Do vegetarian and vegan diets contain the micronutrients our brains need to function properly? Or do they put people at risk for serious deficiencies that increase the likelihood of developing psychiatric disorders?
In my last post, Food Fights: Are Vegan Diets Healthier for the Brain? we compared big-picture health effects of plant-based diets to meat-based diets, and discovered more similarities between them than many might have expected.
Today, we go micro! We will explore each nutrient’s function in the brain, and the psychiatric symptoms that can develop if we don’t get enough of each one. I created this comprehensive summary of the science to be a resource for everyone, regardless of chosen diet, so that we may all improve our understanding of how to help our brains function at their absolute best.
You will notice that certain essential nutrients are missing. This is because those nutrients are generally easy to obtain from both plant-based and omnivorous diets. I also chose to reserve coverage of deficiency testing and supplementation for future posts, as complete discussion of any one of these fascinating nutrients would have been pages long (and believe me, I was going there…)
Most people think of fruits and vegetables as teeming with the vitamins, minerals, and powerful antioxidants we need to live and thrive. It is true that many plant foods are rich in many of these substances, but just because a food contains a particular nutrient doesn’t mean we can access it.
Unfortunately, plant nutrients often suffer from low “bioavailability”—which means that they are hard for us to extract, absorb, and utilize. Below I summarize the most important potential brain nutrient deficiencies that ALL plant-eaters—vegans, vegetarians and omnivores— need to be aware of, as well as key deficiency risks unique to vegans and vegetarians.
Vegan diets increase risk for certain fat-soluble vitamin deficiencies. Low-fat vegan diets further increase risk for these deficiencies, because sufficient fat must be eaten with every meal in order to absorb these vitamins from food.
Contrary to popular belief, plant foods are lousy sources of vitamin A. In fact, they contain no vitamin A at all! Instead, they contain carotenoids, which we must then convert into retinol, the form of vitamin A our bodies can use. This is 12 to 24 times more difficult than obtaining retinol from animal foods. This helps to explain why childhood blindness due to vitamin A deficiency is rampant in dozens of developing countries, particularly in Africa and South-East Asia:
“Poor bioavailability plays a predominant role in the development of [Vitamin A deficiency] among communities that mainly rely on plant-based foods.” 1)
Fortunately, vitamin A deficiency in the U.S. and other developed countries is very rare, due to the abundance of animal foods and because many processed foods are fortified. Although vitamin A is important to many aspects of brain function including vision, learning, and memory, I’m not aware of any human studies demonstrating that vitamin A deficiency causes psychiatric disorders.
The form of vitamin D our bodies need is vitamin D3 (cholecalciferol). We can make Vitamin D3 from sunshine or obtain it from animal foods.
The form of vitamin D found in plant foods is vitamin D2 (ergocalciferol). Our bodies can convert some D2 to D3, but D2 is less potent, doesn’t last as long in the bloodstream, and may be harder to store in our body fat for rainy days and dark winters.2) If we spend enough time in the sun, we don’t need to obtain any vitamin D from our diet at all, yet many people (regardless of chosen diet) are deficient. Most studies have found that vegans have lower blood levels of vitamin D3 and are more likely to drop to deficient levels during winter months than omnivores.3)
Vitamin D3 is important in brain growth and development, regulates calcium levels within the brain, helps protect brain cells from damaging oxidation, and supports the health of the hippocampus (the brain’s memory center). Lower levels of vitamin D3 are associated with an increased risk for schizophrenia, autism, depression, and dementia.4) But that doesn’t mean deficiency causes those conditions. Can D3 supplements help with psychiatric disorders? I did not find any clinical trials studying D3 supplementation and psychosis or dementia, and found only one study showing slight improvement in depression symptoms.5) However, several recent studies of Vitamin D3 supplementation have shown promising improvements in autism symptoms.6)
When most people think of vitamin K they think of vitamin K1, which is abundant in many plant foods, but vitamin K2 is just as important and often overlooked. Vitamin K2 is confusing because it comes in many forms, but the essential form we need is called MK-4, which only exists in animal foods. In the brain, MK-4 is required to build critical cell membrane components called sphingolipids, as well as to support the overall health and function of brain cells.7) It stands to reason that MK-4 deficiency could cause mental health problems, but I’m not aware of any human studies exploring the potential connection between K2 deficiency and any psychiatric disorder (K2 is poorly studied in general).
