Quack Asylum: “Don’t Eat Vegetables”

In Podcasts by Danny Lennon2 Comments

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Introduction

In this episode Alan and Danny aim to address the idea that you shouldn’t eat vegetables, or that they aren’t beneficial. We will specifically look at a number of claims that relate to:

  1. The claim that vegetables aren’t beneficial for health, or that there is no health benefit to high vegetable intake.
  2. The claim that vegetables are actually detrimental to health, and their removal improves health.

Overview

Two related ideas have been circulated in some nutrition/health communities on the internet:

  1. Vegetables aren’t beneficial for health (or that there is no health benefit to high vegetable intake).
  2. Vegetables are actually detrimental to health, and their removal improves health.

Such advice is usually defended through some combination of the following claims, which we examine in this episode:

  1. Humans are naturally carnivores, or have evolved to thrive on animal foods, and only turn to plants in times of famine.
  2. Certain indigenous populations such as the Inuit or the Masai, eat close to no vegetables, yet have robust health.
  3. Many of the nutrients present in vegetables can be obtained from animal foods. And beyond that, these nutrients are more bioavabilable when coming from animal sources.
  4. Fibre is not an essential nutrient, and high-fibre diets don’t lead to the health benefits that are typically claimed.
  5. Certain compounds in plants are actively harmful to us. Some of these compounds are natural pesticides, aimed to hurt us. Others are anti-nutrients, which decrease absorption of other key nutrients.
  6. Plants/vegetables contain compounds/nutrients exacerbate clinical conditions such as IBS or autoimmune disorders, and removing all plants including veg, leads to improved outcomes in these people.
  7. There is no benefit to a diet high in vegetables compared to a diet with low/no vegetable consumption.

Detailed Study Notes

In the episode, we highlighted the following claims, which are usually the basis for those claiming that vegetables are not health-promoting:

  1. Humans are naturally carnivores, or have evolved to thrive on animal foods, and only turn to plants in times of famine.
  2. Certain indigenous populations such as the Inuit or the Masai, eat close to no vegetables, yet have robust health.
  3. Many of the nutrients present in vegetables can be obtained from animal foods. And beyond that, these nutrients are more bioavabilable when coming from animal sources.
  4. Fibre is not an essential nutrient, and high-fibre diets don’t lead to the health benefits that are typically claimed.
  5. Certain compounds in plants are actively harmful to us. Some of these compounds are natural pesticides, aimed to hurt us. Others are anti-nutrients, which decrease absorption of other key nutrients.
  6. Plants/vegetables contain compounds/nutrients exacerbate clinical conditions such as IBS or autoimmune disorders, and removing all plants including veg, leads to improved outcomes in these people.
  7. There is no benefit to a diet high in vegetables compared to a diet with low/no vegetable consumption.

Here are additional study notes relating to each claim.

1: Humans are naturally carnivores, or have evolved to thrive on animal foods, and only turn to plants in times of famine.

This claim has become popular amongst though in the “Carnivore Diet” community. For example, you can see this type of rhetoric in blog posts like this.

As is usually the case with quackery, such claims are based in “half-truths”. For example, some things that we do know (from Mann, 2007):

  1. We developed a larger brain balanced by a smaller, simpler gastrointestinal tract requiring higher-quality foods based around meat protein and fat.
  2. Study of hunter-gatherer societies in recent times shows an extreme reliance on hunted and fished animal foods for survival.
  3. Optimal foraging theory shows that wild plant foods in general give an inadequate energy return for survival, whereas the top-ranking food items for energy return are large hunted animals.

Evidence of early plant consumption:

  1. Multistep food plant processing at Grotta Paglicci (Southern Italy) around 32,600 cal B.P.
    1. Over 30,000 years ago
    2. Residue analyses on grinding tools showing processing of various plants
    3. Oat grains – pestle grinder was used, and thermal treatment before grinding
  2. The plant component of an Acheulian diet at Gesher Benot Ya‘aqov, Israel
    1. “These remains, some 780,000 y old, comprise 55 taxa, including nuts, fruits, seeds, vegetables, and plants producing underground storage organs. They reflect a varied plant diet, staple plant foods, seasonality, and hominins’ environmental knowledge and use of fire in food processing.”
  3. Cooked starchy rhizomes in Africa 170 thousand years ago
    1. “… earliest direct evidence for the cooking of underground storage organs. The edible Hypoxis rhizomes appear to have been cooked and consumed in the cave by the Middle Stone Age humans at the site. Hypoxis has a wide geographical distribution, suggesting that the rhizomes could have been a ready and reliable carbohydrate source for Homo sapiens in Africa, perhaps facilitating the mobility of human populations.”

