Each vitamin has specific functions. If a certain vitamin is deficient, 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 lactation. Beta carotene is a precursor to vitamin A; it has antioxidant properties.

Thiamine (B1) helps the body cells convert carbohydrates into energy. It is also essential for the functioning of the heart and for healthy nerve cells and the brain.

Riboflavin (B2) works with the other B vitamins and is important for body growth and red cell production. Similar to thiamine, it helps in releasing energy from carbohydrates.

Vitamin B6 is also known as pyridoxine. The more protein a person eats, the more B6 is required to use the protein. It helps 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 B12, like the other B vitamins, is important for metabolism. It helps in the formation of red blood cells and in the maintenance of the central nervous system.

Pantothenic acid and biotin: 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.

Folate (Folic acid) works with vitamin B12 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.

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

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

Vitamin E is also known as tocopherol; it 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.

Vitamins are obtained from food, except for vitamin D and vitamin K, which the body can synthesize.

There are 13 vitamins needed by the body: Vitamins A, C, D, E, K, and the B vitamins (thiamine, riboflavin, niacin, pantothenic acid and biotin, vitamin B6, vitamin B12, and folate).

FAT-SOLUBLE VITAMINS
Vitamin A is found in milk, cheese, cream, liver, kidney, and cod and halibut fish oil. All of these sources, except for fortified skim milk, are high in saturated fat and cholesterol.  However, vegetable sources of a vitamin A precursor called beta carotene are fat and cholesterol free. The body regulates the conversion of beta carotene to vitamin A based on the body's needs. Beta carotene comes from carrots, pumpkin, sweet potatoes, winter squashes, cantaloupe, pink grapefruit, apricots, broccoli, spinach and most dark green leafy vegetables. 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, fortified milk (all milk in the United States is fortified with Vitamin D), fish, oysters, and fortified cereals. The body can 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 (corn, sunflower, soybean, and cottonseed) and products made from them such as margarine.

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

WATER-SOLUBLE VITAMINS
Thiamine (vitamin B1) is found in fortified breads, cereals, pasta, whole grains (especially wheat germ), lean meats (especially pork), fish, dried beans, peas, and soybeans. Dairy products and milk, fruits, and vegetables are not very high in thiamine, but when consumed in large amounts they become a significant source.

Niacin (vitamin B3) 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.

Vitamin B12 is found in eggs, meat, poultry, shellfish, and milk and milk products.

Pantothenic acid and biotin are found in eggs, fish, milk and milk 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, 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.

Recommended daily 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 has a PDF file that lists these http://www.nal.usda.gov/fnic/dga/rda.pdfrecommendations.