Vegan Vs Omnivorous Diets: A Review of the Scientific Literature

January 11, 2020

I’ve hesitated for a few months to write this article. 

For a few reasons. 

It’s an emotionally charged topic. It often promotes argument rather than discussion. How we choose to eat is intensely personal. People get easily offended, fiercely protective, and respond on the offensive.

I didn’t want to write it as a knee jerk reaction to a current upward trending in vegan eating.

But a few things have happened that have catalysed this article.

People I care about have begun experimenting with a vegan lifestyle. As have people who rely on my advice, opinions and council for their health decisions. And I appreciate that some people can be susceptible to pseudoscience, media, here-say, single person case studies, personal opinion and rhetoric. Not to say that a vegan diet isn’t valid (we’ll explore that soon), but I’d like the science to do the talking (I’ll just be the somewhat clumsy translator, doing the best job I can).

Further to that, my wife and I have a baby on the way, and I’ll move heaven and earth to ensure I do everything I can to help our little girl grow up happy and healthy. If the science can convince me that a vegan lifestyle is the best way to achieve this, I’ll make the switch tomorrow, support my wife in doing the same, and adopt this as how my child will eat until she’s old enough to make her own decision. 

But I will need the science to convince me.

Before I begin my review, a disclosure. I take an evolutionary perspective on human physiology. I currently subscribe to a school of thought that says that if we can emulate some of the factors in our ‘environment of evolutionary adaptiveness’ (the 40,000 years where 95% of human biology was naturally selected), we can achieve the best expression of our genes. I believe that the selective pressures of the environment in which we evolved (if replicated today) can result in the most favourable expression of our genes (i.e. the optimal expression of health).

As such it is my current (though open-to-change) personal opinion (which as I’ll explain soon, it worthless) that the healthiest human diet (for the most favourable expression of genes to ultimately make humans healthier and happier) is the closest modern equivalent to “animals caught via hunting, and uncultivated plant foods from gathering”.

But this stance is my (current) opinion. And as much as I respect opinions, I often find them worse than useless (and indeed, often dangerous and damaging). People have a tendency to ‘choose sides’, to choose which side of the fence they sit, and once they do, to become blind to anything that supports the other side of the fence. It’s a form of confirmation bias, where people will only consume research, media, conversation or any other form of content that supports their opinion – artificially strengthening their stance. Effectively, we are all guilty of becoming radicalised in our beliefs of something – me as much as the next person.

I’ve also been caused to question (which I relish, and accept with open arms) my opinion. If there are so many people removing animal-based foods from their diet, maybe I’m missing something? Not to say that I’m one for following the herd, but maybe they’re flocking in a certain direction for good reason – a ‘wisdom of the crowd’ phenomenon. So (and possibly above all else), the true reason for this article is purely selfish. It’s for my own self interest (albeit also the interest of the people I care about) and my own education. It’s so I can ascertain to the best of my understanding, and to the best of my abilities, which of two different (though perhaps not opposing) approaches to eating is preferable, at least with the evidence we have on the table with our current means. I’m not sure what I’ll find. And that’s the point.

A further note. I approach this as an Exercise Physiologist and Scientist. Not as an environmental scientist, and absolutely not as a philosopher or moral compass. This review will stay within the realms I best understand. While I don’t deny that there are monumental environmental science considerations, as well as deeply personal moral considerations, they’re outside my ‘sphere’ and my scope. While I hope I can provide value to you as a scientist, I have absolutely no right to force your ethics – only to provide the information to help you develop them. I’ll stay with what I best understand. This is purely a review of health implications.

I also admit that nutritional science (and health science in general) is (at current, and until we can all map our own genomes and hand them over to AI to run instantaneous multi-decade case studies) a ‘soft science’. It’s just not possible to control the countless variables that contribute to human health. 

That being said, even soft science is better than no science. Even soft science is better than baseless opinion. And (soft though it may be) it’s hard enough for me to base the health of my family on it.

Opinion aside (as much as possible, but please consider that I possess an inbuilt bias towards the inclusion of animal-based food), I intend to (as neutrally as I’m able) review the literature comparing an ‘omnivore diet’ with a ‘plant only diet’. I’m reluctant to call it a ‘plant based diet’, because that’s how I’d define how I eat now. Most of what I eat is plants. But I also happen to eat meat and animal products.

So let’s define some terms. I appreciate that there are countless sub categories for each ‘camp’, but generally speaking, I’ll use the research to contrast diets which contain animal products, with those that don’t. 

I appreciate that it’s not always that simple, but I’ll do my best.

While there are thousands of published, peer reviewed research articles on vegan diets (as a stand alone) or omnivorous diets (as a stand alone), I’ve chosen to review only those articles that compare both. I have to draw the line somewhere. For similar reasons, I haven’t looked into any conflicts of interest of the authors or study-funders. I’ve doubtless misinterpreted data at times (human error), though my hope would by that if there is a misinterpretation, it happens in both directions.

