Tuesday 17 July 2018

Can diet affect mental health?

Seaweed meal
Seaweed meal

Recently I had the pleasure of being interviewed by Shane Coleman (standing in for Pat Kenny) on the Pat Kenny Show, Newstalk 106. The topic was diet and mental health and Shane conducted a wide ranging interview and the level of interest shown by the listeners suggests that it is worth highlighting some of the issues that arose.

The stimulus for this interview was a recently published article by dietician Leslie Beck for the Globe and Mail series entitled Open Minds: Better Mental Health Care. It stated that "what you eat and don't eat can have a powerful impact on mental health".

The first question is "Is this true"? To date there is no certainty about this although there are straws in the wind. But caution is still the best approach since diet is a very difficult to study and to suggest that diet of itself is a significant cause of mental health problems is a step too far, at this point. The number of quality studies is still relatively small.

The suggestion that diet may play a role in causing mental illness comes from studies of societies where diet has changed in recent years. By comparing the prevalence of certain illnesses before and after this change allows investigators to study this question. The diet among the polar nations (North and South) has recently undergone major change in content. Traditional diets in the Poles are generally rich in marine mammals, fish, fur-bearing animals, birds and their eggs, plants, and berries.

These foods have high levels of protein, fat (especially omega-3 fatty acids), and antioxidants (e.g., selenium), while being low in carbohydrates. Of course the physical activities associated with hunting for these foods may also have had an impact.

The traditional diet has now been replaced with a Western-style diet in many polar areas. The new foods are high in carbohydrates and saturated fats, and low in essential nutrients such as omega-3 fatty acids.

The impact on mental health has been noted, particularly in the less isolated circumpolar people, who have changed their diet more dramatically than those that are more isolated.

Rates of depression, seasonal affective disorders, anxiety, and other mental disorders have increased in non-isolated areas as have suicide rates.

A 2013 study of different diets in Japan, examined traditional and modern diets and also a diet called "prudent" which combined the best of each and consisted of vegetables, fruits, potatoes, soy products, mushrooms, seaweed and fish. This type of diet was associated with less suicide that the other two. Yet this defies our conventional ideas about the value of the traditional diet.

Why should a traditional Japanese diet be linked to a similar suicide rate to those having a Western diet? One of the difficulties with studies of diet is that if they are based on interviews with individuals then the volunteers may have changed their food preferences many times. It is commonplace for meat eaters to become vegetarian and then later again to revert to carnivorism. Also people's memory for previous diets may be unreliable and not truly reflect the overall dietary pattern. The Japanese study used this methodology.

There are also problems with studies that inquire only about diet. Factors other than diet impinge upon mental illness. Indeed even when humans ate berries, naturally produced food, free from additives, and had a lot of exercise, they still developed mental illness.

Thales, Pythagoras and Hippocrates, living in Greece around 400 BC recognised the existence of mental illness and postulated that its causes were related to physiology rather than demons. It may have been linked to infection, genetics (unknown at that time) or some other internal change in the person.

The problems of extrapolating from population studies to the individual sitting at home cannot be over-estimated. In essence this raises the question of what the findings regarding polar diets mean for individuals. And what about people who have changed their diets and how this will impact on their mental health?

The best evidence is for the role of omega-3 oils in maintaining the sheathings in the neurones throughout the brain, while the vitamin B group influence the concentration of serotonin and other brain chemicals involved in emotional regulation. Some also promote the response to antidepressants.

The only definitive way to show that diet influences mental health is with trials of certain nutrients, such as specific vitamins, administered in randomised controlled clinical trials and these have not yet happened in sufficient number to make for a definitive policy in this regard. The best evidence so far is for the role of omega 3 fatty acids as one of the elements in the causation and treatment of depression.

Psychiatrists are not yet prescribing dietary measures to treat mental illness. If your diet is satisfactory and balanced supplements are unnecessary. There is no need to go "free range organic", at least not just yet.

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