Are vitamins good for you?
In natural form, they're essential to the proper functioning of our bodies. The term "vitamins" covers a diverse array of molecules that fulfill a huge variety of biochemical functions — helping human beings to grow, repair damaged tissue, and avoid such diseases as scurvy, rickets, and pellagra. In the modern world, the abundant supply of a wide variety of foods makes it possible to satisfy virtually all nutritional needs by eating a healthful, balanced diet rich in vegetables, fruit, and protein sources. But based on the idea that more of a good thing is better, companies are now selling Americans $12 billion worth of vitamins a year. Many scientists and doctors, however, question the value of gobbling vitamins — and there is evidence that large doses of some vitamins may actually be harmful. A recent long-term study of more than 400,000 people concluded that "most vitamin supplements [have] no clear benefit" and warned that excess vitamin E and beta-carotene may actually weaken the immune system's ability to kill cancer cells. "The case is closed," the study authors wrote. "Enough is enough."

Does that mean vitamin pills are useless?
There is still conflicting opinion about this. But most doctors and nutritionists agree that people who eat well do not need supplements — and that megadoses of vitamins are actually dangerous. If you eat a balanced diet, your kidneys will flush out any excess water-soluble vitamins (C and B complex), which is why some doctors joke that people who take vitamins have "expensive urine." Fat-soluble vitamins (A, D, E, K) can accumulate to the point of toxicity when taken in larger doses; one meta-analysis of 68 studies found that taking vitamins A and E "may increase mortality." The research indicates that there is an optimum level of any given vitamin in the body; more isn't better. Vitamin-makers nonetheless contend that most people need supplements.

What is their evidence?
It's that most Americans do not eat a balanced diet and are deficient in at least some vital nutrients. "You're not getting any of these micronutrients from Coke and Twinkies," said epidemiologist Gladys Block. A 2012 study of 14,000 men found that a daily multivitamin did reduce the risk of cancer by 8 percent. Still, most nutritionists maintain that good food is the best source of vitamins. "Food contains thousands of phyto-chemicals, fiber, and more that work together to promote good health that cannot be duplicated with a pill," said Karen Ansel, a nutritionist.

When did vitamin use take off?
Supplements have been around since the 1930s, but their popularity exploded with the 1970 publication of the best-selling Vitamin C and the Common Cold. Its author, the renowned chemist and Nobel laureate Linus Pauling, urged people to take 3 grams of vitamin C daily, or 50 times the recommended daily dose. He promised that taking gargantuan amounts of vitamin C would prevent not only colds but also cancer, and enable people to live to 150. Because of his prestige, people trusted him, and within a few years, 50 million Americans were taking his advice. At least 15 studies have shown that vitamin C doesn't prevent the common cold, but Pauling stubbornly denied the evidence. He went on to champion megavitamins — those containing doses many times greater than what's needed — and claimed they could treat everything from warts to AIDS. Today, more than half of American adults take vitamins.

Has the research affected sales?
Not so far. Aging Baby Boomers and Americans who hope to avoid doctors' bills keep sending sales ever upward. Critics say the unshaken faith in vitamin pills is the result of the placebo effect. "It's not surprising that people's beliefs aren't modified by scientific evidence," explains Paul Marantz, an epidemiologist at New York City's Yeshiva University. "People so want to believe there is a magic bullet out there." That timeless desire for a miracle pill has allowed the industry to flourish. "The thing to do with [these studies] is just ride them out," said Joseph Fortunato, CEO of General Nutrition Centers. "We see no impact on our business."

Is the industry regulated?
Barely. Under the Dietary Supplement Health and Education Act of 1994, all supplements — including minerals, medicinal herbs, and protein powders — were categorized as food rather than drugs. As a result, supplement manufacturers can sell their products without proving their safety, so long as their labels don't claim to prevent or treat disease. Critics see the law as a gift to the industry won by relentless lobbyists. "If vitamins were a regulated industry, megavitamins would have a black box warning on them," said public health specialist Dr. Paul Offit of the University of Pennsylvania. "We are the victims of an enormous marketing campaign."

The antioxidant paradox
As our cells metabolize oxygen to produce energy, they produce highly reactive molecular fragments called free radicals. These fragments, which are missing an electron, can assault cells and damage their DNA; over time, accumulated damage can play a role in causing heart disease, Alzheimer's, and cancer. Nature has provided an antidote to free radicals — antioxidants, substances that can donate electrons and neutralize damaged molecules. Fruit and vegetables provide a wide range of antioxidants, and studies show that people whose diet is rich in plants enjoy a strong protective effect. Based on the premise that taking more antioxidants is better, the vitamin industry began making and concentrating various kinds of antioxidants in pill form, including vitamin E, grape seed extract, and resveratrol. But research hasn't found any benefit to these pills. Why? It turns out that at a certain level, free radicals can actually aid the body by helping the immune system kill infections and cancer cells, and by triggering muscles and other parts of the body to grow stronger in response to exercise. "We are left with a delicate balancing act," say nutritionists Aidan Goggins and Glen Matten in their book, The Health Delusion. "Both too many and too few free radicals spell trouble."