The body can convert a little bit of K1 into MK-4 (vitamin K2), but not nearly enough to fully meet our needs. Therefore, savvy vegans turn to natto (fermented soy), which contains a bacterial form of vitamin K that our bodies can turn into MK-4 a little more easily. For an excellent education on vitamin K, please see Chris Masterjohn PhD’s article The Ultimate Vitamin K2 Resource. He recommends that vegans take special supplements instead of relying solely on natto to meet their K2 requirements.
Vegan diets contain virtually no vitamin B12. Without this essential vitamin, the body cannot synthesize DNA, RNA, red blood cells, or myelin (the substance that wraps around and insulates our brain circuitry). Not surprisingly, B12 deficiency can CAUSE a whole host of serious psychiatric problems, including depression, psychosis, memory problems, mania, and changes in behavior or personality.8)
Severe, prolonged B12 deficiency is fatal. Most vegans and vegetarians are aware of this danger and either take supplements or consume fortified yeast (yeast doesn’t naturally contain any vitamin B12). Unfortunately, deficiency is still far more common than it should be, with some studies finding that as many as 86% of adults (regardless of chosen diet) are deficient. Researchers report wide ranges of values, but overall, vegetarians tend to have lower B12 levels than omnivores, and vegans on average tend to have the lowest B12 levels.9) Unfortunately, B12 blood levels often don’t always tell the whole story, so proper assessment requires additional tests, most commonly methylmalonic acid and total homocysteine levels.10) [More on the nuances of proper B12 testing and supplementation in a future post.]
Vitamin B1 (thiamin), B2 (riboflavin), B3 (niacin), B5 (pantothenic acid), B6 (pyridoxine), B7 (biotin) and B9 (folate) all work together to extract energy from food and build vital molecules. The brain is a high-energy organ, so even temporary, mild deficiencies of a single B vitamin can significantly disrupt normal brain function. All B vitamins are water-soluble, so what isn’t used is lost in the urine rather than stored in body fat. This means we need to eat good sources of B vitamins as often as several times per week in order to keep our brains functioning at their best.
All of the B vitamins except for B12 can be found in plant foods, yet some studies find that vegans are more likely to be deficient in vitamin B3 (niacin), B6 (pyridoxine).11), and B2 (riboflavin). Riboflavin seems to be the one that vegans need to pay the most attention to, as studies have more consistently shown higher risk of deficiency of this B vitamin in vegans compared to vegetarians and omnivores.
“Riboflavin deficiency is endemic in populations who exist on diets lacking dairy products and meat.”12)
All three of these B vitamins can be found in plant foods, but often in smaller amounts than animal foods, so it can be challenging to obtain adequate amounts from a vegan diet unless great care is taken to include just the right mixture of foods.
Riboflavin deficiency disrupts normal serotonin metabolism and brain energy production.13) Serotonin is a key neurotransmitter (brain communication chemical) that helps to regulate mood, appetite and sleep, among other things. Deficiency can cause fatigue, personality changes and general brain dysfunction.14)
Niacin is also required for proper serotonin metabolism and brain energy production. Severe niacin deficiency leads to pellagra, a disease which can cause depression, psychosis, delirium and dementia. Pellagra was common over a century ago in communities living on corn-based diets. Corn happens to contain a form of niacin our bodies can’t use, AND is low in tryptophan (the amino acid needed to form serotonin) resulting in a one-two punch to serotonin activity in the brain.15)
Pyridoxine is essential for the synthesis of numerous neurotransmitters, including serotonin, dopamine, GABA, norepinephrine, and melatonin. The primary symptoms of vitamin B6 deficiency are depression, difficulty concentrating, and dementia.16)
Most plant foods are quite low in iodine compared to many animal foods. Iodine is a required building block in thyroid hormone, which is critical in brain development and maintenance. Lack of iodine, particularly in early life, stunts body and brain growth. Iodine deficiency affects two BILLION people, and is the most common preventable cause of intellectual disabilities in the world.
