Why You Should Think Twice About Vegetarian and Vegan Diets


by Chris Kresser




There are many reasons why people choose to go vegetarian or vegan. Some are compelled by the environmental impact of confinement animal feeding operations (CAFO). Others are guided by ethical concerns or religious reasons. I respect these reasons and appreciate anyone who thinks deeply about the social and spiritual impact of their food choices—even if my own exploration of these questions has led me to a different answer.

But many choose a vegetarian diet is because they’re under the impression that it’s a healthier choice from a nutritional perspective. It is this reason that I’d like to address in this article. For the last fifty years, we’ve been told that meat, eggs and animal fats are bad for us and that we’ll live longer and enjoy superior health if we minimize or avoid them. This idea has been so thoroughly drilled into our head that few people even question it anymore. In fact, if you asked the average person on the street whether a vegetarian or vegan diet is healthier than an omnivorous diet, they’d probably say yes. But is this really true?

Plant-based diets emphasize vegetables, which are quite nutrient dense, and fruits, which are somewhat nutrient dense. However, they also typically include large amounts of cereal grains (refined and unrefined) and legumes, both of which are low in bioavailable nutrients and high in anti-nutrients such as phytate, and they eschew organ meats, meats, fish and shellfish, which are among the most nutrient-dense foods you can eat.

Vegan diets, in particular, are almost completely devoid of certain nutrients that are crucial for physiological function. Several studies have shown that both vegetarians and vegans are prone to deficiencies in B12, calcium, iron, zinc, the long-chain fatty acids EPA & DHA, and fat-soluble vitamins like A & D.

Let’s take a closer look at each of these nutrients on a vegetarian or vegan diet.

Are plant-based diets missing nutrients required for optimal health? Find out!

Vitamin B12

B12 deficiency is especially common in vegetarians and vegans. The takeaway is that the most recent studies using more sensitive techniques for detecting B12 deficiency have found that 68% of vegetarians and 83% of vegans are B12 deficient, compared to just 5% of omnivores.

Vitamin B12 works together with folate in the synthesis of DNA and red blood cells. It’s also involved in the production of the myelin sheath around the nerves, and the conduction of nerve impulses. B12 deficiency can cause numerous problems, including:

  • Fatigue
  • Lethargy
  • Weakness
  • Memory loss
  • Neurological and psychiatric problems
  • Anemia
  • And much more…

The effects of B12 deficiency on children are especially alarming. Studies have shown that children raised until age 6 on a vegan diet are still B12 deficient years after adding at least some animal products to their diet. In one study, the researchers found:

…a significant association between cobalamin [B12] status and performance on tests measuring fluid intelligence, spatial ability and short-term memory” with formerly vegan kids scoring lower than omnivorous kids in each case.

The deficit in fluid intelligence is particularly troubling, the researchers said, because:

…it involves reasoning, the capacity to solve complex problems, abstract thinking ability and the ability to learn. Any defect in this area may have far-reaching consequences for individual functioning.

A common myth amongst vegetarians and vegans is that it’s possible to get B12 from plant sources like seaweed, fermented soy, spirulina and brewers yeast. But plant foods said to contain B12 actually contain B12 analogs called cobamides that block the intake of, and increase the need for, true B12.


On paper, calcium intake is similar in vegetarians and omnivores (probably because both eat dairy products), but is much lower in vegans, who are often deficient. However, calcium bioavailability from plant foods is affected by their levels of oxalate and phytate, which are inhibitors of calcium absorption and thus decrease the amount of calcium the body can extract from plant foods. So while leafy greens like spinach and kale have a relatively high calcium content, the calcium is not efficiently absorbed during digestion.

