Entirely vegan diets are unknown among traditional human cultures. Back in the early part of the 20th century, dentist and explorer Weston Price went looking for vegetarians, but found only cannibals*. Since vegan diets in nature provide no vitamin B12 and very little in the way of usable long chain omega3 fatty acids, it is not surprising that humans have continued to eat animals and animal-derived products. Nowadays one can obtain algae-derived DHA (the major long chain omega3 fatty acid present in the brain). and supplement B12. That wasn’t possible until a few years ago, and there’s little evidence that supplementation with DHA alone is helpful for the brain.
We have been encouraged to eat more plants and less animals. Various writers have suggested it is healthier for our bodies and our planet. I have no objections to a mostly plant-based diet as long as attention is paid to protein requirements and micronutrition. However, since little things in animal products (some essential like B12, some that can be created in our bodies but perhaps not in the amounts we need, such as creatine) seem to be very important for the brain, it’s interesting to look at the literature on vegetarian diets and mental health. Here is the latest (and the best) observational study: Vegetarian diet and mental disorders: results from a representative community survey.
It's a German study, and for a large population-based retrospective observational design, it's actually fairly thorough and sensible. And if you are a vegetarian, it certainly doesn't say that vegetarianism causes mental health problems. But in all but two studies done in the past, vegetarianism has been linked with higher rates of depression, anxiety, and particularly eating disorders (bingeing, restricting, and purging behaviors). But to be perfectly honest, all those studies had some serious limitations (they were small, done special populations, and often measures based on just a few answers to general survey questions). I've reviewed a few of them. (My favorite has to be the one where they calculated arachidonic acid ingested to the hundredth of a gram based on data from a food frequency questionnaire, which seems very unlikely to be accurate) I don't think it is a coincidence that the two positive studies were done by the same group of researchers in the Seventh Day Adventist population.
The interesting thing about the general trend that vegetarians aren't quite as mentally healthy as omnivores (in observational studies) is that vegetarians tend to do better in other measures of health. They are better educated, as a population they are generally younger, less likely to smoke or drink, more likely to exercise, and they tend to care about ethics and the quality of their food. However, vegetarians are also more likely to be female (which is more likely to be associated with anxiety, depression, and eating disorders by a long shot).
So this new study has some things to recommend it. For one thing, the mental health diagnoses were determined not by answers to typical questionnaires, but by a full clinical interview using psychologists or physicians, lasting an average of 65 minutes each. (Pretty impressive, considering there were over 4,000 participants in the population-based study). In addition, the researchers matched omnivores to vegetarians based on age, education, sex, and whether they were urban or rural and crunched those numbers as well, so we got a good sample that took out some of the major confounders that dogged the previous studies. Finally, this cohort was a purposeful random sampling of the German adult population (excluding people over 65, however), rather than the Seventh Day Adventists or adolescents and college students sampled in previous studies.
And when the researchers went down the line of depressive disorders, anxiety disorders, somatoform disorders (things like body dysmorphic disorder, health anxiety and hypochondriasis), and eating disorders, the mostly vegetarian were more likely to be afflicted, and the strict vegetarian even more likely.** The full blown eating disorder diagnoses were rare enough, however, that the researchers didn't compute the odds ratios, as they felt the dataset was not robust enough to be fair. Compared to the general population, the vegetarians were more likely to have mental disorders, and compared to the sex and education and population and age matched controls, the risk of mental disorders in vegetarians really shot up, with odds ratios hovering around 2 fold increased risk, some as high as 3 fold.
When the data was taken apart from another direction, it was found that participants in the study with depressive, anxiety, somatoform, and anxiety disorders consumed less meat than people without a mental disorder. The amount of vegetables, fruits, fish, and fast food did not have a consistent pattern separating those with and without mental disorders (except fish consumption was linked with reduced anxiety. Hmmm). In fact, unlike the 2010 Australian study, those with mental disorders in this German population were less likely to consume fast food than the mentally healthy population.
Temporally, the adoption of a vegetarian diet, on average, tended to follow the mental health diagnosis, suggesting that the vegetarian diet was not in fact causal. I know originally the abstract of the article said the opposite, but if you read the full text, you find that the abstract was misrepresentative. A retrospective study isn't the most robust way to determine this issue, but I would tend to believe this timing to be true, particularly for anxiety disorders, which often begin before the age of 10. The main exception to the temporal findings in this study were the eating disorders, which tended to start right around the same time as adoption of a vegetarian diet. As I've reported before, several of my eating disordered patients have told me they adopted vegetarianism so they would have an excuse to restrict food and not have to eat in public.
So what is going on? In Germany, are the neurotic perfectionists who are more likely to be choosey about food (and thus select vegetarianism and eschew fast food) also more vulnerable to depression and anxiety? Sure, could be. Or maybe those with mental troubles try to avoid what is thought to be bad food (meat and fast food). It is also possible that the nutrient deficiencies common in vegetarian diets (the most robustly studied being long chain omega 3 fatty acids and B12, though I think zinc and creatine and even too low a cholesterol could also be issues) could accelerate or worsen pre-existing mental conditions.
A large study comparing choosey, neurotic, perfectionistic omnivores (ahem) with strict vegetarians would be interesting, I think.
*these cannibals preferentially ate fisherman, who would be chock ful of long chain omega3 fatty acids!!
**the German word for "meat" excludes poultry.
Copyright Emily Deans, MD