Are vegan diets bad for fitness?

If you’ve cut out all meat, fish, eggs, and dairy – or are thinking about it – you’ve doubtless heard that it’s good for you.

Once, I was a vegan myself, and considered it not only an ethical choice, but an intrinsically healthier one for the long term, too.

But is this really true?

Now, I’m not a nutritionist or doctor, just an ordinary writer at Personal Trainers London, but what I’ve learned from reading some research and from personal experience is that the vegan diet – at least as it’s commonly and naively practiced – is a nutritional minefield.

If you want to walk the path of fitness and strength for any distance, you had better do it with your eyes open, to avoid falling down one of the pits I’m about to point out for you.

What do we mean by “bad for fitness”?

Health and fitness, diet and training all go together.

  • To be fit, you need energy; without enough get-up-and-go you won’t be able to actually push through your workout.
  • To work out strongly, you need to be in robust health. You can’t work out properly if you have a serious disease, if your joints are inflamed, or even if you have a cold every other week.
  • And of course, to build strong muscles and the healthy connective tissue that supports them and protects you from injury long term, you need all the ingredients for growth.

Your training regime, your genetics, and your attitude influence all of these things, and especially your diet does too, to a huge degree.

Cutting out all animal products, according to the research, can have serious consequences for energy, health, and growth.


Here are just a few of the dietary factors that influence our energy.

Have you got the minerals?

Not just a great quote from “Snatch”, this is actually a serious question if you want to have the energy to train well.

Quote from Snatch: "You wanna see if I've got the minerals?"

One mineral you need is iron, for proper heart function, immunity, and energy. Iron is used to produce red blood cells, which take oxygen from the lungs. It’s also a component of myoglobin, a protein that provides oxygen to muscles. Not surprising that one of the first symptoms of an iron deficiency is tiredness and a lack of energy1.

“Ok!” I thought, “green leafy veg is full of iron, so I’m fine.” Unfortunately, it’s not that easy.

Plants contain less iron

WebMD lists the Top 10 sources of iron, and all of them are animal products2. But since not everyone is eating organ meats, here’s a comparison of some commonly eaten animal and vegetable sources of iron (excluding those with iron artificially added, and those such as herbs and spices which are not eaten in significant weights) in order of mg iron per 100g3:

Animal-derived foodIron content in mg/100gPlant-based foodIron content in mg/100g
Oysters, canned6.7Soybeans, raw3.55
Beef liver6.54Lentils, sprouted, raw3.21
Beef steak, grilled3.24Spinach, raw2.71
Egg, raw1.75Kale, raw1.47
Turkey, roasted1.09Kidney beans, canned1.17

Now, you might think the plants came off ok in that list, but remember, it’s by weight. It’s much easier to eat an 8oz steak than it is 220g of raw spinach or kale – that’s two normal bags of the greens from the supermarket.

It’s hard to reorder the list by serving, because my serving might be different from yours, but unless you eat really outlandishly huge amounts of vegetables every day, the plant sources definitely come off significantly lower in terms of total iron content.

But even if you are eating Popeye-like servings, there’s something else you should know.

Iron in plants is harder to absorb

There are two types of iron in food: heme (the animal version) and non-heme iron (the plant version).

Iron ore

Some iron is not very absorbable

One of the recommendations for making non-heme iron easier to absorb is to combine it with heme iron – i.e. to eat animal products!
Cutting out all animal products can make non-heme iron up to four times harder to absorb4.

Plants contain other compounds that block mineral absorption

The same plant foods that you eat in order to get your minerals are also full of compounds that prevent you absorbing those same minerals in your gut.

It’s not what you eat, it’s what you absorb that counts. Tweet: It's not what you eat, it's what you absorb that counts.[Click to tweet]

Vegans rely on whole grains, legumes, soy, and beans as sources of protein, as well as sources of minerals such as iron. Unfortunately, they’re also rich sources of phytic acid.

Phytic acid, although possibly having beneficial effects in small amounts, is problematic when taken in excess because it binds divalent minerals (iron, zinc, calcium, magnesium, manganese etc.) making them unavailable for absorption or poorly absorbed in the gut.5


Magnesium, mentioned in that list of minerals that are made harder to absorb by phytic acid, is involved in countless chemical reactions in our bodies, including the conversion of glucose to energy.

