The vitamin pill you may pop each morning is a dietary supplement. So are the calcium antacids many American women consider standard nutrition. Echinacea, the herb reputed to short-circuit your winter cold, is, and so is the vanilla-flavored meal-in-a-can liquid your granny chug-a-lugs every afternoon just before setting off on her daily mile power walk.

The FDA classifies each of these as a dietary supplement because they meet the agency’s definition: any pill, tablet, capsule, powder, or liquid you take by mouth that contains a dietary ingredient. Of course, that raises another question: What’s a dietary ingredient? Answer:

• Vitamins
• Minerals
• Herbs
• Amino acids
• Enzymes
• Organ tissue, such as desiccated (dried) liver
• Some hormones, such as melatonin, which is promoted as a sleep aid
• Metabolites (substances produced when nutrients are digested)
• Extracts

Dietary supplements may be single-ingredient products, such as vitamin E capsules, or they may be combination products, such as the nutrient-packed protein powders favored by some athletes.

In a country where food is plentiful and affordable, you have to wonder why so many people opt to scarf down pills instead of just plain food. Many people consider vitamin and mineral supplements a quick and easy way to get nutrients without so much shopping and kitchen time and without all the pesky fat and sugars in food. Others take supplements as nutritional insurance.

And some even use supplements as substitutes for medical drugs. In general, nutrition experts, including the American Dietetic Association, the National Academy of Sciences, and the National Research Council, prefer that you invest your time and money whipping up meals and snacks that supply the nutrients you need in a balanced, tasty diet.

Nonetheless, every expert worth his or her vitamin C admits that in certain circumstances, supplements can be a definite plus. In 2002, the American Medical Association (AMA), which for decades had turned thumbs down on vitamin supplements, changed its collective mind after a review of 26 years’ worth of scientific studies relating vitamin levels to the risk of chronic illness.

Robert H. Fletcher and Kathleen M. Fairfield, the Harvard-based authors of the study, which was published in the Journal of the American Medical Association (JAMA), said, yes, true vitamin-deficiency diseases such as scurvy and beriberi are rare in Western countries.

But suboptimal vitamin levels — sciencespeak for slightly less than you need — are a real problem. If “slightly less than you need” sounds slightly less than important, consider this:

• Suboptimal intake of folate and two other B vitamins (B6 and B12) raises your risk of heart disease, colon cancer, breast cancer, and birth defects.
• Suboptimal vitamin D intake means a higher risk of rickets and osteoporosis.
• Suboptimal levels of antioxidant vitamins A, E, and C are linked to a particular form of heart disease and some forms of cancer.

Hence the new AMA rule: “It’s prudent for all adults to take vitamin supplements.” But just as that bit of info was settling in, a new study said, “Hold it! There’s too much Vitamin A in that pill!” That was followed by more new research on too much vitamin E. As you read this, vitamin manufacturers are tumbling over each other in the race to get new, lower formulations to market.

Illness, age, diet preferences, and some gender-related conditions may put you in a spot where you can’t get all the nutrients you need from food alone. Certain metabolic disorders and diseases of the digestive organs (liver, gallbladder, pancreas, and intestines) interfere with the normal digestion of food and the absorption of nutrients.

Some medicines may also interfere with normal digestion, meaning you need supplements to make up the difference. People who suffer from certain chronic diseases, who have suffered a major injury (such as a serious burn), or who have just been through surgery may need more nutrients than they can get from food.

In these cases, a doctor may prescribe supplements to provide the hard-to-get vitamins, minerals, and other nutrients. Checking with your doctor or pharmacist before opting for a supplement you hope will have medical effects (make you stronger, smooth your skin, ease your anxiety) is a smart idea.

The bad old days when doctors were total ignoramuses about nutrition may not be gone forever, but they’re fading fast. Besides, your doctor is the person most familiar with your health, knows what medications you’re taking, and can warn you of potential side effects.

Vitamin B12 is found only in food from animals, such as meat, milk, and eggs. (Some seaweed does have B12, but the suspicion is that the vitamin comes from microorganisms living in the plant.) Without these foods, vegans — vegetarians who don’t eat any foods of animal origin — almost certainly have to get their vitamin B12 from supplements or fortified foods.

Healthy people who eat a nutritious diet still may want to use supplements to make sure they’re getting adequate nutrition. Plenty of recent research supports their choice. Taking supplements may reduce the likelihood of some types of cancer and other diseases.

After analyzing data from a survey of 871 men and women, epidemiologists at Seattle’s Fred Hutchinson Cancer Center found that people taking a daily multivitamin for more than ten years were 50 percent less likely to develop colon cancer. In addition, selenium supplements seem to reduce the risk of prostate cancer, and vitamin C seems to lower the risk of cataracts.

As you grow older, your appetite may decline and your sense of taste and smell may falter. If food no longer tastes as good as it once did, if you have to eat alone all the time and don’t enjoy cooking for one, or if dentures make chewing difficult, you may not be taking in all the foods that you need to get the nutrients you require.

Dietary supplements to the rescue! If you’re so rushed that you literally never get to eat a full, balanced meal, you may benefit from supplements regardless of your age. And what about women? At various stages of their reproductive lives, they, too, benefit from supplements-as-insurance:

• Before menopause: Women, who lose iron each month through menstrual bleeding, rarely get sufficient amounts of iron from a typical American diet providing fewer than 2,000 calories a day. For them, and for women who are often on a diet to lose weight, iron supplements may be the only practical answer.

Iron is a mineral element, so it may be called “iron” or “elemental iron” on the label. Iron pills contain a compound of elemental iron (“ferrous” or “ferric,” from ferrum, the Latin word for iron), plus an ingredient such as a sulfur derivative or lactic acid to enable your body to use the iron.

On the label, the combination reads “ferrous sulfate” or “ferrous lactate.” Different iron compounds dissolve at different rates in your stomach, yielding different amounts of elemental iron, so supplement labels usually list the iron this way: Ferrous sulfate 325 mg/Elemental iron 65 mg.

Translation? This pill has 325 milligrams of ferrous sulfate, yielding 65 milligrams plain old iron. Sometimes the label omits the first part and simply says: Iron 65 mg. If your doctor says, “Take one 325-milligram pill a day,” she means 325 milligrams iron compound, not plain elemental iron.

• During pregnancy and lactation: Women who are pregnant or nursing often need supplements to provide the nutrients they need to build new maternal and fetal tissue or to produce nutritious breast milk. In addition, supplements of the B vitamin folate now are known to decrease a woman’s risk of giving birth to a child with a neural tube defect (a defect of the spinal cord and column).

Never self-prescribe supplements while you’re pregnant. Large amounts of some nutrients may actually be hazardous for your baby. For example, taking megadoses of vitamin A while you’re pregnant can increase the risk of birth defects.

• Through adulthood: True, women older than 19 can get the calcium they require (1,000 milligrams/day) from four 8-ounce glasses of nonfat skim milk a day, three 8-ounce containers of yogurt made with nonfat milk, 22 ounces of canned salmon (with the soft edible bones; no, you definitely should not eat the hard bones in fresh salmon!), or any combination of the above.

However, expecting women to do this nutritional balancing act every single day may be unrealistic. The simple alternative is calcium supplements.