Peter Barry isn't a doctor, a nutritionist or even an herbal healer, but customers constantly stroll into Baltimore's OK Natural Food Store seeking his advice on vitamins.

What's he got for hypertension? For energy? And the one everyone whispers: sexual enhancement?

A natural foods salesman for 22 years, Barry takes a few supplements of his own, but he thinks consumers are desperate for a magic pill. Standing before a giant wall of bottles bearing such labels as coral calcium, B-50 and Change-O-Life Blend, he offers his standard recommendation: "grandma's cooking."

"I see supplements as something we should work not to take," he said. "We should be concentrating on eating more local, seasonal, whole foods that provide the nutrients we need."

Nearly 40 percent of American adults regularly take a vitamin or mineral supplement, according to the Centers for Disease Control and Prevention, keeping the vitamin industry booming. U.S. sales of supplements totaled about $9.7 million in 2007 alone, according to the Nutrition Business Journal, which keeps tabs on the industry.

But while there is a dizzying variety of products on the market, evidence of their benefits remains murky. While many nutritionists have long promoted vitamins as nutrition's insurance policy, some doctors and researchers say not to bother even with the once-hailed one-a-day multivitamin.

"There's not enough evidence to tell people who are taking them to stop," said Mary Frances Picciano, Ph.D., a senior nutrition research scientist in the Office of Dietary Supplements at the National Institutes of Health. "And there isn't enough evidence to tell people who aren't taking them to start." Her agency concluded in 2006 that without more extensive clinical trials, there was little evidence to support the health benefits of multivitamins.

Still, many experts agree that certain vitamins benefit various groups. Calcium combined with vitamin D is often recommended for post-menopausal women to help prevent bone loss. It's recommended for older men, too. Extra iron can be helpful for women of childbearing age, who are at higher risk of iron deficiency than older people. And folic acid is universally recommended for women who may become pregnant to prevent birth defects.

There is sharp disagreement, though, about many other supplements, including such popular remedies as vitamins E and D.

Nearly everyone agrees on this much: People should get as many nutrients as possible through a healthy diet. Beyond that, they should read as much as they can and consult their doctor. Advice on whether to take vitamins or what kind can vary by gender, age and a person's medical condition.

But it can be difficult to decipher the pros and cons of vitamins-of-the-moment, where new research challenging the benefit of a nutrient can refute long-held beliefs.

It happened in 2005 with vitamin E. Praised since the 1960s for near-miracle properties such as helping ward off cardiovascular disease and cancer, vitamin E's cachet was lost when a study by Dr. Edgar R. Miller III showed high doses (more than 400 international units) increased the risk of death among the sick and elderly.

Miller, an associate professor of medicine and epidemiology at the Johns Hopkins School of Medicine, said the vitamin E story proves that more rigorous research is necessary for many supplements on the market. The foundation of many vitamin E fans' support came from studies that surveyed health-conscious people, a demographic that likely skewed results, he said. The most effective way to evaluate if a supplement is effective is with clinical trials that offer placebos to some subjects and vitamins to others, he said.

"Vitamin E can be sold and promoted without showing evidence," he said. "Companies don't have to show efficacy. It is an unregulated industry and they don't have to back up their research."

Miller urges caution for most supplements, with a few exceptions: Pregnant women should take folic acid to reduce the possibility of neural tube defects, and many people can benefit from Omega-3 fatty acids, which he says can decrease the risk of heart disease.

As for the rest of us: "People would do better by exercising and spending their money on fruits and vegetables."

Dr. Meir Stampfer, professor of nutrition and epidemiology at the Harvard School of Public Health, disagrees with much of what Miller has to say. He wasn't convinced by Miller's findings about the ills of vitamin E. With the average American diet inadequate in many nutrients, Stampfer recommends a multivitamin, takes one himself and does not shun large doses of vitamin E.

"By taking a multivitamin, you have just knocked that whole issue off the table," he said. "Vitamin B-6 is one that is often inadequate. The average person doesn't know where vitamin B-6 comes from - even some experts don't know where it comes from. The point is, why should people have to worry about that? They've got enough to think about when it comes to nutrition."

