Could this popular diet be lowering your IQ?

By Naveed Saleh, MD, MS
Published August 26, 2020

Key Takeaways

Vegetarianism dates back to Pythagoras and his adherents in ancient Greece. Veganism, however, is relatively new and dates to 1944 London, when a clique formed “The Vegan Society.” Veganism grew from ideas presented by the Vegetarian Society in London, members of which had previously contended that animals are also harmed by dairy production and egg farming.

Today, veganism is more akin to a movement that extends beyond the eschewal of eggs and dairy, and now excludes other animal products, such as honey and gelatin, as well as the use of inedible animal products, such as wool, leather, fur, and silk. In summation, a vegan diet comprises only plant foods, such as vegetables, fruits, grains, legumes, seeds, nuts, and vegetable oils. 

And the movement is only gaining momentum. The number of vegans in the United States grew from about 4 million in 2014 to 19.6 million in 2017—a 600% increase. 

While there are many documented health benefits of veganism, it does have its drawbacks. According to a thesis published in the University of Pennsylvania ScholarlyCommons, “a well-rounded vegan diet is healthy and such is evidenced by the variety of whole foods and increased vegetable and fruit intake. Health benefits include a decrease in cholesterol, lipid levels, blood pressure, weight, and a reduced risk for a variety of diseases including obesity, diabetes, cardiovascular disease, and cancer. Despite the benefits, health concerns do exist, especially in regard [to] nutrient deficiencies, without a well-planned and varied diet. Nutrient concerns include calcium, vitamin D, iron, and particularly vitamin B-12 for which supplements should be taken,” wrote author Sarah E. Mann, now a research assistant at the University of Pennsylvania. 

Many of these nutrients are essential for neurological functioning and growth. Deficiencies in certain nutrients can impair cognitive function. People following a vegan diet may contend with these nutrient deficiencies: 


Calcium is necessary for bone health, and concerns have risen over calcium deficiency in vegans. Dairy products contain high calcium levels. To compensate, certain vegan-friendly foods are fortified with calcium, such as soy/almond/rice milk. Additionally, certain foods such as beans, nuts, and seeds do contain calcium, although calcium absorption is somewhat lower due to oxalate, phytate, and fiber present in these foods. Furthermore, low-oxalate vegetables like kale, collard greens, and broccoli contain much less calcium than milk.

Although vegans can fulfill daily calcium requirements by varying vegetable consumption, in those who eat unvaried diets, calcium supplementation between meals may be a good idea.

Vitamin D

Vitamin D is an essential nutrient derived from sunlight exposure as well as certain foods. Vitamin D is found in fortified foods—most commonly in cow milk. Cow milk, however, is verboten for vegans. Soy milk, rice milk, and breakfast cereals are also fortified with calcium, although some vegans don’t eat cereal (because cereal contains small amounts of insect parts.) 

Insufficient levels of vitamin D have been associated with reduced cognitive function as well as higher risk of Alzheimer’s disease. 

Consequently, vegans should get sufficient sun exposure and consume fortified foods to avoid vitamin D deficiency.


Vitamin B2, or riboflavin, metabolizes carbohydrates into glucose, which is an important source of energy. Riboflavin also metabolizes fats and protein. Furthermore, the B-complex vitamins are needed for liver, hair, skin, eye, and nervous system function. The antioxidant effects of riboflavin could decrease free radical damage intrinsic in heart disease and cancer pathology.

Riboflavin deficiency is a concern for those following vegan diets. Nevertheless, in those who consume enough nutritional yeast, wheat germ, mushrooms, leafy green vegetables, avocados, mushrooms, sea vegetables, almonds, fortified cereal, enriched grains, and soy milk, riboflavin intake can reach a level comparable to levels in omnivores.


Zinc is an important player in the immune system, and contributes to cell growth, cell division, wound healing, and carbohydrate metabolism, as well as taste/smell. Zinc plays an endocrine role by enhancing insulin’s effects.

Although zinc levels are high in vegan-friendly foods, such as whole-grain pasta, legumes, wheat germ, fortified cereals, tofu, and nuts, its bioavailability in plant foods may be lower. 

“Vegetarians have lower levels of total zinc as well as plasma zinc concentrations, although most often they are within acceptable ranges,” wrote Mann in her thesis.

“This is due to lower zinc intake and absorption,” she added. “However, it is important to note that over time, vegetarians and vegans may adapt to their lower zinc absorption levels by compensating such that ‘zinc excretion decreases when dietary zinc is low.’ Overall, while zinc is abundant in the vegan diet, high levels of dietary phytates reduce its absorption.”


Choline is an essential nutrient required for neurocognition, liver function, lipid metabolism, and homocysteine regulation. Although most Americans fail to consume enough choline, vegans are at heightened risk because choline is mainly found in eggs, meat, and milk.

“[F]urther movements away from the consumption of these could have unintended consequences for choline intake/status,” per a review article published in BMJ Nutrition, Prevention & Health.

“If choline is not obtained in the levels needed from dietary sources per se then supplementation strategies will be required, especially in relation to key stages of the life-cycle such as pregnancy, when choline intakes are critical to infant development,” wrote author and registered dietician, Emma Derbyshire, PhD. 

In short, if the brain is starved of needed nutrients, it won’t grow or perform to its best ability. 

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