WEDNESDAY, March 4 (HealthDay News) -- Many middle-age and older Americans aren't getting enough micronutrients such as calcium, magnesium, potassium and vitamin C, all of which play an important role in maintaining health, a new study finds.

Researchers analyzed data from more than 6,200 people in four ethnic groups -- white, black, Hispanic and Chinese. More than half of the participants took supplements, and those most likely to use supplements were older, women, white and college-educated. The most common supplements were calcium and vitamin C.

Dietary intake of calcium, magnesium, potassium and vitamin C was similar between those who took supplements and those who didn't, but there were differences in median dietary intake levels between the different ethnic groups. Chinese Americans tended to have the lowest dietary intakes of all micronutrients, particularly calcium. Blacks also had significantly lower dietary intakes of calcium than whites and Hispanics.

The researchers also evaluated differences between high-dose supplements and multivitamins. High-dose calcium was associated with users meeting recommended daily allowances (RDA) or adequate intake (AI) for all ethnic groups.

However, some high-dose supplements could cause users to exceed their Tolerable Upper Intake Levels (ULs). For calcium, 15 percent of high-dose users exceeded the UL compared with 1.9 percent of multivitamin users and 2.1 percent of non-users. For magnesium, 35.3 percent of high-dose supplement users exceeded the UL, compared with zero percent of both multivitamin users and non-users. For vitamin C, 6.6 percent of high-dose users exceeded the UL, compared with zero percent of both multivitamin users and non-users.

The researchers also found that potassium intake was well below the RDA in both supplement users and non-users. This suggests it may be necessary to reformulate supplements to deliver higher potassium doses, they said.

"The present study indicates a clear association between meeting RDA/AIs and supplement use for calcium, magnesium and vitamin C," wrote Pamela J. Schreiner, professor and director of graduate studies in the division of epidemiology and community health at the University of Minnesota, and her colleagues.

"However, even with the assistance of dietary supplements, many middle-aged and older Americans are not getting adequate nutrition, and there was no association between supplement use and meeting the AI for potassium," the researchers concluded. "In addition, those taking high-dose vitamin supplements were more likely to exceed the UL for that nutrient. Future studies should explore dietary supplementation along with other methods to improve nutrition in middle-aged and older Americans."

The study was published in the March issue of the Journal of the American Dietetic Association.