Definition

Vitamins are a group of substances essential for normal metabolism, growth and development, and regulation of cell function.

Vitamins work together with enzymes, co-factors (substances that assist enzymes), and other substances necessary for healthy life.

Function

Each vitamin has specific functions. If levels of a particular vitamin are inadequate, a deficiency disease results.

Vitamin A helps in the formation and maintenance of healthy teeth, skeletal and soft tissue, mucous membranes, and skin. It is also known as retinol because it generates the pigments that are necessary for the working of the retina. It promotes good vision, especially in dim light. Vitamin A may also be required for reproduction and breast-feeding. Beta-carotene is a precursor to vitamin A that has antioxidant properties, helping the body deal with unstable chemicals called free radicals.

Thiamine (B-1) helps the body cells convert carbohydrates into energy. It is also essential for the functioning of the heart and for healthy nerve cells, including those in the brain.

Riboflavin (B-2) works with the other B vitamins and is important for body growth and red blood cell production. Similar to thiamine, it helps in releasing energy from carbohydrates.

Niacin is a B vitamin that helps maintain healthy skin and nerves. It is also important for the conversion of food to energy and may have cholesterol-lowering effects.

Vitamin B-6 is also known as pyridoxine. The more protein a person eats, the more vitamin B-6 is required to help the body use the protein. It aids in the formation of red blood cells and in the maintenance of normal brain function. It also assists in the synthesizing of antibodies in the immune system.

Vitamin B-12, like the other B vitamins, is important for metabolism. It, too, helps in the formation of red blood cells and in the maintenance of the central nervous system.

Pantothenic acid is essential for the metabolism of food. It is also essential in the synthesis of hormones and cholesterol. Biotin is essential for the metabolism of proteins and carbohydrates, and in the synthesis of hormones and cholesterol. Cholesterol is needed for the functioning of cell membranes, particularly in the brain.

Folate (folic acid) works with vitamin B-12 in the production of red blood cells. It is necessary for the synthesis of DNA, which controls heredity as well as tissue growth and cell function. Any woman who may become pregnant should be sure to consume enough folate -- low levels of this substance are associated with devastating birth defects such as spina bifida. Many foods are now fortified with folic acid to help reduce the level of such birth defects.

Vitamin C, also called ascorbic acid, promotes healthy teeth and gums, helps in the absorption of iron, and helps maintain normal connective tissue. It also promotes wound healing and is an antioxidant.

Vitamin D is also known as the "sunshine vitamin," since it is manufactured by the body after being exposed to sunshine. Ten to 15 minutes of sunshine three times per week is adequate to produce the body's requirement of vitamin D. This vitamin promotes the body's absorption of calcium, which is essential for the normal development and maintenance of healthy teeth and bones. It also helps maintain adequate blood levels of calcium and phosphorus, which are minerals necessary for many functions.

Vitamin E is also known as tocopherol and is an antioxidant. It is also important in the formation of red blood cells and the use of vitamin K.

Vitamin K is known as the clotting vitamin, because without it blood would not coagulate. Some studies indicate that it helps in maintaining strong bones in the elderly.

Food Sources

There are 13 vitamins essential for bodily functions: Vitamins A, C, D, E, K, and the B vitamins (thiamine, riboflavin, niacin, pantothenic acid, biotin, vitamin B-6, vitamin B-12, and folate). They all can be obtained from food, and vitamin D and vitamin K can be synthesized by the body.

FAT-SOLUBLE VITAMINS

Vitamin A is found in milk, cheese, cream, liver, kidney, and cod and halibut fish oils. Because most of these sources are high in saturated fat and cholesterol, vegetable sources of a vitamin A precursor called beta-carotene may be a better choice. Beta-carotene comes from carrots, pumpkin, sweet potatoes, winter squashes, cantaloupe, pink grapefruit, apricots, broccoli, and spinach. The more intense the color of a fruit or vegetable, the higher the beta-carotene content.

Vitamin D is found in cheese, butter, margarine, cream, fish, oysters, and fortified milk and cereals. The body can also synthesize vitamin D when the skin is exposed to sunshine.

Vitamin E is found in wheat germ, corn, nuts, seeds, olives, spinach, asparagus, and other green leafy vegetables, vegetable oils, and products made from vegetable oils, such as margarine.

Vitamin K is found in cabbage, cauliflower, spinach, soybeans, and cereals. Bacteria in the intestines normally also produce vitamin K.

WATER-SOLUBLE VITAMINS

Thiamine (vitamin B-1) is found in fortified breads, cereals, pasta, whole grains, lean meats, fish, dried beans, peas, and soybeans. Dairy products, fruits, and vegetables contain some thiamine as well.

Niacin (vitamin B-3) is found in dairy products, poultry, fish, lean meats, nuts, and eggs. Legumes and enriched breads and cereals also supply some niacin.

Folate is found in green, leafy vegetables and many foods are now fortified with it as well.

Vitamin B-12 is found in eggs, meat, poultry, shellfish, milk and milk products.

Pantothenic acid and biotin are found in eggs, fish, dairy products, whole-grain cereals, legumes, yeast, broccoli and other vegetables in the cabbage family, white and sweet potatoes, lean beef, and other foods.

Vitamin C (ascorbic acid) is found in citrus fruits and their juices, strawberries, tomatoes, broccoli, turnip greens and other greens, sweet and white potatoes, and cantaloupe. Most other fruits and vegetables contain some vitamin C; fish and milk contain small amounts.

Side Effects

See the individual vitamins.

Recommendations

Recommended dietary allowances (RDAs), are defined as the levels of intake of essential nutrients that, on the basis of scientific knowledge, the Food and Nutrition Board judges to be adequate to meet the known nutritional needs of practically all healthy people.

The best way to get the daily requirement of essential vitamins is to eat a balanced diet that contains a variety of foods from the food guide pyramid.

Specific recommendations for each vitamin depend on age, gender, and other factors (such as pregnancy). The U.S. Department of Agriculture offers a PDF file that lists these recommendations.

Note: Many people think that if some is good, a lot is better. This is not always the case, and high doses of certain vitamins are actually toxic. Read about the specific vitamins and check with your health care provider if you are unsure about how much to take -- and how much may be too much.