According to a study published in The American Journal of Clinical Nutrition, people who take dietary supplements also tend to get more nutrients from their food than those who don't take supplements -- suggesting that vitamins may be taken by the people who need them least.

In some cases, supplement users may actually be overloading on minerals, such as iron, that can cause potentially serious health problems, researchers said.

"People need to choose supplements to help meet, but not exceed, the recommended daily intake levels," said Regan Bailey, a nutrition research at the National Institutes of Health, who led the study.

Bailey and her colleagues used dietary surveys to assess mineral intake among 8,860 men and women who participated in a major government health survey between 2003 and 2006.

Men and women who reported using dietary supplements containing eight important minerals -- calcium, iron, magnesium, zinc, phosphorus, copper, potassium and selenium -- were much less likely to be getting inadequate amounts of those minerals from the foods they ate than were people who said they didn't take supplements, the study found.

The link was strongest for women, who are more likely than men to take supplements.

Supplement users, in turn, tended to eat better and live healthier lifestyles than non-users, Bailey noted.

The NIH team also found that calcium intake often fell below recommended levels, even among professed supplement users.

Roughly a quarter of supplement users, and 71 percent of nonusers, did not receive the recommended daily amount of calcium -- 800 to 1,000 milligrams a day for men over age 51 and 1,000 to 1,200 mg per day for women of the same age. Calcium is necessary for the healthy formation of bone.

Older people were much more likely to fall short of their daily calcium requirements -- but also to exceed them.

That's because people tend to use more supplements as they age, which helps explain why nearly 16 per cent of women between the ages of 51 and 70 reported daily calcium intakes that exceeded the recommended upper limit. Too much supplemental calcium has been linked to kidney stones.

Supplement users were also more likely to boost their intake of magnesium and zinc above recommended upper limits, although the health consequences, if any, of consuming too much of those minerals are unknown.