Stampfer also takes a supplement of vitamin D, the so-called "sunshine vitamin" and the subject of the latest debate among researchers. Vitamin D, which is made when the skin is exposed to sunlight, has long been seen as essential in building strong bones. Recent findings suggest it could also play a role in reducing heart disease, preeclampsia in pregnant women and a variety of cancers.

In addition, pediatric researchers think vitamin D deficiency may be more common than they realize, especially in breast-fed infants, since breast milk contains low levels of the nutrient. The American Academy of Pediatrics is considering increasing its recommended vitamin D supplementation in infants from 200 IU to 400 IU after a recent resurfacing of rickets, a softening of the bones that takes place when children do not get enough vitamin D and a condition that was thought to be wiped out with the fortification of milk.

But Picciano points out there is no evidence that supplemental vitamin D is beneficial for adults. "We need many more rigorous studies before decisions can be drawn," she said. "The horizon looks promising, but we may get there and see a storm. We don't know what we may find."

Stampfer disagrees and says such studies can take decades. For now, he thinks, the evidence is clear that a vitamin D supplement is beneficial, especially to those like him who live in northern climates with limited sun exposure in the winter. He takes a daily multivitamin with 1,000 IUs of vitamin E as well as a vitamin E supplement.

"I figure the risk is about zero, the cost is very low, and the potential for benefit is substantial," he said. "You could either wait for the perfect data, or you make the best judgment you can, recognizing it's imperfect."

Of course, it's possible to get too much of a good thing. Excessive intake of such vitamins as beta-carotene and calcium have been found to be harmful. Trials have shown too much beta carotene, once thought to help lung health, can be toxic, while excessive calcium can lead to elevated levels in the blood and cause kidney problems.

And even helpful vitamins are not a substitute for healthy living, Stampfer said. "I think it's a small part of a healthy lifestyle."

With so many products on the market, Jennifer Brewington is not sure if vitamins do any good. Brewington, 55, keeps the books for her husband Rob's software consulting business in Howard County. He is convinced that a multivitamin has helped keep him relatively healthy since he started taking one eight years ago, when robust business meant working grueling hours, skipping meals and getting little sleep.

When his go-to multivitamin was recently discontinued, Jennifer Brewington scoured the Internet to help him find a replacement. But she wasn't interested in finding one for herself.

"I'm still confused," she said. "It's hard for a consumer like me to sift through and understand or even believe it all."

Rob Brewington, 59, admits he doesn't feel different, but he's certain the vitamins are doing their job.

"I could never say it was something noticeable," he said. "But mentally, I felt better."

the lowdown on common vitamins and mineral supplements

Multivitamin

The most common supplement taken nationwide, one-a-day multivitamins are used not only for nutritional supplementation but for prevention and treatment of disease. While many nutritionists say they can help round out a person's diet, health experts in an Office of Dietary Supplements report concluded more research is needed to determine whether they are truly effective.

Calcium

Calcium, the most abundant mineral in the body, is essential for bone health. Post-menopausal women, vegetarians and people who are lactose intolerant are most at risk for deficiencies. But too much calcium is associated with kidney problems.

Vitamin D

The "sunshine vitamin," long known for aiding the absorption of calcium and the promotion of strong bones, is the nutrient du jour. New studies say increased vitamin D may reduce the risk of heart disease, cancer and diabetes. Researchers are concerned that recommended levels are not sufficient because vitamin D is found in only a handful of foods and added to milk. But others warn more studies are necessary.

Folic Acid

Fortified in bread, pasta and flour, folic acid is a member of the B vitamin family and a synthetic form of the nutrient folate. It's universally accepted by health experts as a must for women who may become pregnant to prevent birth defects. New research, however, cautions that too much might accelerate development of precancerous growths.

Vitamin E

Found in nuts, green leafy vegetables and fortified foods, vitamin E is an antioxidant that protects cells. It was once hailed as a wonder nutrient that could help prevent cardiovascular disease and cancer. Studies several years ago warned that excessive vitamin E could cause increased risk of death in certain populations.

Omega-3 Fatty Acids

Known as fish oils, these nutrients are found in fish, organ meat, green leafy vegetables and vegetable oils. Many health experts believe American diets are too low in Omega-3 fatty acids, which are important for brain development and can help prevent heart disease. Still, some experts question whether supplementation is necessary and call for more studies on its effect on specific diseases.

Source: the Office of Dietary Supplements at the National Institutes of Health