Vitamins are organic chemicals, substances that contain carbon, hydrogen, and oxygen. They occur naturally in all living things, plants and animals alike: flowers, trees, fruits, vegetables, chickens, fish, cows — and you. Vitamins regulate a variety of bodily functions.

They’re essential for building body tissues such as bones, skin, glands, nerves, and blood. They assist in metabolizing (digesting) proteins, fats, and carbohydrates so that you can get energy from food. They prevent nutritional deficiency diseases, promote healing, and encourage good health.

This article is a guide to where the vitamins are, how you can add them to your diet, how to tell how much is more than enough of any specific vitamin, and ever so much more . . . maybe more than you really want to know.

Your body needs at least 11 specific vitamins: vitamin A, vitamin D, vitamin E, vitamin K, vitamin C, and the members of the B vitamin family: thiamin (vitamin B1), riboflavin (B2), niacin, vitamin B6, folate, and vitamin B12. Two more B vitamins — biotin and pantothenic acid — are now believed valuable to your well-being as well.

And one unusual compound called choline has recently received some favorable mention. You need only miniscule quantities of vitamins for good health. In some cases, the recommended dietary allowances (RDAs), determined by the National Research Council, may be as small as several micrograms (1⁄1,000,000 – that’s one one-millionth — of a gram).

Nutritionists classify vitamins as either fat soluble or water soluble, meaning that they dissolve either in fat or in water. If you consume larger amounts of fat-soluble vitamins than your body needs, the excess is stored in body fat. Excess water-soluble vitamins are eliminated in urine.

Large amounts of fat-soluble vitamins stored in your body may cause problems. With watersoluble vitamins, your body simply shrugs its shoulders, so to speak, and urinates away most of the excess.

Medical students often use mnemonic devices — memory joggers — to remember complicated lists of body parts and symptoms of diseases. Here’s one I use to remember which vitamins are fat-soluble: “All Dogs Eat Kidneys.” This saying helps me remember that vitamins A, D, E, and K are fat-soluble. All the rest dissolve in water.

Fat-soluble vitamins

Vitamin A, vitamin D, vitamin E, and vitamin K are relatives that have two characteristics in common: All dissolve in fat, and all are stored in your fatty tissues.

But like members of any family, they also have distinct personalities. One keeps your skin moist. Another protects your bones. A third keeps reproductive organs purring happily. And the fourth enables you to make special proteins. Which does what? Read on.

Vitamin A

Vitamin A is the moisturizing nutrient that keeps your skin and mucous membranes (the slick tissue that lines the eyes, nose, mouth, throat, vagina, and rectum) smooth and supple.

Vitamin A is also the vision vitamin, a constituent of 11-cis retinol, a protein in the rods (cells in the back of your eye that enable you to see even when the lights are low) that prevents or slows the development of age-related macular degeneration, or progressive damage to the retina of the eye, which can cause the loss of central vision (the ability to see clearly enough to read or do fine work).

Finally, vitamin A promotes the growth of healthy bones and teeth, keeps your reproductive system humming, and encourages your immune system to churn out the cells you need to fight off infection.

Two chemicals provide vitamin A: retinoids and carotenoids. Retinoids are compounds whose names all start with ret: retinol, retinaldehyde, retinoic acid, and so on. These fat-soluble substances are found in several foods of animal origin: liver (again!) and whole milk, eggs, and butter.

Retinoids give you preformed vitamin A, the kind of nutrient your body can use right away. The second form of vitamin A is the vitamin A precursor, a chemical such as beta-carotene, a deep yellow carotenoid (pigment) found in dark green and bright yellow fruits and vegetables.

Your body transforms a vitamin A precursor into a retinol-like substance. So far, scientists have identified at least 500 different carotenoids. Only 1 in 10 — about 50 altogether — are considered, like beta-carotene, to be sources of vitamin A. Traditionally, the recommended dietary allowances of vitamin A are measured in International Units (IU).

However, because retinol is the most efficient source of vitamin A, the modern way to measure the RDA for vitamin A is as retinol equivalents, abbreviated as RE. One microgram (mcg) RE = 3.3 IU. However, many vitamin products still list the RDA for vitamin A in IUs.