One study suggests that it would take 16 servings of spinach to get the same amount of absorbable calcium as an 8 ounce glass of milk. That would be 33 cups of baby spinach or around 5-6 cups of cooked spinach. There are a few vegetables listed in this paper that have higher levels of bioavailable calcium, but it’s important to note that all of the vegetables tested required multiple servings to achieve the same amount of usable calcium as one single serving of milk, cheese, or yogurt. This suggests that trying to meet your daily calcium needs from plant foods alone (rather than dairy products or bone-in fish) might not be a great strategy.


Vegetarians eat a similar amount of iron to omnivores, but as with calcium, the bioavailability of the iron in plant foods is much lower than in animal foods. Plant-based forms of iron are also inhibited by other commonly consumed substances, such as coffee, tea, dairy products, supplemental fiber, and supplemental calcium. This explains why vegetarians and vegans have lower iron stores than omnivores, and why vegetarian diets have been shown to reduce non-heme iron absorption by 70% and total iron absorption by 85%.


Overt zinc deficiency is not often seen in Western vegetarians, but their intake often falls below recommendations. This is another case where bioavailability is important; many plant foods that contain zinc also contain phytate, which inhibits zinc absorption. Vegetarian diets tend to reduce zinc absorption by about 35% compared with omniovorous diet. Thus, even when the diet meets or exceeds the RDA for zinc, deficiency may still occur. One study suggested that vegetarians may require up to 50% more zinc than omnivores for this reason.


Plant foods do contain linoleic acid (omega-6) and alpha-linolenic acid (omega-3), both of which are considered essential fatty acids. In this context, an essential fatty acid is one that can’t be synthesized by the body and must be obtained in the diet. However, an increasing body of research has highlighted the benefits of the long-chain omega-3 fatty acids EPA & DHA. These fatty acids play a protective and therapeutic role in a wide range of diseases: cancer, asthma, depression, cardiovascular disease, ADHD, and autoimmune diseases, such as rheumatoid arthritis.

While it is possible for some alpha-linolenic acid from plant foods to be converted into EPA & DHA, that conversion is poor in humans: between 5-10% for EPA and 2-5% for DHA. Vegetarians have 30% lower levels of EPA & DHA than omnivores, while vegans have 50% lower EPA and nearly 60% lower DHA. Moreover, the conversion of ALA to DHA depends on zinc, iron and pyridoxine—nutrients which vegetarians and vegans are less likely than omnivores to get enough of.

Fat-soluble vitamins: A and D

Perhaps the biggest problem with vegetarian and vegan diets, however, is their near total lack of two fat-soluble vitamins: A and D. Fat-soluble vitamins play numerous and critical roles in human health. Vitamin A promotes healthy immune function, fertility, eyesight and skin. Vitamin D regulates calcium metabolism, regulates immune function, reduces inflammation and protects against some forms of cancer.

These important fat-soluble vitamins are concentrated, and in some cases found almost exclusively, in animal foods: primarily seafood, organ meats, eggs and dairy products. Some obscure species of mushrooms can provide large amounts of vitamin D, but these mushrooms are rarely consumed and often difficult to obtain. (This explains why vitamin D levels are 58% lower in vegetarians and 74% lower in vegans than in omnivores.)

The idea that plant foods contain vitamin A is a common misconception. Plants contain beta-carotene, the precursor to active vitamin A (retinol). While beta-carotene is converted into vitamin A in humans, the conversion is inefficient. For example, a single serving of liver per week would meet the RDA of 3,000 IU. To get the same amount from plant foods, you’d have to eat 2 cups of carrots, one cup of sweet potatoes or 2 cups of kale every day. Moreover, traditional cultures consumed up to 10 times the RDA for vitamin A. It would be nearly impossible to get this amount of vitamin A from plant foods without juicing or taking supplements.

But don’t vegetarians live longer than omnivores?

At this point you might be thinking, “Well, so what if plant-based diets are lower in some nutrients. Everyone knows vegetarians live longer than omnivores!” While it’s true that some observational studies suggest that vegetarians and vegans enjoy longer lifespans, these studies were plagued by the “healthy user bias”. The healthy user bias is the scientific way of saying that people who engage in one behavior that is perceived as healthy (whether it is or not) are more likely to engage in other behaviors that are healthy. For example, vegetarians tend to be more health conscious on average than general population; they are less likely to smoke or drink excessively and more likely to exercise, eat fruits and vegetables and take care of themselves.

