Thirteen compounds have been classified as vitamins. Of these, the five most important for disease protection, according to the Harvard School of Public Health, are vitamins B6, B12, D, E, and folic acid.

Vitamin A helps you see in the dark. It stimulates production of white blood cells and regulates cell growth and division.

The body can manufacture vitamin A from beta carotene, which is found in carrots and many other fruits and vegetables.

Vitamin A can build up in the body, so it is possible to take too much. Excessive amounts can increase the risk of hip fracture, and some birth defects.

The recommended daily intake of vitamin A is 5,000 IU (international units) for men and 4,000 IU for women.

There are eight B vitamins, of which the most important are B6, B12, and folic acid. All three play key roles in recycling homocysteine (a protein breakdown product) into methionine, one of the building blocks for new proteins. Adequate amounts help prevent strokes.

In addition, folic acid has been shown to reduce the risks of colon and breast cancer (especially for those who drink alcohol), and to prevent birth defects such as spina bifida.

The recommended daily intake for folic acid is 400 micrograms per day. For vitamin B6, it is 1.3 to 1.7 milligrams per day. For vitamin B12, it's six micrograms per day.

Vitamin C is a powerful antioxidant that can neutralize harmful free radicals (which cause aging and cancer). It helps make collagen, important for healthy bones and teeth. Vitamin C has shown cold-fighting potential, but there is as yet little evidence to indicate megadoses are helpful. The recommended daily intake is 90 milligrams for men and 75 milligrams for women (more for smokers).

Vitamin D helps ensure the body absorbs and retains calcium and phosphorus, critical for building bone. It also keeps cancer cells from growing and dividing.

Fatty fish such as tuna and salmon are good sources of vitamin D, but few foods naturally contain it. Sunlight is critical to the formation of vitamin D. People who live in northern climates and African-Americans often don't get enough.

The recommended daily intake for vitamin D is 5 micrograms up to age 50; 10 micrograms to age 70; 15 micrograms for people older than 70.

Vitamin E is the family name for eight antioxidants. It can help prevent heart attacks and strokes. Nuts and tomatoes are good natural sources. The recommended daily intake is 15 milligrams from food, or 33 IUs of the synthetic form.

Vitamin K helps make six of the 13 proteins needed for blood clotting, and is involved in building bone. Lettuce and other green, leafy vegetables are a good source. The recommended daily intake is 80 micrograms for men and 65 for women.