A study published in the British Journal of Nutrition found an association between vitamin B12 deficiency and the incidence of depression in older people.

Older adults with low levels had a 51% increased risk of developing depression during the study. Low levels of vitamin B12 may result from poor diet, or poor absorption related to lower levels of pepsin secretion that releases vitamin B12 from food.

Pepsin is one of the main digestive enzymes that breaks down proteins into smaller peptides in the stomach.

Other nutrient deficiencies are also associated with depression, including vitamin D and a low omega-3 index. Omega-3 fatty acids are essential for brain health, blood clotting, muscle activity and more and deficiency contributes to mood disorders, including depression.