Interestingly, the study 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) which may suggest that mental disorders cause vegetarianism which then makes the mental disorders worse.
Entirely vegan diets are unknown among traditional human cultures. Back in the early part of the 19th century, dentist and explorer Weston Price went looking for vegans, 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. …
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.