What do anthropologists say? It’s as close to a complete consensus as you can get, that humans are omnivorous and are highly adaptable animals, being able to thrive on a variety of different foods.

Arguments based on logic that “humans evolved on animal foods”

As I (Danny) mentioned in this episode, I totally understand how people may see the argument as logical, because when I first heard the evolutionary biology argument it also made complete logical sense. And in fact, I do think evolutionary biology does tell us quite a lot. But… there are two main issues:

  1. Is the idea humans evolved on a carnivore diet actually accurate?
  2. Even if we were to grant that first point, is that the best way to assess the health impact on us modern day individuals?
    • E.g. if we stumble across a bee hive, should we gorge on as much honey as we possibly can, simply becuase that’s what we would have done in pre-agricultural times?
    • Many aspects of modern civilization, society and law are in opposition to how our ancestors were “hard-wired” or would have acted.

Mann, 2007: “Carnivores tend to have a well-developed acid stomach and long small intestine. The human gut with its simple stomach, relatively elongated small intestine and reduced caecum and colon, does not fit any one group but lies between the frugivore and faunivore groups.”

#2: Certain indigenous populations such as the Inuit or the Masai, eat close to no vegetables, yet have robust health.

This claim dismisses the human ability to adapt, and that diets consumed in one area or by one demographic do not allow us to extrapolate health claims beyond that.

Cordain et al., 2005: “Similar to historically studied hunter-gatherers, there would have been no single universal diet consumed by all extinct hominin species. Rather, diets would have varied by geographic locale, climate, and specific ecologic niche.”

Inuit:

  • Have a mild form of CPT-1a deficiency known as “the arctic variant”
    • True CPT-1a deficiency is a rare genetic disorder and is potentially fatal
    • CPT1 helps transport fatty acids into the mitochondira
    • It has a few isoforms (1a, 1b, 1c) – with CPT 1a being involved in ketogensis
  • So the Inuit have low ketone production relative to what one would expect – i.e. don’t enter “deep” ketosis

Individuals from populations with high-starch diets have, on average, more salivary amylase gene AMY1 copies than those with traditionally low-starch diets (Perry et al., 2007)

#3: Many of the nutrients present in vegetables can be obtained from animal foods. And beyond that, these nutrients are more bioavabilable when coming from animal sources.

An example of this claim is that put forward by Paul Saladino, between 3.13 and 3.27 of this clip:

A number of examples of higher nutrient content and/or bioavailability from animal sources are often given. For example:

  • Direct DHA (omega-3 found in fish) consumption is proposed to be more beneficial than ALA (omega-3 in plant foods that must be converted in the body to EPA and DHA)
  • Vitamin A retinol is more bioavailable than Vitamin A carotenoids
    • Retinol = preformed vitamin A
      • From animal foods: liver, eggs, oily fish, etc.
    • Carotenoids (e.g. beta-carotene) = provitamin A
      • Found in carrots and other vegetables and fruits
    • Carotenoids converted to retinol in the body
    • The vast majority of vitamin A intake is from carotenoids in plants
  • Vitamin K2 has greater bioavailability than Vitamin K1
    • K1 = found in leafy green vegetables
    • K2 = found in fermented foods and some animal products
  • Heme Iron is more bioavailable than non-heme iron
    • RDA for vegetarians is 1.8 times higher than for people who eat meat (18mg vs. 33mg)
    • The bioavailability of iron is approximately
      • 14% to 18% from mixed diets
      • 5% to 12% from vegetarian diets

The idea that we can get all the nutrients we get from plants from animal sources may be technically correct in relation to essential nutrients. However, it does not account for non-essential nutrients that can still have health-promoting effects (e.g. phytonutrients, including polyphenols). These compounds (and their metabolites) are know to have beneficial health effects.

Additionally, “essential” doesn’t tell us about optimal health or disease risk, it simply tells us about survival.

This focus on the nutrients in animal foods being more bioavailable than those in plant foods is reductionist as it over-indexes on nutrient density, at the expense of other important aspects of diet. One example is the non-essential bioactive compounds like polyphenols, mentioned above. Additionally, aspects like fibre intake, caloric density, variety, etc. all impact health.

