activity thermogenesis (including things like subconcious fidgeting) and exercise intensity all increase.

So an increase in intake of 1,000 kcal from the diet does not equal an extra 1,000 kcal of energy that will be stored in the body.

The tricky thing is working out exactly the role of carbohydrate intake in affecting the energy balanace equation, if it does at all.

We know that diets made up of different macronutrient composition will affect the energy balance equation. As a crude example, a high-protein, moderate-fat, low-carb diet will have a very different metabolic response to a very high-carb, low-fat, low-protein diet. The problem with such comparisons, that should be screaming out to you right now, is that we’re not changing one variable. In other words, are those differences down to the carb restriction or due to the difference in protein intake?

From my blog post I referenced above, I concluded that from the evidence we have before us right now, there doesn’t seem to be a metabolic advantage to a LC diet over a higher carb diet that is matched for calories and protein. That may have left people with the opinion that I don’t think a LC diet can be effective or that I’m anti low-carb.

That is incorrect and so let me set the record straight.

At this point I hope we’re in agreement that carbohydrates are not “evil”. And that you don’t *need* to be on a low-carb diet to lose body fat. If not, go read some of my previous posts.

So while you don’t need to be low-carb, a common question for people to want to investigate is:

Is going on a low-carbohydrate the most effective diet for losing body fat?

To answer this we need to first clarify what do we mean by “effective”?

What Do We Mean By “Effective”?

Before I give my thoughts, I’m going to lead with an extract from a piece by Drs. Allen Last & Stephen Wilson, as it sums up the state of things pretty well (bolded text is mine for emphasis):

“Short-term studies comparing traditional low-fat diets with low-carbohydrate diets found lower triglyceride levels, higher high-density lipoprotein cholesterol levels, similar low-density lipoprotein cholesterol levels, and lower A1C levels in persons on low-carbohydrate diets. These diets induce greater weight loss at three and six months than traditional low-fat diets; however, by one year there is no significant difference in maintained weight loss. Weight loss is directly related to calorie content and the ability to maintain caloric restriction; the proportions of nutrients in the diet are irrelevant. Low-carbohydrate diets had lower dropout rates than low-fat diets in several studies, possibly because of the high protein content and low glycemic index, which can be appetite suppressing. Data indicate that low-carbohydrate diets are a safe, reasonable alternative to low-fat diets for weight loss. “

Last, A. & Wilson, S.,  ‘Low-Carbohydrate Diets’, Am Fam Physician, 2006 Jun 1;73(11):1942-1948

That whole section is a good representation of the overall findings of research data so far. The bolded comments are particularly important when we consider the potential effectiveness of a LC diet.

Back in 2003 a systematic review of the efficacy of LC diets carried out by Bravata et al. concluded that that lower-carbohydrate (≤ 60 g/d) diets were associated with reduced calorie intake and that weight loss was predicted by calorie intake, diet duration, and baseline body weight, but not by carbohydrate content.

Similarly, a full 11 years later, a recent meta-analysis by Johnston et al. seems to also to point to calorie intake and adherence as the key factors for sustained weight loss, as opposed to macronutrient breakdown.

In another paper from earlier this year, which sparked a lot of discussion, Naude et al.  concluded that there was no real advantage for low-carb diets over isocaloric diets. However, there are plenty of caveats that should be pointed out with regard to that paper. I’ll talk about that in more detail in the next blog post later this week.

While meta-analyses are extremely helpful and are certainly at the top of the hierarchy of evidence, sometimes I think it can be problematic to base our conclusions solely on them. Sometimes it may be more insightful to look at individual randomized controlled trials (RCT) to see if we can determine effectiveness.

In trials of 6 months and less, LC diets seem to be more effective than low-fat diets for fat loss (or weight loss as is more commonly measured). One such example is this 2003 paper by Brehm et al., which is helpful as it used healthy subjects as opposed to people with significant metabolic derangement. In that trial, the LC group lost more body fat (4.8 +/- 0.67 vs. 2.0 +/- 0.75 kg; P < 0.01) than the low fat diet group, despite it being reported that ” women on both diets reduced calorie consumption by comparable amounts at 3 and 6 months”.