3D molecule of glucose

Mmmm… glucose…

Athletes who’re absorbing magnesium poorly to the point where they have a mild deficiency “wouldn’t be able to work or train as long as they would if they had better magnesium levels.”6


And lastly, to round off this bleak picture of mineral absorption in the vegan diet, there’s calcium. Again, veggie favourites like spinach do contain lots of calcium, but they also contain lots of oxalates.

Oxalates, apart from not being so healthy in themselves in large amounts, block calcium absorbtion. According to one scientific study, it takes 16 servings of spinach to equal the bioabsorbable calcium from one glass of milk7.

Calcium is not only needed for the increase in bone density that is one of the advantages of engaging in weight-bearing exercise, but also plays a key role in muscle contraction and cardiovascular function.

There’s another reason that a purely plant-based diet can be prone to mineral deficiency – and we’ll talk about Vitamin D in a bit. But first, another essential vitamin.

You really, really need Vitamin B12

This is one you’ve heard of before, but there’s a couple of things you might not know. Firstly, the consequences for screwing up your B12 intake are not just being a bit tired.

The B vitamins in general are known for their role in energy production. And B12 in particular is necessary not just for general metabolic function and the production of red blood cells, but also for maintaining the health of the nervous system and spinal cord8.

Vitamin B12 keeps nerves healthy

Lack of Vitamin B12 damages nerves

If you don’t get enough B12, you can get nerve problems. This can include pins and needles, disturbed vision, and more9.

Most of us know that B12 is present in good amounts in animal products such as eggs, dairy, and meat. And of course vitamin supplements, and foods with vitamins added. But you might not know that this is literally the only way to get B12.

Some vegans are under the misapprehension that there are some plant sources of B12, like chlorella, a kind of algae.

There really aren’t.

What there are, though, is poorly executed or interpreted methods of measuring analogues, compounds called cobamides that are chemically similar to vitamin B12 but have not been shown to improve B12 deficiency10.

Unscrupulous or just ignorant manufacturers can make the scientifically unjustifiable leap from there being “B12” detected in a food to that food actually being a source of B12 for humans. Again – seaweed will not protect you here.

Seaweed mask

Seaweed – good for many things, just not Vitamin B12

For optimum health, intake of B12 actually needs to be higher than just the minimum required to avoid deficiency. Interestingly, the venerable Vegan Society itself mentions, in its informative article on B12, that “repeated observations of elevated homocysteine in vegans… show conclusively that B12 intake needs to be adequate … to avoid unnecessary risk”11.

We’ll come on to inflammation later, but for now just know that homocysteine is something you do not want lots of in your body.

So you must supplement your way out of this situation12 (or just go to work on an egg).

Why some people do well for years on a vegan diet

Now, it does take some months or years for the body’s stores of certain nutrients to become exhausted, once you stop eating animal produce. But once you’ve used up what you’d stored from eating non-vegan food in the past, long-term, any deficiencies predispose you to fatigue, poor endurance, inflammation, nerve problems, and heart disease.

But that’s not all.


Many foods that are included in a vegan diet are healthy – mainly fresh vegetables. And if vegans eschew processed foods, they can cut out many disease-promoting foods.

But by doing this to the exclusion of all meat, fish, eggs and dairy, many health-promoting nutrients become very hard to come by.

Vitamin D

This vitamin – actually a hormone – performs hundreds of functions. It protects against cancer, helps absorb calcium, and reduces inflammation throughout the body.

There are no fruit or vegetable sources of Vitamin D. It’s added in small amounts to “fortified” – i.e. processed, therefore usually not particularly healthy – vegan foods such as margarine and breakfast cereals.

It’s also contained in algae, and certain mushrooms. These sources, though, are so unimpressive that the British Dietetic Association lists ZERO vegan, non-fortified sources in its list of Vitamin D rich foods13.

Not everyone lives in foggy London town. If you’re in southern latitudes, great – you can make all your Vitamin D from the sun. For the rest of us, take a look at the following table of non-fortified food sources of Vitamin D14:

FoodVitamin D in International Units per 100g
Salmon, canned841
Egg, fried88
Cheese, cheddar24
Shiitake mushrooms, stir-fried21
White mushrooms, raw7

Now imagine how many mushrooms you’d have to eat to get your recommended 400IU a day.

And there’s another catch.

Plant Vitamin D is inferior to animal Vitamin D

The type of Vitamin D found in plants, and the type most commonly added to fortified foods (because it’s cheaper) is D2, ergocalciferol.

