Insulin resistance may be involved in the pathology of bipolar disorder. There is preliminary evidence that the ketogenic diet may improve insulin resistance and thus help treat bipolar disorder.
Mental health refers to emotional and psychological well-being. Diet and dietary supplements can be helpful in maintaining mental health.
Mental health can be defined as having the necessary emotional and psychological well-being to enjoy life, manage stress, and develop meaningful relationships. Disorders such as depression, anxiety, schizophrenia, and ADHD negatively impact mental health and can be difficult to manage. Additionally, having a mental health disorder may increase the risk of developing other diseases, including type 2 diabetes and coronary artery disease. 
There does appear to be a relationship between diet and mental health. However, the effect of diet on mental health is highly complex, and its mechanisms are yet to be fully understood. Similar to other aspects of health, a diet rich in fruits, vegetables, nuts, legumes, fish, and lean protein can benefit mental health.
The supplements that are of interest for mental health include ashwagandha, curcumin, D-serine, fish oil, kava, saffron, and sarcosine. Before starting any supplement for mental health, it is important to consult a healthcare professional because concurrently taking a drug for mental health disorders and a dietary supplement could lead to adverse effects.
Supplement GuideClick here to read the Stress & Anxiety Supplement Guide
Many people get depressed during the winter months, when we produce less vitamin D. So, can supplemental vitamin D cure seasonal depression, and maybe other types of depression? No, alas — but it may help.