A two-year study comparing a very low-carb (initially < 20 g/d) and a low-fat diet, published by Foster et al.  in 2010, tells a similar story. In that study the low-carb group had slightly more weight loss after 3 months but for the rest of the study weight loss in the groups was pretty much the same.

Effectiveness: Physiologically vs Pragmatically

The research currently weighs in the favour of low-carb and low-fat diets being equally effective for weight loss; once calories, protein, diet duration and adherence are matched. So we certainly can’t conclude that there is a metabolic advantage for them, at least based on the totality of the literature.

However, as I’ve said in previous posts, the question we must always strive to answer is “what is the practical significance of this?”

Now, consider the following thought experiment. We create two diets that are matched for calories and protein. The dieters are metabolically healthy. And hypothetically, we know the dieters will stick 100% to the prescribed diet. One diet has a higher amount of carbs and less dietary fat. The other diet, the reverse.

It’s safe to assume that the results a given individual will obtain from being placed on either one of those diets will be the same (provided all other variables, like activity and training, are the same).

So the low-carb diet is technically no more (or no less) effective than the higher-carb diet. From a phyiological perspective.

But we’re missing a key piece from a practical point of view; How many people who start implementing  a low-fat diet, will end up with the same calorie and protein intake as if they had decided to cut carbs? Is the probability of long-term adherence (which is the most important factor for diet success) the same for both diets?

Calorie Intake

When a paper is published showing support for LC diets being more effective for weight loss, one of the common points that critics make is “Of course it was seen to be more effective! When people reduce carb intake they subconsciously reduce calorie intake”.

This is often thought of as a point to disprove the efficacy of a LC diet. But if we consider this from a pragmatic perspective, rather than a metabolic one, then is it in fact supporting the action of recommending overweight populations to lower carbohydrate intake?

I mean, if we agree that energy balance is the key to body composition change, and we can get someone to subconsciously reduce caloric intake by simply telling them to reduce the amount of carb-rich foods in their diet, isn’t that a pretty easy way for the average person (who is not receiving coaching) to drop weight?

A good example of this is shown in that Foster et al. paper I referenced earlier. Recall that that was a two-year study, which showed the same weight loss for both a very low-carb group and a low-fat group. But what is interesting about that study is that while the low-fat group’s diet was calorie-controlled (i.e. they had to conciously restrict calories), the low-carb group did not have to track calories (they were free to eat as much protein and fat as they felt like).

So whilst their weight loss was the same, one group didn’t have to think about monitoring calories. Depending on the type of person undertaking a diet, this could be a hugely important factor.

For the person who has no problem tracking/logging their dietary intake, then they can use whatever they prefer. But for someone who absolutely despises having to consciously track their intake, doesn’t want any education around diet and just wants some stupid simple ‘write-on-your-palm’ advice of what to do, then perhaps they are a candidate to be simply told “go low-carb”.

Note, that I’m not arguing for or against anything here, simply pointing out some things to consider.

Protein Intake

Protein produces a high satiety response, has a higher thermic effect of feeding (requires more energy to be used in metabolizing it) than the other macronutrients and an adequate intake is important for maximising lean mass retention/growth, which correlates well with the ability to maintain a healthy body composition.

Similar to the calorie issue, when low-carb diets are seen to be “more effective”, the point is made is that it is the increased protein intake that usually occurs on a carbohydrate restricted diet that is the reason for the better outcomes, rather than the carbohydrate restriction itself.

And it’s true. If you take out carb-rich foods from the diet, what foods make up a large amount of intake? Meat, fish, fibrous vegetables, eggs, yogurt, cheese, nuts, et cetera. Protein everywhere!

Again though, I’ll pose the same question as I did in relation to calories. If simply getting someone to go on a low-carb diet can lead to a subconscious increase in protein (and therefore decreased hunger and increase energy expenditure) then do we need to care that it is not the carb restriction directly that is having the beneficial impact?


