People who take dietary supplements to boost their intake of minerals tend also to get more nutrients from their food than those who don't take supplements, according to a new study that suggests vitamins are often taken by the people who need them least.

Indeed, in some cases, supplement users may be getting too much of a good thing by 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 of nutrients, Regan Bailey, a nutrition researcher at the National Institutes of Health and leader of the study, told Reuters Health.

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 researchers found.

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

Supplement users, in turn, tend to eat better and live healthier lifestyles than nonusers, Bailey noted.

The NIH team also found that calcium intake often fell below recommended levels, even among professed supplement users, the researchers reported in The American Journal of Clinical Nutrition.

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 per 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 -- and also to exceed them. That's because people tend to use more supplements as they age, the researchers said, which helps explain why nearly 16 percent 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 also were 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.

Cheryl Rock, a nutrition researcher at the University of California, San Diego, said the results are not surprising in light of previous research, including her own, into the dietary habits of supplement users. We always would hope that the people who are taking dietary supplements are the ones who need it the most, but it doesn't seem to be true, Rock said.

When it comes to overconsumption, Rock added, we have been telling people clinically for years that the daily value cut point is not your minimum requirement. Having a dietary assessment is definitely a good idea to determine where one's nutrient intake might be inadequate, if at all, she said.

The National Institutes of Health provides information on nutritional supplements here:

Source: The American Journal of Clinical Nutrition, online September 28, 2011.