The importance of diet for health has been known since ancient times, with Hippocrates and Galen1. In his writings, Hippocrates recommended the following treatment for melancholia (severe depression he attributed to an excess of black bile): “Donkey’s milk should be prescribed. The patient should eat very little food, if not weak; this food should be cold, relaxing, nothing sour, nothing salty, nothing oily, nothing sweet2”.
Since Hippocrates, medical and scientific knowledge has advanced. In recent years, numerous studies have focused on the impact of nutrition on health and, in particular, on mental health; some people, like Dr Guillaume Fond, psychiatrist and researcher, who published “Bien manger pour ne plus déprimer – Prendre soin de son intestin pour prendre soin de son cerveau” [Eating well to avoid depression – Taking care of your gut to take care of your brain] in 2022, refer to this as “psychonutrition”. This is not to say that diet or nutritional supplements will prevent or treat mental health problems in 100% of cases, but they can be used in addition to other approaches.
Defining depression
It’s important to define the terms we use. The term “mental health” is vague, as is “well-being’. Most studies on nutrition concern psychiatric disorders, such as depression, schizophrenia, bipolar disorder, etc. For depression, we’re not talking about the meaning used by the general population, but what appears in classification systems under the term “major depressive disorder” – diagnostic criteria for major depressive disorder according to the American classification (DSM 5).
Which supplements for which symptoms?
For example, vitamin B9 (folic acid) supplementation is recommended in pregnant women for the development of the baby. However, it has also been shown that insufficient plasma folate levels are associated with an increased risk of depression, and that folate supplementation in addition to antidepressant treatment can reduce depressive symptoms3.
In 2022, the World Federation of Societies for Biological Psychiatry (WFSBP) and the Canadian Network for Mood and Anxiety Management (CANMAT) published guidelines for the treatment of psychiatric disorders using nutraceuticals and phytoceuticals. In the case of unipolar depression, they recommend supplementation with omega-3 fatty acids4. Other products may be effective (zinc, probiotics), but not at the same level (they recommend them provisionally). According to a meta-analysis published in 2023, antioxidant supplementation (tea, coffee, zinc, etc.) reduces symptoms of anxiety and depression5. According to a meta-analysis published in 2024, with the exception of depression, current evidence is insufficient to support interventions with prebiotics, synbiotics and fermented foods in the treatment of other mental illnesses6.
The impact of nutrition on depressive symptoms
Apart from supplementation, we can also ask what impact our daily diet has and what mechanisms are involved.
In a Swedish study including 33,000 women, the risk of high-level psychotic symptoms was 53% lower (95% CI, 30-69%) in women who ate fish three or four times a week than in women who never ate fish7. Omega-3s are linked to various biological pathways (inflammation, oxidative stress, etc.)8. It is often their anti-inflammatory effect that is put forward to explain their usefulness in depression.
Data in the literature suggest a protective link between lithium intake from public drinking water and suicide mortality at population level. Lithium naturally present in drinking water may reduce the risk of suicide and help stabilise mood9. Some researchers are even calling for water to be supplemented with lithium to reduce the risk of suicide10.
The mechanisms involved are complex. Numerous studies have found immune system abnormalities, inflammation or oxidative stress in psychiatric disorders. In recent years, there has been an abundance of literature on the role of the gut microbiota in psychiatry, but the results are inconclusive11-14. The positive impact of certain diets could be linked to their anti-inflammatory effect15.
Some nutritional recommendations…
From a practical point of view, there is no simple list of recommended foods, since everything depends on the quality of the food, and the diet must also be adapted to each individual. However, on the basis of the data in the literature, a few broad principles can be outlined. According to data from the French NutriNet-Santé cohort, which began in 2009, consumption of ultra-processed foods (UPF) is associated with chronic insomnia16 and the onset of depressive symptoms17. According to a recent meta-analysis, a low-carbohydrate diet, such as the ketogenic diet, has no significant effect on depression or anxiety18. On the other hand, data suggest that Mediterranean and anti-inflammatory (Dietary Inflammatory Index) diets are associated with a reduced risk of depression19. The Mediterranean diet is a dietary model based on limited consumption of red meat and a high intake of fruit and vegetables, legumes, fish, wholegrain cereals, extra-virgin olive oil and nuts, guaranteeing a high intake of unsaturated omega-3, 6 and 9 fatty acids, folates and fibre20, etc. The Dietary Inflammatory Index, created in the 2010s, is a validated tool for measuring the inflammatory potential of the overall diet, based on 45 parameters21.
In conclusion, current scientific knowledge suggests that diet can have an impact on mental health. Firstly, it can reduce or increase the risk of psychiatric disorders. Secondly, when added to psychiatric treatment, certain nutritional supplements, such as omega-3s, may enhance the improvement of symptoms. In all cases, dietary supplements must be approved by a doctor. People must not follow a diet that could be life-threatening or lead to deficiencies. Dietary measures are not a substitute for medication and regular physical activity is also important.
Sources
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