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.

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. 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?

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.

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.

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. 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 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.

Water-Soluble Vitamins

Vitamin C and the entire roster of B vitamins (thiamin, riboflavin, niacin, vitamin B6, folate, biotin, and pantothenic acid) are usually grouped together simply because they all dissolve in water. The ability to dissolve in water is an important point, because that means large amounts of these nutrients can’t be stored in your body.

If you take in more than you need to perform specific bodily tasks, you will simply pee away virtually all the excess. The good news is that these vitamins rarely cause side effects. The bad news is that you have to take enough of these vitamins every day to protect yourself against deficiencies.

Vitamin C, which also is referred to as ascorbic acid, is essential for the development and maintenance of connective tissue (the fat, muscle, and bone framework of the human body). Vitamin C speeds the production of new cells in wound healing, protects your immune system, helps you fight off infection, reduces the severity of allergic reactions, and plays a role in the syntheses of hormones and other body chemicals.

Call it thiamin. Call it B1. Just don’t call it late for lunch (or any other meal). This sulfur (thia) and nitrogen (amin) compound, the first of the B vitamins to be isolated and identified, helps ensure a healthy appetite. It acts as a coenzyme (a substance that works along with other enzymes) essential to at least four different processes by which your body extracts energy from carbohydrates.

And thiamin also is a mild diuretic (something that makes you urinate more). Although thiamin is found in every body tissue, the highest concentrations are in your vital organs — heart, liver, and kidneys. The richest dietary sources of thiamin are unrefined cereals and grains, lean pork, beans, nuts, and seeds.

In the United States, refined flours, stripped of their thiamin, are a nutritional reality, so most Americans get most of their thiamin from breads and cereals enriched with additional B1.

Riboflavin (vitamin B2), the second B vitamin to be identified, was once called “vitamin G.” Its present name is derivative of its chemical structure, a carbon-hydrogen-oxygen skeleton that includes ribitol (a sugar) attached to a flavonoid (a substance from plants containing a pigment called flavone).

Like thiamin, riboflavin is a coenzyme. Without it, your body can’t digest and use proteins and carbohydrates. Like vitamin A, it protects the health of mucous membranes — the moist tissues that line the eyes, mouth, nose, throat, vagina, and rectum. You get riboflavin from foods of animal origin (meat, fish, poultry, eggs, and milk), whole or enriched grain products, brewer’s yeast, and dark green vegetables (like broccoli and spinach).

Niacin is one name for a pair of naturally occurring nutrients, nicotinic acid and nicotinamide. Niacin is essential for proper growth, and like other B vitamins, it’s intimately involved in enzyme reactions. In fact, it’s an integral part of an enzyme that enables oxygen to flow into body tissues.

Like thiamin, it gives you a healthy appetite and participates in the metabolism of sugars and fats. Niacin is available either as a preformed nutrient or via the conversion of the amino acid tryptophan. Preformed niacin comes from meat; tryptophan comes from milk and dairy foods.

Some niacin is present in grains, but your body can’t absorb it efficiently unless the grain has been treated with lime — the mineral, not the fruit. This is a common practice in Central American and South American countries, where lime is added to cornmeal in making tortillas.

In the United States, breads and cereals are routinely fortified with niacin. Your body easily absorbs the added niacin. The term used to describe the niacin RDA is NE (niacin equivalent): 60 milligrams tryptophan = 1 milligram niacin = 1 niacin equivalent (NE).

Vitamin B6 is another multiple compound, this one comprising three related chemicals: pyridoxine, pyridoxal, and pyridoxamine. Vitamin B6, a component of enzymes that metabolizes proteins and fats, is essential for getting energy and nutrients from food. It also helps lower blood levels of homocysteine, an amino acid produced when you digest proteins.

The American Heart Association calls a high level of homocysteine an independent (but not major) risk factor for heart disease, and the American Journal of Clinical Nutrition reported in 2005 that a high homocysteine level may be associated with an age-related decline in memory.

Alas, follow up studies show no reduction in the risk of heart disease or improvement in memory in those who reduce their blood levels of homocysteine.

