Studies have revealed: 
-  A reduced likelihood of depression is associated with increased intake of a ‘healthy diet’, defined as a diet high in fruit, vegetables, fish and whole grains.
- A reduced risk of depression is linked with adherence to the Mediterranean diet.
- A Mediterranean diet can confer a protective effect against depression.
- Dietary interventions hold promise to reduce depression incidence.

Link Between Depression & Diet

Highest impairments in cognition are caused by deficiencies in:
- Vitamin B12
- Thiamine
- Folic Acid 
- Niacin

Vitamin Deficiencies & Mental Health

Several studies have reported strong correlations between a healthy diet and mental health. For example, increased consumption of a diet rich in fresh fruits and vegetables has been associated with increased reported happiness and higher levels of mental health and well-being. 

Saint Louis University

In this meta-analysis, researchers provide an overview of the emerging field of nutrition psychiatry. They also synthesize current findings on diet and nutrition and their impact on mood and mental health.

1. Nutrition is crucial in addressing the burden of mental disorders, and its importance should be recognized in psychiatry.
2. Unhealthy dietary patterns are linked to a higher prevalence of mental health issues, while healthy dietary patterns reduce the risk of depression and suicide.
3. Nutrient-based supplements, such as omega-3 fatty acids, SAMe, NAC, zinc, B vitamins, and vitamin D, show potential in managing mental disorders.
4. Future research should focus on identifying the impact of overall diet and specific nutritional elements on mental health and the efficacy of nutrient supplements.
5. Governments should take action to improve food quality, promote healthier dietary practices, and educate the public and clinicians about the role of nutrition in mental health.

Nutritional Psychiatry & the Impact of Food on Mental Health

Adan, R. A., van der Beek, E. M., Buitelaar, J. K., Cryan, J. F., Hebebrand, J., Higgs, S., ... & Dickson, S. L. (2019). Nutritional psychiatry: Towards improving mental health by what you eat. European Neuropsychopharmacology, 29(12), 1321-1332.

Nutrition Psychiatry: Towards Improving Mental Health by What You Eat

The brain's composition, structure and ability to function, are dependent on the availability of appropriate nutrients: including lipids, amino acids, vitamins and minerals. Therefore, food intake and quality have an impact on brain function, which makes diet a modifiable variable to target mental health, mood and cognitive performance.

Likewise, endogenous gut hormones, neuropeptides, neurotransmitters, and the gut microbiota, are affected directly by the composition of one's diet

Role of Nutrition Regarding the Brain & Body

Correlations Between Diet & Mental Health

Research is sparse and cannot be generalized but researchers have found:
- Rigorous elimination diets (including elimination of food additives) reduced symptoms in 1/3 of participants
- Supplementation of free fatty acids aids in symptom reduction
- Supplementation with micronutrients (i.e. vitamins and minerals) is associated with less aggression and better emotion-regulation
- Diets high in refined sugar and saturated fat may increase risk for ADHD in contrast to the potential protective effect of a diet high in fruits and vegetables

Diet Interventions for ADHD

Results of nutritional interventions (from supplementation with vitamin D and micronutrients to gluten-free and casein-free) in autism are also very diverse, and have not been subjected to rigorous meta-analyses. With autism, better controlled studies are required and multiple mechanisms may explain efficacy

Diet Interventions for Autism

Few studies have centered on the relationship between diet and bipolar disease, but researchers have found that:

- Higher seafood consumption is associated with lower bipolar disease incidents
- People with bipolar disease more frequently report eating one meal per day and having trouble sourcing food and cooking
- An Australian study found that bipolar patients had a higher rate of energy consumption and more often ate typical western diet foods
- A study on ambulatory bipolar patients in Japan indicated higher symptom severity in a group who reported less frequent consumption of Mediterranean foods.

Diet Interventions for Bipolar Disease

A study on young adults with moderate or high levels of depression symptoms found that:

- brief diet intervention improved depression symptoms in the participants a part of the treatment group
- the results are the first to show that compliance with a recommended intake of fruits and vegetables, as well as other dietary interventions, can reduce symptoms of depression in young adults

Diet Interventions for Depression in Young Adults

A meta-analysis is a quantitative type of review article that provides a statistical summary of all previous results on a specific topic. It synthesizes findings from multiple studies into a single combined result or distribution of effect sizes, making it an ideal method for comprehensively evaluating a body of literature's overall impact and implications.

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