Vitamin D3, cholecalciferol, found in animal produce, is 70% better at raising blood concentrations of 25-hydroxyvitamin D, the modern marker of Vitamin D status in humans. The American Journal of Clinical Nutrition goes on to say that “Vitamin D2, or ergocalciferol, should not be regarded as a nutrient suitable for supplementation or fortification.”15

If it’s not suitable for fortifying foods, should we rethink whether it’s really a suitable choice to rely on in our food?

Conjugated Linoleic Acid (CLA)

Let’s take a break from deficiencies and look at something that’s just good for you. Butter.

Homer Simpson smoking butter

Don’t try this at home, kids

Yes, you read that right, butter, dairy in general, and meat from ruminant animals, especially when it’s grass-fed, is an excellent source of this natural, healthy trans fat (not to be confused with the artificially created, horrendous-for-your-health trans fats found in non-dairy butter substitutes).

Unfortunately, it seems that plant foods are very poor sources of this nutrient. And when it does occur, it’s in a different form, which is not as healthy.

CLA has been studied in animals and humans for preventing heart disease16, improving insulin sensitivity17, and reducing the risk of cancer18.

Supplements, wisely used, can be a good way to fill in some nutritional gaps. But think twice before try to supplement this one. CLA supplements are made by chemically altering safflower and sunflower oils, resulting in different proportions of the sub-types of CLA (specifically, too much t10,c12-CLA)19. Small differences in chemical composition can have huge differences in the effect of such compounds.

How to destroy your Omega 3:6 balance in 3 easy steps

  1. Cut out the best sources of Omega 3: meat, fish, butter, and eggs
  2. Overdose on Omega 6 from vegetable oils
  3. Supplement with poorly-converted Omega 3 from plants

The very conservative report by the US Dept. of Health20 says: “According to both primary and secondary prevention studies, consumption of omega-3 fatty acids, fish, and fish oil reduces all-cause mortality and various CVD outcomes such as sudden death, cardiac death, and myocardial infarction.”

Reducing all-cause mortality basically means you live longer if you eat O3.

Old couple holding hands sitting on deck chairs


Actually we’ve all heard that we need to eat more Omega 3 essential fatty acids. Well, that’s a bit of a simplification.

What we really need is to keep total O3 and O6 in a good range (too much total easily oxidised polyunsaturated fatty acids is itself a health risk), and most importantly, keep them in balance.

Roughly speaking, while O6 is pro-inflammatory, O3 is anti-inflammatory. It interferes with the conversion of O6 to inflammatory compounds because it competes for the same conversion enzymes, besides having other active anti-inflammatory effects itself through chemicals called resolvins and protectins21.

Both O3 and O6 are needed for health, but pretty much everyone in the West eats far, far too much O6 compared to O3. The best guess for the ideal ratio (governments won’t be drawn on this) is 1:1, or maybe 2:1. The average ratio for Americans is estimated to be between 10:1 and 20:122, and you can bet the Brits aren’t faring too much better.

Remember the report from the US Dept. of Health that said Omega 3 was very important? Unfortunately, they fail to sufficiently distinguish between the different forms of O3, including EPA, DHA, and ALA. So let’s look at that.

The plant form of Omega 3 is… wait for it… much harder to absorb

The healthful effects of fish and fish oil are so well-studied I’m not even going to give any links here. Those effects are due to the long-chain fatty acids eicosapentanaenoic acid (EPA) and (more importantly) docosahexaenoic acid (DHA), also contained in smaller amounts in meat and eggs.

Now, the critical mistake that vegetarians make is to assume that the plant form of O3, alpha-linolenic acid (ALA), is equivalent, and that therefore you can get the same benefits from consuming, say, hemp, or flax.

Turns out it’s not.

And the reason is that the conversion steps to get from ALA (which the body cannot use directly) to DHA are very inefficient.

Only around 4% of ALA is converted to DHA. And in unfavourable conditions – such as those provided by a vegan diet high in O6 from sunflower oil, walnuts, etc – the ratio can get worse still23.

In other words, it’s up to 50 times better to eat DHA than ALA.

Omega 3 supplements

DHA can be supplemented in fish oil or algae-derived forms

When vegan foods are measured for nutrient content, the picture is often misleadingly positive. But as we’ve seen, that’s not a good methodology. When the blood of vegans and, to a lesser extent, vegetarians, is measured, deficiencies of various kinds can surface.

And DHA is a prime example – several studies have demonstrated that vegetarians and vegans have much lower plasma concentrations of EPA and DHA when compared to those who eat fish24.

