The case for dietary supplements is crumbling.
A series of human studies on a large scale, including two that appeared earlier this month in major medical journals, suggests that multivitamins and many other dietary supplements do not contribute to health, and in some cases may even cause harm.
The data have led some researchers to argue that taking vitamins is a waste of money for those who do not have a specific nutritional deficiency or chronic illness. Such results have also stoked the debate over whether it should continue to undertake costly human testing to see if certain supplements can affect health.
Several studies have concluded that vitamin C reduces the risk of catching a cold.
"The better the quality of research, the lower the profit showing (supplements)," says Marion Nestle, professor of nutrition, food studies and public health at the University of New York. "It's fair to say from the research that supplements do not make healthy people more healthy."
For example, vitamins B-6 and B-12 are often recommended as good for the heart, but several studies have concluded that reduce the risk of cardiovascular disease, according to the Office of Dietary Supplements, part of the National Institutes of Health (NIH, for its acronym in English) of the United States. The same goes for vitamin C and its supposed quality to reduce the risk that a person will contract a cold. Calcium, while important for bone health, it reduces the risk of heart disease or cancer, and may increase the risk of kidney stones.
"We have a huge volume of data that tells us that diets rich in plants are very healthy," says Josephine Briggs, head of the National Center for Complementary and Alternative Medicine, one NIH center. "As soon as we get these different antioxidants (and other nutrients) and put them in a pill, do not get a benefit on an ongoing basis."
Researchers and nutritionists still recommend dietary supplements for people with malnutrition or certain diseases or nutritional deficiencies. For example, folic acid reduces the likelihood of a common birth defect if taken by pregnant women.
Studying the effects of vitamins and supplements is difficult, because people eat food containing multiple nutrients supplements can interact with and skew the results. And observational evidence can only show association, not cause and effect.
That's one reason why the Council on Responsible Nutrition, which represents supplement companies, says it's premature to say that supplements do not have health benefits. Duffy MacKay, the deputy director of regulatory and scientific affairs of the group, says that studies may be needed longer to show the benefits of some supplements.
Micronutrients, including antioxidants such as vitamin C, vitamin D hormones and metals such as iron, are essential to the body in small amounts because they help to facilitate reactions in cells and between them. Too many micronutrients, however, can cause problems.
A topic of growing concern to many scientists are increasing signs of damage vitamins.
Roberta Anding, a nutritionist at Texas Children's Hospital in Houston, says some people need multivitamins and other supplements, but warns about the risk of high doses of micronutrients. "It's not nutrition when doses become high, it's pharmacology," he says.
The best way to get micronutrients is through a balanced diet, he says.
"If you think, 'At least I'm (taking a multivitamin') but is not exercising or eating well, then it is a waste of money," says Anding.