Vitamin D

If I say “bones” or “teeth,” what nutrient springs most quickly to mind? If you answer calcium, you’re giving only a partial picture. True, calcium is essential for hardening teeth and bones.

But no matter how much calcium you consume, without vitamin D, your body can’t absorb and use the mineral. So vitamin D is vital for building — and holding — strong bones and teeth.

Researchers at the Bone Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University in Boston say vitamin D may also reduce the risk of tooth loss by preventing the inflammatory response that leads to periodontal disease, a condition that destroys the thin tissue (ligaments) that connects the teeth to the surrounding jawbone.

Finally, a report in the February 2006 issue of The American Journal of Public Health suggests that taking 1,000 international units (IU) of vitamin D a day may cut in half a person’s risk of developing some forms of cancer, including cancer of the colon, breast, or ovaries.

Vitamin D comes in three forms: calciferol, cholecalciferol, and ergocalciferol. Calciferol occurs naturally in fish oils and egg yolk. In the United States, it’s added to margarines and milk. Cholecalciferol is created when sunlight hits your skin and ultraviolet rays react with steroid chemicals in body fat just underneath.

Ergocalciferol is synthesized in plants exposed to sunlight. Cholecalciferol and ergocalciferol justify vitamin D’s nickname: the Sunshine Vitamin. The RDA for vitamin D is measured either in International Units (IUs) or micrograms (mcg) of cholecalciferol: 10 mcg cholecalciferol = 400 IU vitamin D.

Vitamin E

Every animal, including you, needs vitamin E to maintain a healthy reproductive system, nerves, and muscles. You get vitamin E from tocopherols and tocotrienols, two families of naturally occurring chemicals in vegetable oils, nuts, whole grains, and green leafy vegetables — your best natural sources of vitamin E.

Tocopherols, the more important source, have two sterling characteristics: They’re anticoagulants and antioxidants that reduce blood’s ability to clot, thus reducing the risk of clot-related stroke and heart attack.

Antioxidants prevent free radicals (incomplete pieces of molecules) from hooking up with other molecules or fragments of molecules to form toxic substances that can attack tissues in your body.

In fact, nutrition scientists at Purdue University released a study showing that vitamin E promotes bone growth by stopping free radicals from reacting with polyunsaturated fatty acids to create molecules that interfere with the formation of new bone cells.

But some claims about E’s heart health benefits are now considered iffy. True, a recent clinical trial at Cambridge University in England showed that taking 800 IU (International Units) of vitamin E, two times the RDA, may reduce the risk of nonfatal heart attacks for people who already have heart disease.

And, yes, the federal Women’s Health Study found that older women taking 600 IU vitamin E per day had a lower risk of heart attack and a lower risk of death from heart disease.

But the Heart Outcomes Prevention Evaluation (HOPE) study showed no such benefits. In fact, people taking 400 IU per day vitamin E were more likely to develop heart failure. No one (and no study) has found similar problems among those taking less vitamin E, say 100 IU/day. Whew.

The best sources of vitamin E are vegetables, oils, nuts, and seeds. The RDA is expressed as milligrams a-tocopherol equivalents (abbreviated as a-TE).

Vitamin K

Vitamin K is a group of chemicals that your body uses to make specialized proteins found in blood plasma (the clear fluid in blood), such as prothrombin, the protein chiefly responsible for blood clotting. You also need vitamin K to make bone and kidney tissues.

Like vitamin D, vitamin K is essential for healthy bones. Vitamin D increases calcium absorption; vitamin K activates at least three different proteins that take part in forming new bone cells.

For example, a report on 888 men and women from the long-running Framingham (Massachusetts) Heart Study shows that those who consumed the least vitamin K each day had the highest incidence of broken bones. The same was true for a 1999 analysis of data from the Nurses’ Health Study.

Vitamin K is found in dark green leafy vegetables (broccoli, cabbage, kale, lettuce, spinach, and turnip greens), cheese, liver, cereals, and fruits, but most of what you need comes from resident colonies of friendly bacteria in your intestines, an assembly line of busy bugs churning out the vitamin day and night.