Little is known about how simple iodine deficiency (without hypothyroidism) might affect brain function beyond infancy, although a recent Polish study found a correlation between lower iodine levels and certain symptoms of autism.17) Two randomized placebo-controlled clinical trials found that replenishing iodine to normal levels in schoolchildren improved their cognitive function.18)
Fortunately, iodine deficiency in the United States is uncommon due to the widespread use of iodized salt. Vegans, on average, have lower iodine levels than vegetarians, and are more likely to meet criteria for iodine deficiency on paper, but nevertheless appear to have normal thyroid function.19)
Many plant foods are lower in iron than animal foods, and to make matters worse, plants contain a form of iron that is far more difficult to absorb than heme iron, the form found in animal foods.20) Most vegans and vegetarians have about the same amount of iron in their blood as omnivores do, but their total body iron stores (how much they have in reserve) do tend to be lower.21)
When people think of iron deficiency, they think of anemia (lower numbers of red blood cells in the circulation), but the truth is that the brain needs iron just as much as red blood cells do. Iron is required for neurotransmitter production (serotonin, dopamine and norepinephrine), generation of brain energy, hippocampal function (memory!), cell signaling, and infant brain development (which means that expectant and breastfeeding mothers must take in plenty of iron). People with iron deficiency may be more likely to have psychiatric problems such as depression and schizophrenia,22) but we don’t yet have studies that can tell us whether iron deficiency alone can cause these conditions.
Plant foods are far lower in zinc than animal foods. Zinc deficiency is much more common among vegans than iron deficiency, and yet gets far less attention, perhaps because it is difficult to accurately measure zinc levels in the body. A 2017 Swiss study found that 47% of vegans had inadequate zinc levels compared to only 10% of omnivores.23)The brain requires zinc for serotonin synthesis, vitamin B6 activation, and cell signaling.24) People with low zinc levels are more likely to be depressed, but more importantly, some clinical trials show that combining zinc supplements with antidepressants improves outcomes.25)There’s even been one randomized controlled trial demonstrating that zinc supplements alone can reduce severity of depression symptoms.26)
Some studies suggest that zinc deficiency may play a role in childhood ADHD symptoms, but the jury is still out.27) Older people with zinc deficiency are more likely to have poor overall mental health, but no causal relationship between the two has been established.
Our bodies can store iron, but we have no good way to store zinc, so we must eat high quality sources of zinc on a regular basis, or take supplements. The challenge for those choosing a vegan diet is that the plant foods richest in zinc also happen to contain compounds which interfere with zinc absorption—which brings us to the fascinating topic of antinutrients!
The grains, beans, nuts and seeds that vegan diets rely upon for protein are technically all seeds. For example, grains are the seeds of grasses. Seeds are higher in protein than most other plant parts, but they are unfortunately also high in phytic acid, a mineral magnet that interferes with our ability to absorb iron, zinc, calcium and magnesium.
Seeds use this clever molecule to hold on to precious minerals so they can’t be washed away in the soil while waiting to sprout. Unfortunately, seeds don’t just selfishly hoard their own minerals. Phytic acid also steals minerals from other unsuspecting foods you may eat with your seeds, making it more difficult for you to extract minerals from everything you eat. Take a look at how much black beans (a legume) and corn (a grain) interfere with zinc absorption!
A wide variety of plant foods contain goitrogens which interfere with iodine metabolism and thyroid hormone production. Examples include soy, millet, cassava, rutabaga, and the entire cruciferous vegetable family, including kale and broccoli. [I have a whole post dedicated to this topic if you are curious to learn more.]
Oxalates are corrosive compounds which naturally occur in plant foods and interfere with iron, calcium and magnesium absorption. Plants use oxalates to regulate their own internal mineral content and help defend against predators.28) Foods rich in oxalates include cocoa, beets, sesame seeds, rhubarb, sweet potato, coriander, currants, and spinach. In fact, thanks to oxalates, virtually none of the iron present in spinach makes it into Popeye’s body.29) If only steak came in a can…
Tannins are darkly-pigmented astringent compounds which naturally occur in plant foods and interfere with iron absorption.30) Tannins are found in a wide variety of foods including legumes, nuts, cocoa, wine, tea, berries, pomegranates, and many other fruits.
Of course, most omnivores eat plenty of grains, beans, high-tannin, high-oxalate and high-goitrogen plant foods too, which may help to explain why so many omnivores are also deficient in key minerals.