I’ll try to refrain from commentating on the findings, or injecting my pre-held opinions, but I will summarise or simplify where I think necessary or useful.

All references are included at the end of this review.

Let’s review the scientific literature:

Comparison of Nutritional Quality of the Vegan, Vegetarian, Semi-Vegetarian, Pesco-Vegetarian and Omnivorous Diet found “…the vegan diet, had the lowest total energy intake, better fat intake profile, lowest protein and highest dietary fiber intake in contrast to the omnivorous diet.” (Clarys et al., 2014). When scoring the alternatives, they also went on to state that “Typical aspects of a vegan diet (high fruit and vegetable intake)… contributed substantially to the total score…”. This suggests not an absence of animal products, but the presence of plant matter as the major ‘health improving’ factor. Elements of these findings were replicated in 2016, “Vegetarian respondents consumed more leafy and non-leafy vegetables…” (Akther, 2016).

Research published in 2001 by Larsson, stated that “Young vegans (16-20 year-olds) were seen to have a higher calculated intake of vegetables, legumes, and dietary supplements and a lower intake of ice creams, cakes/cookies, and candies/chocolate than omnivores.”

Janelle and Barr (1995) examined nutrient intake in non vegetarians, vegetarians and vegans. Counterintuitively, they found more differences between the intakes of vegetarians and vegans, than they did between vegetarians and non vegetarians.

“Vegetarians had lower protein and cholesterol intakes and higher percentage of energy as carbohydrate, ratio of polyunsaturated fat to saturated fat (P:S ratio), and fiber intake than nonvegetarians. Vegetarians had lower riboflavin, niacin, vitamin B-12, zinc, and sodium intakes and higher folate, vitamin C, and copper intakes.” (Janelle & Barr, 1995).

They went on to state that “Vegans’ calcium and vitamin B-12 intakes may need attention.”

How we eat causes chemical and hormonal changes in our body, and an omnivore versus vegan diet results in different changes.

One such change relates to homocysteine levels, caused by deficiencies in various B vitamins. This excessive elevation can result in cardiovascular disease, blood clotting, dementia, Alzheimer’s Disease, cognitive impairment and loss of bone mineral density.

“The plasma homocysteine level was measured in alternative nutrition groups of adults…” including vegans and omnivores (Krajčovičová-Kudláčková, et. al., 2000). The research found “…a deficit… in 78% of the vegans vs. 0% in omnivores.”

This was echoed by Majchrzak et. al. (2006), “Exclusion of animal products in vegetarian diets may affect the status of certain B-vitamins, and further cause the rise of plasma homocysteine concentration.”

Of course, Hyperhomocysteinemia can be combated by supplementing a vegan diet with these B vitamins in supplement form. 

(Apologies, I need to provide a personal opinion here. I don’t use supplements, preferring to get what I need from real foods. The fact that supplements are needed makes me wonder what else we are yet to discover should be supplemented due to deficiencies associated with a vegan diet. Can a diet be classified as healthy if the fine print says *when used in conjunction with xyz supplements?).

A vegan diet outperformed an omnivorous diet when it came to levels of folate, with ”a deficient status of folate… found in 18% of omnivores and in approximately 10% of vegans and vegetarians.” (Majchrzak et. al., 2006). I suggest these healthy folate levels in a vegan diet come from the legumes, nuts and seeds that are often used as protein sources. The inclusion of these sources in an omnivorous diet (as well as cruciferous and leafy green vegetables) should help to mitigate this deficiency.

The same study found “The status of riboflavin is considered to be deficient in about 10% of omnivores and vegetarians and in over 30% of vegans.” This is most likely due to the nature of foods high in riboflavin, eggs, organ meats (kidneys and liver), lean meats, and milk. Riboflavin supplementation would be recommended if these food sources are deficient.

These findings were echoed by Larsson (2001). “The dietary intake was below the average requirements of riboflavin for 73% of the vegans, vitamin B12 for all vegans, vitamin D for 43% of the vegans, calcium for 77% of the vegans and selenium for all vegans and 43% of the omnivores. If intake of supplements was included the intake of e.g. calcium and selenium was still lower than the average requirements for 67% and 73% of the vegans respectively.”

In “Antioxidant status in vegetarians versus omnivores: a mechanism for longer life?” (2000), the authors stated that a vegetarian (note, not vegan) diet “…contains more antioxidant vitamins (vitamin C, vitamin E, and beta-carotene) and copper than that of omnivores.” They also warned that the “…bioavailability of zinc in vegetarian diets is generally lower than that of omnivores.”

In “Assessment of fatty acid intakes in vegans and omnivores” (2019), the authors stated that “…vegan diets were devoid of arachidonic acid, eicosapentaenoic and docosahexaenoic acids.” These essential fatty acids were present in an omnivorous diet consisting of poultry, eggs and seafood.

So what does the research tell us about the acute and chronic health effects of these changes in nutrient intake and blood chemistry?

