While there are many factors that could make a person moody or irritable, depleted levels of certain vitamins and other nutrients are among the physiological causes, according to "The Food-Mood Solution" by Jack Challem and Melvyn R. Werbach. Those who are deficient may notice mood improvement upon increasing their intake of these essential vitamins, and it's possible that supplementation with these vitamins may also generate favorable results among those whose irritability has other causes.
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Challem and Werbach state that vitamin B12 has been studied in numerous clinical trials focused on depression, anxiety and other mood-related problems, and has often been found to improve these conditions when administered to test subjects. The National Institutes of Health's Office of Dietary Supplements lists depression and confusion among the symptoms of vitamin B12 deficiency. According to "User's Guide to Natural Remedies for Depression" by Linda Knittel, B12 deficiency may be related to substance abuse, high stress, recent surgery or even vegetarian diets.
According to Knittel, folic acid is another essential vitamin related to mood management. Also known as vitamin B9 or folate, folic acid is needed only in small amounts, but it is crucial to the generation of serotonin and norepinephrine in the brain. Many prescription antidepressants also aim to create these brain chemicals. Folic acid is available as a supplement, is a common ingredient in multivitamins and can be obtained by eating leafy green vegetables, pineapple, oranges, bananas and asparagus.
Niacin, also known as vitamin B3, is responsible for breaking down fats, proteins and carbohydrates and converting them into energy, according to Knittel. Low levels of this vitamin have been linked with depression and other mood abnormalities, in part because of niacin's relationship with the amino acid tryptophan. As an essential amino acid, one of tryptophan's roles is to produce niacin, but it is also responsible for producing serotonin, a neurotransmitter that stabilizes mood. When tryptophan levels are low, as is commonly observed in people with depression, too much of the existing tryptophan is dedicated to niacin production, leaving serotonin production neglected. Supplementation with niacin from multivitamins, whole grains and organ meats like kidney and liver can help offset this effect.
According to a study published in the December 2006 issue of "The American Journal of Geriatric Psychiatry," vitamin D deficiency is associated with poor mood and cognitive performance among older adults. "Health and Fitness Times" cites another study that suggests supplementation with vitamin D may be an effective alternative treatment for periodic affective disorder, a type of depression that affects sufferers during the winter months. Fish is a good dietary source of vitamin D, and milk and breakfast cereals are often fortified with this vitamin, though you'll want to carefully check the label to be sure.