What exactly should we eat? You would think that after literally thousands and thousands of studies that have been published on all aspects of nutrition, we would have an answer to this question. In fact, we do. Perhaps not “exactly,” given that there is such a diversity of nutritional studies, some of which can be classified as good, some as bad, and most as mediocre. Nevertheless, with a little scientific research, the chaff can be separated from the wheat. While separating the chaff from the wheat is a good idiom for differentiating information from misinformation, it’s not so great when it comes to nutrition. We’ve learned that eating whole grains is in many ways better than eating refined grains. And what else have we learned?

In addition to replacing the refined with whole grains, we should increase our consumption of legumes, eat five to seven servings a day from the group of fruits, berries and vegetables, favor fish and poultry over red meat, reduce sugar intake and ignore “superfoods”. It’s hard to glean much more detail than that because nutritional studies that would provide definitive answers are impossible to do. This would require a trial in which an experimental group and a control group are followed for decades, the only difference being the diet of the subjects. They would only be entitled to food provided by the study designers who would also monitor their health over the years.

Given the financial and logistical barriers to such a trial, most studies are “observational”. Groups of volunteers fill out detailed food frequency questionnaires and, years later, are asked to report any health problems they encounter. Often the questionnaire is completed at some point and it is assumed, somewhat precariously, that the same diet is followed during the experimental period. Another complication is that people have trouble remembering exactly what they ate and judging how much. Sometimes they may also report what they think they should have eaten instead of what they actually ate.

Another type of study, called case-control, asks patients suffering from a specific disease to describe the diet they followed. It is notoriously difficult to remember the details of foods and the amount of each eaten over the years. Another problem is determining whether a certain effect is due to what is eaten or what is not eaten. For example, the disadvantages of a diet high in red meat are due to the meat or the low consumption of fruits and vegetables. There’s also the problem that people don’t react to diets uniformly. With the same amount of sugar, blood insulin levels can vary significantly.

Nevertheless, when all the studies are compiled, whether they relate to cardiovascular disease, cancer or diabetes, a pattern emerges. It emphasizes plant foods. And it appears this even extends to mental health, a condition that is gaining more and more attention in light of an increased risk of depression related to COVID-19. Admittedly, the data linking diet to mental health is weak, but there are indications of a relationship from observational studies. A number of them have linked the Mediterranean diet to lower rates of depression compared to the traditional “Western” diet. Although there is no specific Mediterranean diet, after all Italians do not eat exactly like Greeks, we can describe the Mediterranean diet in general as being based on fruits, vegetables, whole grains, fish, olive oil, nuts and low in red meat and processed foods. The Western diet, in turn, is high in red meat and processed foods, refined grains, potatoes, dairy products, and a low intake of fruits and vegetables.

Of course, observational studies can show an association but cannot prove a causal relationship. It may be that other aspects of a Mediterranean lifestyle, perhaps reduced calorie intake, greater level of social interaction, more cohesive family structure, better sleeping habits or activity level higher lead to a lower rate of depression. The only way to establish a causal relationship is through randomized controlled trials. These, as already mentioned, are difficult to perform but some attempts have been made with promising results. In one trial, people diagnosed with depression were split into two groups, one following a Mediterranean diet, the other following a Western diet. After three months, 8% of individuals in the Western group achieved remission, compared to 32% of those in the Mediterranean group. But the study was small and has not been replicated by others.

Then there is the question of why diet should have an effect on mental health. Is it due to too much of a nutrient, perhaps saturated fat or sugar? These can deplete dopamine in the brain, a neurotransmitter associated with good moods. Or could the problem be a deficiency in certain minerals or vitamins like magnesium, iron, zinc, vitamin B6, B12 or D? Depressed people may also have reduced levels of the neurotransmitter serotonin, much of which is produced by bacteria in the gut and the ratio of “good” to “bad” bacteria that make up the “microbiome” depends on food. Perhaps the Mediterranean diet being higher in pre and probiotics which promote beneficial bacteria is a factor. Then there is the matter of omega-3 fatty acids which are important in brain development and also have an anti-inflammatory effect. Some studies have linked depression to inflammation based on finding higher blood levels of inflammatory markers.

Finally, if depression is caused by low levels of certain nutrients, there is the question of treatment with supplements. There are no convincing studies that show effectiveness. Perhaps vitamin, mineral, or omega-3 supplements can help some people who have extremely poor diets, but not those who already have sufficient intake. There are also a variety of “natural” products that are promoted, usually through vigorous advertising, as being effective in reducing depression or anxiety. St. John’s wort, S-adenosyl methionine (SAMe), 5-hydroxytryptophan (5-HTP), dehydroepiandrosterone (DHEA), saffron, turmeric, lavender, tyrosine, folic acid and inositol go into this category. Although some studies have shown benefits, the evidence is far from convincing.

So what is the bottom line when it comes to any sort of recommendation on diet and mental health? Same as what has already been discussed for other conditions. Look askance at supplements, cut back on red and processed meats in favor of fish and poultry, replace refined grains with whole grains, increase legumes, and eat at least 5 servings of fruits and vegetables a day. So that’s the link between diet and health in a nutshell. And when you shell those nuts, snack on what’s in them, just like the Mediterranean people.

@JoeSchwarcz