Bone mineral density and bone fractures we examined in 34,696 subjects in “Comparative fracture risk in vegetarians and nonvegetarians in EPIC-Oxford” (Appleby, Roddam, Allen & Key, 2007). They found a 30% increased risk of fracture in a vegan diet when compared to ‘meat eaters’.

They went on to state that “In this population, fracture risk was similar for meat eaters, fish eaters and vegetarians. The higher fracture risk in the vegans appeared to be a consequence of their considerably lower mean calcium intake.”

Once again, a vegan diet would require supplementation to mitigate this heightened risk.

When comparing blood pressure levels of females with the two diet types, the vegan diet came out on top in research from Appleby et. al. in 2006. They found hypertension was reported at “…12.1% in female meat eaters to 7.7% in female vegans, with fish eaters and vegetarians having similar and intermediate prevalences.” 

In females with no self reported hypertension, the difference in diastolic blood pressure between the two groups was 1.7mmHg – statistically insignificant (the equivalent of a blood pressure of 120/80 versus 120/81.7). This study also indicated that “much of the variation was attributable to differences in body mass index between the diet groups.” We need to be careful of not using correlation to imply causation, and need to examine the research into the effects of these different diet types on body mass index, seeing as THAT is what seems to cause the hypertension.

Further to the conversation on body weight, “…the mean BMI (body mass index) of the vegans was significantly lower than that of the nonvegetarians” (Haddad et. al., 1999). This was attributed by the research to “…reduced food intake”.

This same study went on to caution that a vegan diet was often associated with lower levels of protein ingestion: “…the protein contents of the vegan diets of women were significantly lower than those of the nonvegetarians, and 10 of the 25 vegan women failed to meet the recommended dietary allowance…”.

The “Dietary intake and biochemical, hematologic, and immune status of vegans compared with nonvegetarians” study also went on to indicate that a vegan diet was often associated with lower levels of B-12, Iron (in females only) and zinc (though not statistically significant).

In a five year study, Rosell et. al., (2006) found that although there was an average increase in weight over the five years for all diet-groups (average annual weight gain was 389g in men and 398g in women) those following a vegan diet had lower average increases in body weight (284g in men and 303g in women) than those following an omnivorous diet.

The researchers go on to mention though, that “…people who become vegetarians are more health conscious and therefore likely to be relatively slim before they change their diet.” This points to further confounding variables that may skew the data.

From an immunological perspective, “The question was raised as to whether the immune status results observed in this study are a consequence of the relatively low body weights of the vegans.”

In terms of growth rates (height) in children, while not examining a vegan diet, but instead a vegetarian diet, “Growth of Vegetarian Children: The Farm Study” (“Growth of Vegetarian Children”, 2009) found that below ten years of age, there was a 0.2 to 2.1cm stunting of growth in vegetarian children when compared to the National average.

From a female hormonal and fertility perspective, “Dieting influences the menstrual cycle: vegetarian versus nonvegetarian diet” (1986) examined the presence of ovulation, and the levels of luteinizing hormone, estradiol and progesterone. 

“Seven of nine women in the vegetarian group became anovulatory” (ovulation does not occur). They went on to state that, “During the vegetarian diet the average luteinizing hormone (LH) values were significantly decreased during the midcycle and the luteal phase. Estradiol (E2) and progesterone (P) values were significantly lower during the luteal phase. In contrast, the nonvegetarian group did not show significant reduction of LH, E2, and P values during any part of the menstrual cycle.”

In contrast with 78% of women ceasing ovulation in the vegetarian group, “Seven of nine women in the nonvegetarian diet group MAINTAINED ovulatory cycles with no changes in cycle length or in the length of the follicular phase.”

This means women who follow a standard vegan diet experience loss of ovulation at 354% higher rates than women who follow a ‘non vegetarian diet’.

Note the year of this study however (1986), and consider this for context. Also bear in mind this was for a less extreme vegetarian diet, not a vegan diet.

I did say I wouldn’t delve into the environmental considerations of these diet types, but, as a product of my research, I did develop an interest in ‘forage-ruminant-based agriculture’. If you’re interested in researching this further, it might be a good topic to explore.

“(We) may actually be better served using food production systems that include both plant-based agriculture and a forage-ruminant-based agriculture as compared to a strict plant-based (vegan) system. Perhaps we are morally obligated to consume a diet containing both plants and ruminant (particularly cattle) animal products.” (Davis, 2003).


I can’t claim to have been completely impartial in this review, I honestly don’t believe it’s possible for any human to be. But I did my best.

To summarise my key findings (chronologically, as covered in this literature review):

Dan Williams

Dan Williams


Dan Williams is the Director of Range of Motion and leads a team of Exercise Physiologists, Sports Scientists, Physiotherapists and Coaches. He has a Bachelor of Science (Exercise and Health Science) and a Postgraduate Bachelor of Exercise Rehabilitation Science from The University of Western Australia, with minors in Biomechanics and Sport Psychology.

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