#4: Fibre is not an essential nutrient, and high-fibre diets don’t lead to the health benefits that are typically claimed.

Some prominent figures in the carnivore community make the point that“most plants aren’t edible, but animals are”. However, such a point really tells us nothing about health impacts

For their consistency with what is “natural” and how we’re “evolved”, I referenced this tweet by Alex Leaf discussing prebiotic oligosaccharides in breast milk:

Gorilla diet and SCFA (based on Popovich et al., 1997):

  • Mean macronutrient concentrations (% of calories) were:
    • Fat 0.5 ± 0.4
    • Protein 11.8 ± 8.2
    • Available carbohydrate 7.7 ± 6.3
    • Dietary fiber 74.0 ± 12.9
  • So where do they get their calories from? Much of the fibre is converted into short-chain fatty acids (SCFAs), which provide “metabolizable” energy. So the metabolizable energy comes from:
    • Fat = 2.5%
    • Protein = 24.3%
    • Available carbohydrate = 15.8%
    • SCFA, derived from colonic fermentation of fiber = 57.3%
  • The authors of the paper state: “We suggest that humans also evolved consuming similar high foliage, high fiber diets, which were low in fat and dietary cholesterol.”
  • But advocates for low-carb and high-animal fat diets have instead twisted such an observation to back up their claims that high-fat (particularly high saturated fat diets) are healthy based on evolutionary reasons are this example from our primate relatives.
    • For example, see the 1 minute of commentary betwen 9.20 and 10.14 of the below lecture by a guy called Barry Groves, titled “Homo Carnivorus What We Are Designed to Eat

This same erronenous argument gets parroted elsewhere by other pushing a high animal-fat diet:

… even though it supports nothing of the sort.

#5: Certain compounds in plants are actively harmful to us. Some of these compounds are natural pesticides, aimed to hurt us. Others are anti-nutrients, which decrease absorption of other key nutrients.

“Vegetables contain TOXIC compounds!”

  • Based on idea that compounds like lectins, saponins, phytate, and oxalates are all defense mechanisms to dissuade animals from eating them.

Carnivores regularly reference Bruce Ames’ toxicology work from 1990. Reading that paper, consider some key points made:

  • “We calculate that 99.99% (by weight) of the pesticides in the American diet are chemicals that plants produce to defend themselves.”
  • “About half of all chemicals tested chronically in animal cancer tests at the maximum tolerated dose are carcinogens. The MTD of the test chemical is a near-toxic dose that can cause chronic mitogenesis, often as a result of cell killing.”
  • “… it is probable that almost every fruit and vegetable in the supermarket contains natural plant pesticides that are rodent carcinogens”
  • The paper estimates Americans (as of 1990) eat about 1.5g of “natural pesticides” per day. But “natural pesticides” as used in this study include: chlorogenic acids, caffeine, phenolic compounds, flavonoids, as well some of the anti-nutrients we’ll discuss in a moment.
    • Side note: chlorogenic acid was mentioned in our coffee episode (358), while phenolic compounds and flavanoids are discussed in our polyphenol episodes (406 and 407).
  • But here’s the real kicker, directly from that paper:
    • “Caution is necessary in interpreting the implications of the occurrence in the diet of natural pesticides that are rodent carcinogens. It is not argued here that these dietary exposures are necessarily of much relevance to human cancer. Indeed, a diet rich in fruit and vegetables is associated with lower cancer rates. This may be because anticarcinogenic vitamins and antioxidants come from plants. What is important in our analysis is that exposures to natural rodent carcinogens may cast doubt on the relevance of far lower levels of exposures to synthetic rodent carcinogens”.

Much of the claims around the harm of “toxic” compounds in vegetables amount to referencing rodent studies with high-dose intakes of these compounds showing detrimental impacts.

But… the real question is: Does the presence of such compounds mean that eating vegetables is bad for you? And in general the answer is no.

Knowing if a food simply has such compounds tells us nothing about long-term health effects for humans. We need to look at outcome data, specifically trials in humans with actual health outcomes measured.

We need to ask about actual whole food plant consumption, not isolated compounds in super high-doses impacting a mechanism that may (or may not) lead to certain issues.

