Deficiencies in omega-3, vitamin D, B12, folate, zinc, magnesium & iron can lead to mental illness including depression.

FTA:

The importance of nutrition for maintaining mental health has been highlighted by recent research. The human brain needs an adequate intake of key nutrients, such as polyunsaturated fatty acids Omega-3, essential amino acids, B-group vitamins (B12 and folate), vitamin D and minerals like zinc, magnesium and iron. A balanced and high-quality diet, such as the Mediterranean, provides all of these, the researchers note.