Vegan diets contain absolutely no DHA or EPA, the forms of essential omega-3 fatty acids required for brain and immune system function (and vegetarian diets contain only small amounts from eggs and dairy). The brain is extremely rich in DHA, which is required to make myelin (the material that wraps around nerve cells, insulating brain circuits), and to keep cell membranes fluid and flexible enough to pass neurotransmitters back and forth. DHA is critical in the formation of healthy synapses (connections between brain cells), therefore infant brains require lots of DHA to develop properly. In short, DHA plays a “unique and indispensable role” in the “cohesive, organized neural signaling essential for higher intelligence.”31)
In comparison to omnivores, EPA and DHA levels can be about 30% lower in vegetarians and more than 50% lower in vegans.32) This is primarily because the form of omega-3 (ALA) found in plant foods (flax, chia, walnut, etc.) is very difficult for the body to convert into the DHA our brains need. At best, women convert only about 9% of the ALA they consume into DHA, whereas men convert a dismal 0-4%.33)
The only veg-friendly supplements on the market that contain any EPA or DHA are those derived from algae. [Plant-sourced omega-3 supplements such as those from flax or chia contain ALA only]. Therefore, I strongly recommend that every vegan and vegetarian take an algae-sourced omega-3 supplement. EPA and DHA are hard to come by even in most omnivorous diets, because the foods that naturally contain them (oily fish, pastured animal fats/organs) are ones that people don’t eat much of these days, so even omnivores may need to supplement.
There have been numerous studies demonstrating that people with lower levels of omega-3 fatty acids in their bodies are more likely to suffer from ADHD, depression, bipolar disorder, and schizophrenia, but these are just associations. What’s more important is that omega-3 supplements rich in EPA seem to have positive effects on people with major depression, bipolar disorder, and those at risk for psychotic disorders.34)
It is unclear why EPA seems to work better as a supplement than DHA, but it clearly does.35)
At the recent International Society for Nutritional Psychiatry research meeting, I spoke with leading fatty acid researchers Michael Crawford PhD and Alex Richardson PhD about this, and they suggested that EPA’s superior effectiveness in short-term studies is likely due to its fast-acting anti-inflammatory properties. They emphasized that DHA remains vitally important long-term to brain structure and function despite its weaker performance in clinical trials. Some suggest that the effects of DHA supplements may take years to notice, so most clinical trials are far too short to demonstrate its importance.
Contrary to popular belief, there is no evidence that vegans suffer from true protein deficiency, despite the fact that plant proteins are more challenging to digest and absorb than animal proteins.36) Fortunately, it appears that most people in the developed world meet their daily minimum requirement for protein without even trying. That doesn’t mean that some of us aren’t walking around with less than optimal protein intake, it’s just that we don’t have a shared understanding of what the symptoms of mild protein deficiency are or what our optimal daily protein intake should be.37) This is an area of great controversy in the nutrition nerd networks (of which I’m a proud card-carrying member). Please see my Protein Page for more information.
We are often told that one advantage of a vegan diet is that all plant foods are naturally 100% cholesterol-free. Yet the brain (and indeed, every cell in the human body) requires cholesterol to function properly.
The human brain is loaded with cholesterol—on purpose. Although the brain represents only 2% of total body weight, it contains 20% of the body’s cholesterol. Cholesterol is required for brain cell membrane structure and function, and is a vital component of myelin (brain cell insulation). This leads some to worry that vegans are at risk for low brain cholesterol, but is there really any cause for alarm?
Even though vegan diets contain zero grams of cholesterol, there is good reason to believe that vegans might not suffer from low brain cholesterol after all. This is because the brain does not obtain its cholesterol from the foods we eat! Cholesterol is too big and bulky to cross the blood-brain barrier—so the brain makes all of its own cholesterol on site. Cholesterol can be manufactured out of anything—carbohydrate, protein, or fat—so regardless of what we eat, we can always make plenty of cholesterol. Case in point: plenty of vegans have “high cholesterol” despite eating zero grams of cholesterol per day–if you don’t believe me, simply google “vegan high cholesterol” or see this interesting case study.
There are some (epidemiological) studies weakly suggesting that people with lower total cholesterol levels may be more likely to have depression and suicidal ideation,38)but these cannot show a cause and effect relationship. And again, just because blood cholesterol may be low doesn’t mean that brain cholesterol is low. Unfortunately there is currently no way to directly measure brain cholesterol content in living human beings.39) Furthermore, there is no consensus about what constitutes a low LDL level in the first place. In other words, we aren’t sure how low is too low. 40) For more information, please see my Cholesterol Page.
Yes, unfortunately they do.
Taking all of the above information in, you may have noticed two interesting things:
If you were a curious alien who had just landed on planet Earth and were reviewing the facts about human nutrition for the first time (this would absolutely be the very first thing I would do upon arriving on a foreign planet), you would logically conclude that consuming animal foods is critical to human health. You may even go so far as to wonder—as I do—whether we would be better off not eating any plants at all! While we don’t have the clinical studies to prove it, my suspicion, after having studied this topic for nearly ten years now, is that the more plants we eat, the harder it is for our brains (a