Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.
As I noted in the last video, the association between vegetarian diets and depressive symptoms is ambiguous and lacks consensus. Some systematic reviews found no association between vegetarian diets and symptoms of depression, while others suggest that those following a vegetarian diet are less likely to be depressed, but even that doesn’t mean that eating vegetarian makes people feel better. Maybe those who feel better are just more likely to eat healthier. Maybe we’re less likely to need comfort foods if we’re already comforted. Or, it could go the other way, where individuals who experience problems may be more inclined to follow a vegetarian or vegan diet in order to improve their mental health. For example, most medications used to treat depression cause weight gain; so, maybe depression led to antidepressants, which led to weight gain, which then led people to try eating healthier diets. What we need are prospective studies that follow people over time to see, in the very least, which came first. There’s been a few.
This study in Germany found that the mental disorders came first, followed by the adoption of the vegetarian diet, suggesting that those with mental health issues may try to clean up their diets in hopes it will benefit their condition. It was the same in the UK; psychological distress predicted later vegetarianism, and not the other way around.
By far the biggest study was in Taiwan, a prospective cohort study following more than 3,500 vegetarians for about nine years, and compared with non-vegetarians, those in the vegetarian group had a 30% lower chance of developing a depressive disorder in that time. Thus, Taiwanese vegetarians were at lower risk. As you can see, rates remained about the same for the first two years, and then the curves separate. What might be the mechanism? Well, the sleep quality of vegetarians may be significantly better, and maybe that helps mood, or maybe they’re just sleeping better because they’re less depressed.
On the one hand, consumption of vegetarian diets could increase the risk of depression if the vegetarians weren’t getting enough vitamin B12 or long-chain omega-3 fats. At the same time, there are aspects about their diet that could reduce the risk of depression. Like all the antioxidant, anti-inflammatory foods, like fruits and vegetables, may help to counteract the inflammation in the brain associated with depression.
In the United States, those eating vegetarian may only have half the odds of depression. But if you adjust for factors such as inflammation and body weight, the association is no longer significant. Now, it’s totally legitimate to control for things like educational level and smoking status, but since vegetarians are less likely to be obese and inflamed, maybe that played a role in their better mental health. So, by effectively erasing those benefits, one may be undercutting part of the causal pathway.
It’s not actually clear what role inflammation plays in depression, though. Check out this study: a double-blind, randomized, placebo-controlled trial on the effect of the anti-inflammatory drug aspirin for the long-term management of depression in older people. Those randomized to aspirin actually had a worsening of their depression, and diminished quality of life compared to placebo.
There are certainly confounding factors that need to be taken into account. Otherwise, we don’t know if some of the associated lifestyle factors are the cause, rather than the diet itself. For example, in this study of plant-based health professionals, besides eating fewer animal foods, they also tended not to smoke or drink as much, exercised and slept regularly, and also ate less junk. And one can imagine them feeling better mentally if only because they felt better physically. They lost weight, up to 120 pounds (55 kg); some reversed their health conditions and were able to stop medications. Here are some of the conditions that reportedly improved or reversed on a plant-based diet—no wonder they felt less depressed.
It’s like when you look at the effectiveness of plant-based diets in promoting well-being in the management of type 2 diabetes. Yes, compared to traditional diets for diabetes, eating plant-based was associated with significant improvement in emotional well-being, depression, and quality of life, but maybe it’s just because their diabetes improved significantly more.
And finally, when we’re talking about well-being, we’re talking about well-being through self-report. So, for example, in this study, the vegetarians reported significantly better well-being than omnivores, and vegans were about twice as likely to report good well-being, but maybe they were just ideologically motivated to say so. It’s like this study on the self-reported health status of those following the carnivore diet. If you do a social media survey of communities like the World Carnivore Tribe, they also report feeling great. They also reported shockingly high LDL cholesterol levels, but you can see how self-selection bias in various communities could skew the results. This is all to say why it’s critical we have interventional trials. Randomize people to eat different diets and see what happens: interventional trials, which I’ll cover, next.
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