Vitamins serve a wide variety of functions.
Vitamin A aids in the maintenance of epithelial tissues, boosts the body's resistance to infection, and is essential to good eyesight. Vitamin D promotes the absorption of calcium, vitamin K is necessary for blood clotting, and vitamin C acts as a potent antioxidant. The B vitamins assist enzymes in the breakdown and formation of proteins, carbohydrates, and fats. Unlike most of the other nutrients that have been discussed, vitamins cannot serve as energy sou rces and they contain no calories, although several of the B vitamins do play a role in taking energy out of other nutrients. Being soluble in water, these vitamins circulate freel y in the bloodstream without the need for protein carriers.