Even though vitamins are not a source of energy, we need them to extract energy from macronutrients, as well as to provide many regulatory and metabolic functions in the body. Vitamins are also necessary for proper cell functioning, development, and growth. The bioavailability, or the degree to which nutrients can be absorbed and utilized by the body, is influenced by many factors, including physiological and dietary conditions. The presence of disease, chronic alcohol abuse, and age-related physiological changes decrease bioavailability. The bioavailability of nutrients is also affected by the presence of other nutrients or food components that can enhance or reduce absorption, or by food-handling practices, such as whether you cook a food or eat it raw.

Vitamins are commonly grouped according to their solubility—or ability to disperse or dissolve in water. Solubility is an important determinant of how vitamins are absorbed and transported in the body. Solubility also affects where the vitamins are stored in the body and, potentially, the risk of toxicity (adverse effects from getting too much). Of the 14 currently identified vitamins, four dissolve in fat and are stored in body tissue. These fat-soluble vitamins, namely vitamins A, D, E, and K, are the subject of this chapter. The B vitamins, choline, and vitamin C—which disperse easily in water-based solutions, such as blood—are called water-soluble vitamins and are the subject of Chapter 11.