Of course the flip-side is also true: those that engage in behaviors perceived to be unhealthy are more likely to engage in other unhealthy behaviors. The healthy user bias is one of the main reasons it’s so difficult to infer causality from observational studies. For example, say a study shows that eating processed meats like bacon and hot dogs increases your risk of heart disease. Let’s also say, as the healthy user bias predicts, that those who eat more bacon and hot dogs also eat a lot more refined flour (hot dog and hamburger buns), sugar and industrial seed oils, and a lot less fresh fruits, vegetables and soluble fiber. They also drink and smoke more, exercise less and generally do not take care of themselves very well. How do we know, then, that it’s the processed meat that is increasing the risk of heart disease rather than these other things—or perhaps some combination of these other things and the processed meat?

One way to answer that question is to design a study that attempts to control for at least some of the healthy user bias. In other words, instead of comparing the “average” meat eater (who tends to be less health conscious) with the “average” vegetarian (who tends to be more health conscious), what happens when you compare vegetarians and omnivores that are both health-conscious?

Thankfully, we have a study that did just that. It compared the mortality of people who shopped in health food stores (both vegetarians and omnivores) to people in the general population. This was a clever study design. People who shop in health food stores are more likely to be health conscious, regardless of whether they eat meat, which reduces the likelihood that the study results will be thrown off by the “healthy user bias”. What did the researchers find? Both vegetarians and omnivores in the health food store group lived longer than people in the general population—not surprising given their higher level of health consciousness—but there was no survival difference between vegetarians or omnivores. Nor was there any difference in rates of heart disease or stroke between the two groups. (15) In other words, omnivores who are health conscious live just as long as vegetarians that are health conscious.

Final thoughts

With care and attention, I think it’s possible to meet nutrient needs with a vegetarian diet that includes liberal amounts of pasture-raised, full-fat dairy and eggs, with one exception: EPA and DHA. These long-chain omega fats are found exclusively in marine algae and fish and shellfish, so the only way to get them on a vegetarian diet would be to take a microalgae supplement (which contains DHA) or bend the rules and take fish oil or cod liver oil as a supplement. Still, while it may be possible to obtain adequate nutrition on a vegetarian diet, it is not optimal—as the research above indicates.

I do not think it’s possible to meet nutrient needs on a vegan diet without supplements—and quite a few of them. Vegan diets are low in B12, biovailable iron and zinc, choline, vitamin A & D, calcium, and EPA and DHA. So if you’re intent on following a vegan diet, make sure you are supplementing with those nutrients.

It’s worth pointing out that there are genetic differences that affect the conversion of certain nutrient precursors (like beta-carotene and alpha-linolenic acid) into the active forms of those nutrients (like retinol and EPA and DHA, respectively), and these differences may affect how long someone will be able to follow a vegetarian or vegan diet before they develop nutrient deficiencies. This explains why some people seem to do well for years on these diets, while others develop problems very quickly.

From an evolutionary perspective, is difficult to justify a diet with low levels of several nutrients critical to human function. While it may be possible to address these shortcomings through targeted supplementation (an issue that is still debated), it makes far more sense to meet nutritional needs from food. This is especially important for children, who are still developing and are even more sensitive to suboptimal intake of the nutrients discussed in this article. Like all parents, vegetarians and vegans want the best for their children. Unfortunately, many are not aware of the potential for nutrient deficiencies posed by their dietary choices.

I hope this article can serve as a resource for anyone on a plant-based diet, whether they choose to start eating meat (or animal products, in the case of vegans) again or not.

by Chris Kresser


One Comment Add yours

  1. really interesting with lots of info, thanks!

    Liked by 1 person

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