Go back to that Last &  Wilson piece: “Low-carbohydrate diets had lower dropout rates than low-fat diets in several studies, possibly because of the high protein content and low glycemic index, which can be appetite suppressing.”

An interesting point about that Johnston et al. systemic review we looked at earlier, is that as part of the inclusion criteria was that only subjects who stuck with a diet for the full length of a given study were included. But what about those who dropped out?

I’m a big believer that THE most important characteristics of a successful diet are adherence and consistency. So anything that increases adherance is going to be a useful strategy. Hence why I never give set macronutrient breakdowns for what a “good diet” needs to be. There is no such thing.

There are only two types of diets; ones that work and ones that don’t. And the only characteristic that can be 100% accurate in distinguishing between them is consistent adherence.

A recent review paper from November 2014, found that very low-carb ketogenic diets were able to suppress appetite even in hypocaloric conditions where weigh loss was occuring. Feeling less hungry and more satisfied whilst dieting obviously has practical implications for the ability to adhere to a calorie-restricted diet. So a potential positive there.

However, I would hypothesise that this potential beneficial effect on adherence is more than cancelled out by the logistics of being on a true ketogenic diet. You need to monitor carbohydrate and protein intake at meals carefully, measure blood/breath ketone levels, avoid a wide variety of foods, etc.) and how those factors affect adherence.

I’m not saying people can’t do it but we have to think about who will be on the receiving end of our recommendations. The recipient’s psychology is probably more important to consider than their physiology, especially if it’s a ketogenic diet that is being proposed. You can’t half-ass a ketogenic diet. You’re either all-in or don’t go near it. (Hint: if you’re coaching, the % of people that you will ever place on a ketogenic diet should be extremely small).

As an aside, I would be confident in estimating that a large number who claim to be on a ketogenic diet are in fact not in ketosis. They just don’t know how to put together a well-formulated  ketogenic diet.

Interestingly, in contrast to some of the other meta-analyses listed, when we look a truly very low-carb/ketogenic diet, a different conclusion was drawn. A 2013 meta-analysis of randomised controlled trials by Bueno et al., compared very low-carb ketogenic diets (VLCKD) to low-fat diets for long-term weight loss (12 months or more follow up). And this analysis showed that those on a VLCKD achieved greater weight loss in the long-term than those on the low-fat diet. As a full disclosure, I haven’t had a chance to go through every individual trial that the study examined yet.

 Take-Home Message

  • I stand by my point from a previous post, that there is no metabolic advantage for body composition to a very low-carbohydrate diet over a diet higher in carbohydrate, when calories and protein are matched.
  • However, low-carb diets can be a great recommendation for certain people.
  • Low-carb diets can be an effective fat-loss tool, especially if they cause a subconcious drop in calories, a subconcious increase in protein, increased satiety and improved likelihood of adherence, or some combination of those.
  • If you prefer lower-carb dieting, feel good, perform well and you get benefit from it, then awesome, stay doing it!
  • If you are on a low-carb diet because you feel you *have* to be on one in order to lose fat, then please realise this is not the case (for most people at least. Health issues can complicate things).
  • If you tell people that everyone should be low-carb, then stop it. You’re being a dick.
  • If you mock people who are on a low-carb diet because they say it’s effective, then stop it. You’re being a dick.
  • I addressed those above two points in THIS POST.

Right tool, for the right job, at the right time.

You may have noticed that in the original question “Is going on a low-carbohydrate the most effective diet for losing body fat?”, whilst we tried to clarify “effective”, we didn’t address perhaps an even bigger issue: what is “low-carb”? That will the addressed in the next blog post out later this week. (Make sure you’re on the Sigma mailing list so you don’t miss it. Click Here to Subscribe)


In the comments below, tell me what your current carb intake looks like and WHY.

No judgement, I just find practical n=1 anecdotes really interesting. I’ll start with my current situation as the the first comment…

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