The best food sources of vitamin B6 are liver, chicken, fish, pork, lamb, milk, eggs, unmilled rice, whole grains, soybeans, potatoes, beans, nuts, seeds, and dark green vegetables such as turnip greens. In the United States, bread and other products made with refined grains have added vitamin B6.

Folate, or folic acid, is an essential nutrient for human beings and other vertebrates (animals with backbones). Folate takes part in the synthesis of DNA, the metabolism of proteins, and the subsequent synthesis of amino acids used to produce new body cells and tissues.

Folate is vital for normal growth and wound healing. An adequate supply of the vitamin is essential for pregnant women to enable them to create new maternal tissue as well as fetal tissue.

In addition, an adequate supply of folate dramatically reduces the risk of spinal cord birth defects. Beans, dark green leafy vegetables, liver, yeast, and various fruits are excellent food sources of folate, and all multivitamin supplements must now provide 400 mcg of folate per dose.

Vitamin B12 (cyanocobalamin) makes healthy red blood cells. Vitamin B12 protects myelin, the fatty material that covers your nerves and enables you to transmit electrical impulses (messages) between nerve cells. These messages make it possible for you to see, hear, think, move, and do all the things a healthy body does each day.

In 2005, the Canadian Medical Association Journal reported that low blood levels of B12 in older people are linked to higher levels of homocysteine (a minor risk factor for heart disease; see previous, “Vitamin B6 [Pyridoxine]”). Vitamin B12 is unique. First, it’s the only vitamin that contains a mineral, cobalt.

Second, it’s a vitamin that can’t be made by higher plants (the ones that give us fruits and vegetables). Like vitamin K, vitamin B12 is made by beneficial bacteria living in your small intestine. Meat, fish, poultry, milk products, and eggs are good sources of vitamin B12. Grains don’t naturally contain vitamin B12, but like other B vitamins, it’s added to grain products in the United States.

Biotin is a B-vitamin, a component of enzymes that ferry carbon and oxygen atoms between cells. Biotin helps you metabolize fats and carbohydrates and is essential for synthesizing fatty acids and amino acids needed for healthy growth. And it seems to prevent a buildup of fat deposits that may interfere with the proper functioning of liver and kidneys.

The best food sources of biotin are liver, egg yolk, yeast, nuts, and beans. If your diet doesn’t give you all the biotin you need, bacteria in your gut will synthesize enough to make up the difference. No RDA exists for biotin, but the Food and Nutrition Board has established an Adequate Intake (AI), which means a safe and effective daily dose.

Pantothenic acid, another B-vitamin, is vital to enzyme reactions that enable you to use carbohydrates and create steroid biochemicals such as hormones. Pantothenic acid also helps stabilize blood sugar levels, defends against infection, and protects hemoglobin, as well as nerve, brain, and muscle tissue.

You get pantothenic acid from meat, fish and poultry, beans, whole grain cereals, and fortified grain products. As with biotin, the Food and Nutrition Board has established an Adequate Intake (AI) for pantothenic acid.

Choline is not a vitamin, a mineral, a protein, a carbohydrate, or a fat, but it’s usually lumped in with the B-vitamins, so heeeeere’s choline! In 1998, 138 years after this nutrient first was identified, the Institute of Medicine (IOM) finally declared it essential for human beings. The IOM had good reasons for doing so.

Choline keeps body cells healthy. It’s used to make acetylcholine, a chemical that enables brain cells to exchange messages. It protects the heart and lowers the risk of liver cancer.

And new research at the University of North Carolina (Chapel Hill) shows that choline plays a role in developing and maintaining the ability to think and remember, at least among rat pups and other beasties born to lab animals that were given choline supplements while pregnant. Follow-up studies showed that prenatal choline supplements helped the animals grow bigger brain cells.

True, no one knows whether this would also be true for human pups, er, babies, but some researchers advise pregnant women to eat a varied diet, because getting choline from basic stuff like eggs, meat, and milk is so easy. IOM’s Food and Nutrition Board, the group that sets the RDAs, has established an AI (Adequate Intake) for choline.