What exactly should we eat? You would think that after literally thousands and thousands of studies that have been published on every aspect of nutrition, we would have an answer to that question. Actually, we do. Maybe 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 bit of scientific sleuthing the chaff can be separated from the wheat. While separating the chaff from the wheat makes for a good idiom when differentiating information from misinformation, it is not that great when it comes to nutrition. We have learned that eating whole grains is in many ways preferable to eating refined grains. And what else have we learned?
Besides replacing refined with whole grains, we should increase our intake of legumes, eat five to seven servings daily from the fruit, berry and vegetable group, favour fish and poultry over red meat, curb sugar consumption, and ignore talk of “superfoods.” It is hard to glean much more detail than that because nutritional studies that would yield definitive answers are impossible to carry out. That would require a trial in which an experimental group and a control group is followed for decades with the sole difference being the subjects’ diet. They would be allowed only foods provided by the designers of the study who would also monitor their health status over the years.
Given the financial and logistic barriers for 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 issues they encountered. Often the questionnaire is filled out at a single point in time and it is assumed, somewhat precariously, that the same diet is followed over the experimental period. A further complication is that people have a hard time remembering exactly what they ate and judging amounts. Sometimes they may also report what they think they should have eaten instead of what they really ate.
Another type of study, termed case-control, asks patients suffering from a specific disease to describe the dietary pattern they followed. Recalling specifics of what foods and how much of each was consumed over years is notoriously difficult. Another problem is determining if a certain effect is due to what is eaten or to what is not eaten. For example, are the detriments of a diet high in red meat due to the meat or to the low consumption of fruits and vegetables. There is also the problem that people do not respond to diets in a uniform fashion. Given the same amount of sugar, blood insulin levels can vary dramatically.
Nevertheless, when all the studies are compiled, whether about cardiovascular disease, cancer, or diabetes, a pattern emerges. It is an emphasis on plant foods. And it seems that even extends to mental health, a condition that is getting more and more attention in light of an increased risk of depression being linked with COVID-19. Admittedly, the data linking diet to mental health is soft, but there are indications of a relationship from observational studies. A number of these have linked the Mediterranean diet with lower rates of depression when compared with the traditional "western" diet. Although there isn't one specific Mediterranean diet, after all, Italians don't eat exactly like the Greeks, we can describe the Mediterranean diet in general as being based on fruit, vegetables, whole grains, fish, olive oil, nuts, and low in red meat and processed foods. The western diet in turn has lots of red meat and processed foods, refined grains, potatoes, dairy products, and low intake of fruits and vegetables.
Of course, observational studies can show an association but cannot prove a cause and effect relationship. It may be that other aspects of a Mediterranean lifestyle, perhaps reduced calorie intake, a greater level of social interaction, more cohesive family structure, better sleep habits, or a higher activity level result in a reduced rate of depression. The only way a cause and effect relationship can be established is by means of randomized, controlled trials. These, as already mentioned, are difficult to carry out but some attempts have been made with promising results. In one trial, people diagnosed with depression were divided into two groups, one adhering to a Mediterranean diet, the other to a western diet. After three months, 8% of individuals in the western group achieved remission, compared with 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 one nutrient, perhaps saturated fat or sugar? These can deplete dopamine in the brain, a neurotransmitter associated with good mood. Or could the problem be or a deficiency in some minerals or vitamins such as magnesium, iron, zinc, vitamin B6, B12 or D? Depressed people can also have reduced levels of the neurotransmitter serotonin, much of which is produced by bacteria in the gut and the ratio of "good" versus "bad" bacteria that make up the "microbiome" depends on diet. Perhaps the Mediterranean diet being richer in pre and probiotics that favour beneficial bacteria is a factor. Then there is the issue of omega-3 fatty acids that are important in brain development and which also have an anti-inflammatory effect. Some studies have linked depression with inflammation based on finding higher blood levels of inflammatory markers.
Finally, if depression is caused by low levels of some nutrients, there is the question of treatment with supplements. There are no compelling studies that show efficacy. Perhaps vitamin, mineral, or omega-3 supplements may help some people who have an extremely poor diet, but not those who have an adequate intake already. 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 fall into this category. While there are some studies that have shown benefits, the evidence is less than compelling.
What then is the bottom line when it comes to some sort of recommendation about diet and mental health? Same as already discussed for other conditions. Look askew at supplements, reduce intake of red and processed meats in favour of fish and poultry, replace refined grains with whole grains, increase legumes, and eat at least 5 servings of fruits and vegetables a day. That then is the link between diet and health in a nutshell. And when you shell those nuts, snack on what's inside them, just like the Mediterraneans.
