NEW YORK (Reuters Health) - People seeking to improve their health with vitamin supplements may want to think twice before popping pills containing vitamin A, vitamin E, and beta-carotene. New research suggests that these antioxidants may actually increase the risk of death by 5 percent.

In contrast, the Danish researchers did not discern any significant influence of vitamin C and selenium on longevity, according to the report in the Journal of the American Medical Association.

Antioxidant supplements are popular among consumers, based on studies claiming that antioxidants improve health and prevent disease, Dr. Goran Bjelakovic and his colleagues note. But other reviews and guidelines suggest that antioxidant supplements may be of no benefit.

Bjelakovic and associates searched the medical literature through 2005 to identify trials involving adult subjects comparing beta-carotene, vitamin A, vitamin C, vitamin E, and selenium, singly or combined, versus inactive "placebo" or versus no treatment.

Their search turned up 68 trials with 232,606 participants.

When all trials were considered, antioxidant supplements had no effect on the risk of death. However, when the analysis was limited to only the highest quality studies, the supplements increased the risk of death by 5 percent.

Further analysis showed that only vitamin A, vitamin E, and beta carotene seemed to increase the risk of death, not vitamin C or selenium. The untoward effect of these antioxidants was noted whether they were taken separately or together.

It is estimated that up to 20 percent of adults in North America and Europe consume antioxidant supplements, so "the public health consequences may be substantial," Bjelakovic's team maintains.

The authors stress, however, that they evaluated only synthetic antioxidants, so their "findings should not be translated to potential effects of fruits and vegetables."