New research is prompting a fresh look at the value of vitamin supplements, with some surprising results indicating that taking too many supplements of some could be harmful.
The research is forcing scientists to rethink the use of supplements with antioxidants, which had been seen as beneficial in preventing cancer, heart disease and other ailments.
"Everybody is confused," admitted Toren Finkel, head of the Center for Molecular Medicine at the National Institutes of Health (NIH).
While logic would seem to dictate that taking vitamins and antioxidants should help fight illness and disease, Finkel said in an interview that the clinical data "are pretty consistently showing no benefit."
"So that means we have to go back and think about some of the assumptions we have made along the way in terms of what the mechanism for these diseases are and how things like oxidants play a role in those diseases," he told AFP.
Finkel explained that while it has long been believed that oxidants -- free radicals produced by the body or introduced through external sources such as pollution -- were unhealthy, the research paints a more complex picture.
"You have to go back to the lab and try to design experiments you can do a little simpler with cells or animals... to better understand the role of oxidants and vitamins," he said.
"For years, we were using these supplements without knowing the effects on the body."
A study published on October 11 in the United States indicates a 17 percent increase in the risk of developing prostate cancer among men who take high doses of vitamin E.
Another recent US study conducted among women and published on October 10 revealed that multivitamins -- commonly taken in the United States -- were useless and actually gradually contributed to a higher risk of mortality.
As far back as 2007, researchers had established a link between taking selenium supplements and an increased risk of adult diabetes.
For David Schardt, a nutritionist at the non-profit Center for Science in the Public Interest, the problem is that "people think more is better and that (supplements) are harmless."
"We are finding out that some of these vitamins taken alone in large amounts may have effects we did not predict, we did not understand and we did not anticipate," Schardt said.
He added that the country has "a lot of people who believe, almost like a religion, in their vitamins" -- a faith encouraged by an industry that generates $20 billion a year in the United States, where more than half of the population takes some kind of vitamin supplement.
Moreover, US manufacturers are "free to say almost anything they want" about the virtues of supplements, Schardt explained, adding: "The only thing the Food and Drug Administration does not let them make are claims about disease."
Patsy Brannon, a Cornell University professor who served on an NIH panel on multivitamins, mineral supplements and chronic diseases, said that those who take supplements are often those already getting nutrients from their food.
"So the people who are choosing supplements are maybe the people who don't need them," she said. "So they are not thinking about their total intake and that I think is a concern."
Experts agree that multivitamins and other supplements are useful for certain groups such as pregnant women and elderly people suffering from vitamin deficiencies.
But for the general population, a healthy balanced diet rich in fruits, vegetables and fiber as well as animal-based protein will provide the proper vitamins and nutrients needed, Brannon said.
A study published in late August in the Journal of Nutrition showed that a significant number of Americans did not eat well and did not take vitamin supplements.
Twenty-five percent of respondents had a vitamin C deficiency, 34 percent were not getting enough vitamin A and 60 percent needed more vitamin E. All three are found in fruits and vegetables.
Explore further: Worried about vitamin safety? Experts offer advice