So unfortunately, non-fish eaters are, by default, putting themselves at more risk from all kinds of inflammatory disease.


There are so many biological factors that affect growth, but let’s look at the simplest one – protein.

For sure, vegans can get a certain amount of protein, by including beans, pulses, tofu, and alternative protein sources in their diet. Life’s too short to argue about whether this is a sufficient amount. But if you do eat a plant-based diet and you actually audit the amount of protein you eat, you’ll probably be surprised at how little it is.

In any case, all protein is not created equal.

Eggs are literally the standard by which the bioavailability of all other protein is measured. They score the highest possible – 1.00 – on the protein digestibility-corrected amino acid score (PDCAAS), the currently preferred scientific measure.

Here are some more scores:

PDCAAS scoreFoodstuff
1.00 casein (milk protein)
1.00 egg white
1.00 soy protein
1.00 whey (milk protein)
0.99 mycoprotein
0.92 beef
0.78 chickpeas
0.75 black beans
0.73 vegetables
0.52 peanuts
0.50 rice
0.42 whole wheat

Again, you might be thinking that soy comes out pretty well. However, the scores are capped at 1.00, and the scores of animal proteins are actually likely higher. Regardless, it should be clear that vegetable sources of protein are in general not as complete.

More than that, the scores don’t take account of antinutrients.

Why plant protein is harder to digest

Soy, the go-to protein of so many vegans, contains substances that have been shown to decrease protein absorption. It contains protease inhibitors and lectins, which are destroyed by cooking, and goitrogens, tannins, phytoestrogens, flatus-producing oligosaccharides, phytate, and saponins, which are not25.

In animals, the trypsin inhibitors in soybeans and other plant sources of protein can cause reductions of up to 50% in protein and amino acid digestibilities26.

Alternative protein sources: fake meat and inflammation

We hinted before that inflammation is bad. But how bad? Chronic inflammation is increasingly thought to be behind most of the diseases of industrialised countries like cancer, diabetes, heart disease, and chronic auto-immune disease27.

Inflammation is super bad for you

Inflammation is super bad for you

So how do vegan meat substitutes affect inflammation?

As we’ve seen, common substitutions in the vegan diet decrease essential nutrients such as Vitamin D3 and DHA that protect the body from inflammation. But they also often add more of something the body doesn’t need.

Vegan meat substitutes are often high in wheat protein, which contains gluten. Some people tolerate it better than others, but there is more and more evidence that gluten (and the gliadins it contains) causes systemic inflammation28, even in those aren’t clinically “intolerant”.

Seitan, an entirely vegan tinned “mock duck” style food found in most supermarkets, is literally made of gluten29. Seitan is the main ingredient in Tofurky30, and is used by Asian restaurants to make fake meat.

Even if vegans avoid these particular products, vegetarian meat substitutes are almost always highly processed and stuffed with if not wheat protein, then other inflammation-causing substances.

Grain Brain

“Vegan diets are based on grains and other seeds, legumes (particularly beans), fruits, vegetables, edible mushrooms, and nuts.”31.

Based on grains. Almost all grains contain lots of gluten or similar anti-nutrients such as lectins, prolamins, and saponins, which are less well studied.

Those looking to avoid the consequences of systemic inflammation would do well to minimise or avoid grains, not base their diet on them.


Before making any modification in your diet, well, I should say talk to a doctor or qualified nutritionist. And then double-check what they’re telling you (it could be 20 years out of date). Don’t take a stranger’s word on the internet. Do your own research! Read widely.

I hope this article served as a useful provocation to look twice at health claims of any diet in general, of a purely plant-based diet specifically, and to underscore the need to really think about what you eat.

What do you think?

Has this article caused you to re-evaluate some long-held beliefs? Do you wildly disagree? Let us know in the comments below.

Finally, please note this is all just the opinion of the author (not necessarily of everyone at Personal Trainers London Ltd), and balance is a good thing.

So if you write a response to this article on your own site, let us know and we’ll give you a shout out. We might well invite the author of the most convincing response to contribute on this website.

This article written by: Paul

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About Author

Staff writer, Personal Trainers London

Having recently joined the fitness industry, Natasha is already making waves with her dedication to publishing detailed articles, with a special interest in eating for performance and career progression for trainers.