Let’s consider some specific points in relation to:

  1. Phytates
  2. Lectins

Phytates

  • Phytates (phytic acid) present in whole grains, seeds, legumes, some nuts
  • Can decrease the absorption of iron, zinc, magnesium, and calcium.
  • Cooking the food will reduce the phytate content to some degree. Other methods can reduce this even further; e.g. soaking, sprouting, fermentation, pickling.
  • A high proportion of the phytate (37-66%) is degraded in the stomach and small intestines.
  • But as we noted with phytonutrients, phytic acid can also have benefits through it’s metabolites having secondary messenger roles in cells
  • Foods higher in phytate seem to enhance the activity of natural killer cells, which could potentially have beneficial impacts on reducing tumor growth
  • In fact, if you’ve ever seen a IP6 supplement, that’s phytate: inositol hexaphosphate
  • Similarly, you may have seen myo-inositol supplements, which have decent evidence in PCOS, and this inositol is obviously one of the main components of inositol phosphates, including IP6.

Lectins

  • Lectins are found in legumes (beans, peanuts, soybeans) and whole grains
  • They potentially can interfere with the absorption of calcium, iron, phosphorus, and zinc.
  • The Plant Paradox (a popular diet book by Steven Gundry) describes a diet and lifestyle program intended to prevent and reverse most chronic diseases by eliminating their supposed root cause: plant lectins. – see this excellent Red Pen Review. From that review:
    • “Current scientific evidence on the health effects of lectins in humans is very limited, so the claim that consuming vegetables on this basis is not evidence-based.”
    • “We were unable to identify even modest evidence that lectins may be a root cause of obesity or any of the chronic diseases mentioned in the book.”
  • If you are really concerned about decreased absorption of minerals:
    • Consider cooking and prep methods
    • Have certain meals containing those nutrients that are not high in the anti-nutrients
    • Food combining – balance out the effect by including enhancers of mineral-absorption, e.g.: vitamin C for iron, or garlic/onions for zinc/iron.

Do plants contain “toxins”?

Indeed there are some compounds in food that one could legitmately call a toxin, for example:

  • Various beans, including kidney beans, contain a “toxin” known as phytohaemagglutinin.
    • Phytohaemagglutinin can be destroyed by soaking and/or cooking thoroughly at boiling temperature.
    • So eating raw or inadequately-cooked beans can cause food poisoning within 1-3 hours
    • Tinned beans are safe to eat without heating, as they have already undergone thorough heat-treatment.
  • Apple seeds and cassavascontain cyanogenic glycoside.
    • When raw or inadequately-cooked cassavas are ingested, the toxin will be transformed into a chemical called hydrogen cyanide, which may result in food poisoning.

Anthropologist Mary Douglas wrote a book Purity and Danger in 1966, examining the concepts of purity, pollution or what is unclean in a society: deeming one thing natural and another toxic is about imposing a moral order on the world around you.

#6: Plants/vegetables contain compounds/nutrients exacerbate clinical conditions such as IBS or autoimmune disorders, and removing all plants including veg, leads to improved outcomes in these people.

This may be the area where there is the strongest argument for a diet that restricts or eliminates vegetables, as so little is known about diet and autoimmune disease. In addition, it is known that short-term low-fibre diets can play a role in treating certain sub-types of IBS or in the context of acute symptom management for gastrointestinal disorders.

However, it’s not accurate to claim there is good evidence for the use of such diets to treat certain disorders either.

See previous podcast episodes that discuss these issues in more detail:

#7: There is no benefit to a diet high in vegetables compared to a diet with low/no vegetable consumption.

This claim tends to get based on individual studies that either focus on isolated components (e.g. green tea extract) or on individual mechanisms (e.g. impact on DNA damage/repair).

Perhaps the most consistent finding in all of nutrition science, in relation to healthy diet patterns or food groups, is the health benefit of vegetable consumption.

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Comments

  1. Putting transcripts of podcasts behind a paywall is discriminating against those who, for whatever reason, cannot listen to podcasts (deaf, hard of hearing, or otherwise unable to listen due to chronic pain etc.).

    Please reconsider this policy.

    1. Author

      Hi Jane,

      I appreciate the comment and understand that perspective. And this was not a decision made lightly.

      However, the transcripts to the episodes are an add-on to a podcast, rather than a core part. They are done at a relatively significant cost, given that an individual is paid to transcribe hours of audio each week.

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