Vitamins are organic compounds that are essential in small amounts for normal metabolism. Because vitamins (with the exception of vitamin D) cannot be synthesized by humans, they need to be ingested in the diet to prevent disorders of metabolism. They should be distinguished from minerals (such as calcium and iron), some of which are also essential micronutrients. "Food supplements" is a general term for vitamins, minerals, herbs, and other natural compounds that may improve health when taken in small amounts.

The concept of vitamin deficiency has evolved since vitamins were first discovered, from obvious vitamin deficiency syndromes to the subtle effects of suboptimal vitamin intake on chronic diseases. Gross vitamin deficiency may be recognized by obvious clinical syndromes. These syndromes are still seen in areas of the world with very poor diets. Current interest in vitamins centers on whether optimizing the daily ingestion of vitamins can prevent chronic disease (eg, vitamin D supplementation and osteoporotic fractures; vitamin B12 and dementia).

Measurements of serum levels of several vitamins are commercially available. They are useful if clearly very high or very low to diagnose or rule out gross deficiency. However, the meaning of a "normal" value is uncertain; it is defined as the range of usual values in the general population, but many of these people may have suboptimal intake (just as, for example, commonly occurring levels of cholesterol or blood pressure may be harmful).

Intake or serum levels of some vitamins have been related to biochemical abnormalities. As examples, the serum concentration of homocysteine rises with diets low in folic acid, methylmalonic acid rises with low intake of vitamin B12, and parathyroid hormone rises with low intake of vitamin D. These biochemical abnormalities generally improve with increasing intake suggesting a correctable metabolic disorder.