  1. NHS UK: “Iron deficiency anaemia”
  2. WebMD: “Iron-rich foods”
  3. United States Department of Agriculture: Food Composition Databases
  4. National Institutes of Health: Iron Dietary Supplement Fact Sheet”
  5. Dr Loren Cordain, PH.D, Professor Emeritus Colorado State University: “Phytate content within nuts”, “Vegetarian and vegan diets: nutritional disasters”
  6. United States Department of Agriculture Research Magazine: “Lack Energy? Maybe It’s Your Magnesium Level”
  7. The American Journal of Clinical Nutrition: “Choices for achieving adequate dietary calcium with a vegetarian diet”
  8. “What Is Vitamin B12?”
  9. NHS UK: Vitamin B12 deficiency
  10. “B12 in plant foods”
  11. The Vegan Society: What Every Vegan Should Know About Vitamin B12
  12. “In subjects who did not consume vitamins, low holotranscobalamin II (< 35 pmol/L) was found in 11% of the omnivores, 77% of the LV-LOV [Lacto-ovo-vegetarians] group, and 92% of the vegans.” – Vitamin B-12 status, particularly holotranscobalamin II and methylmalonic acid concentrations, and hyperhomocysteinemia in vegetarians., by Herrmann W1, Schorr H, Obeid R, Geisel J., American Journal of Clinical Nutrition 2003 Jul;78(1):131-6.
  13. The British Dietetic Association: Vitamin D Fact Sheet
  14. United States Department of Agriculture: Food Composition Databases
  15. American Society for Clinical Nutrition: “The case against ergocalciferol (vitamin D2) as a vitamin supplement”
  16. “Conjugated linoleic acid (CLA) is a potent anti-atherogenic dietary fatty acid in animal models of atherosclerosis.” – Conjugated linoleic acid and atherosclerosis: no effect on molecular markers of cholesterol homeostasis in THP-1 macrophages, in Atherosclerosis
    Volume 174, Issue 2, June 2004, Pages 261–273
  17. “Isomer-Specific Antidiabetic Properties of Conjugated Linoleic Acid” in Diabetes, 2001 May; 50(5): 1149-1157
  18. “Conjugated linoleic acids (CLAs) decrease prostate cancer cell proliferation: different molecular mechanisms for cis-9, trans-11 and trans-10, cis-12 isomers.”, by Ochoa JJ1, Farquharson AJ, Grant I, Moffat LE, Heys SD, Wahle KW., in Carcinogenesis 2004 Jul;25(7):1185-91. Epub 2004 Feb 19
  19. “Most of the CLA preparations currently available are composed of 2 major CLA isomers (c9,t11- and t10,c12-CLA) … The milk fat from cows fed a normal total mixed ration consisted mainly of c9,t11-CLA … Cheese prepared from the milk of cows grazed at high altitudes in the Alps showed a high content of the t11-containing CLA isomers c9,t11-, t11,c13-, and t11,c13-CLA.” – “Analysis of conjugated linoleic acid and trans 18:1 isomers in synthetic and animal products” in the American Journal of Clinical Nutrition
  20. US Dept of Health, “Omega-3 Fatty Acids and Health: Fact Sheet for Health Professionals”
  21. British Journal of Pharmacology, 2009 Oct; 158(4): 960–971: “Resolvins and protectins: mediating solutions to inflammation” by Payal Kohli and Bruce D Levy
  22., “How too much omega-6 and not enough omega-3 is making us sick”
  23. International Journal for Vitamin and Nutrition Research, by Gerster H. “Can adults adequately convert alpha-linolenic acid (18:3n-3) to eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3)?”
  24. Clinical Nutrition Volume 34, Issue 2, April 2015, Pages 212–218: Blood docosahexaenoic acid and eicosapentaenoic acid in vegans: Associations with age and gender and effects of an algal-derived omega-3 fatty acid supplement by Barbara Sartera, Kristine S. Kelsey, Todd A. Schwartz, William S. Harris
  25. Liener IE, “Implications of antinutritional components in soybean foods”
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  27. Khansari N, Shakiba Y, Mahmoudi M. “Chronic inflammation and oxidative stress as a major cause of age-related diseases and cancer”
  28. “Both in vitro and in vivo studies demonstrate that gliadin and WGA can both increase intestinal permeability and activate the immune system. The effects of gliadin on intestinal permeability and the immune system have also been confirmed in humans.” Karin de Punder and Leo Pruimboom: “The Dietary Intake of Wheat and other Cereal Grains and Their Role in Inflammation”
  29. “What is Seitan?”
  30. Miami New Times: “Tofurky: What the Hell Is It?”
  31. Wikipedia: “Veganism”