Vitamin E, vitamin A and beta-carotene may not be the life-prolonging, antioxidant wonders they were once believed to be, suggests new research.

A study review conducted by researchers at the Copenhagen University Hospital in Denmark found that these antioxidants actually appeared to increase mortality. The scientists examined the data of 67 randomized clinical trials with 232,550 participants.

The doses of the supplements were as follows:

  • Beta carotene: 1.2 to 50 mg, with a mean dose of 18 mg.
  • Vitamin A: 1,333 to 200,000 IU, with mean dose of 20,219 IU.
  • Vitamin C: 60 to 2,000 mg, with a mean dose of 497 mg.
  • Vitamin E: 10 to 5,000 IU, with a mean dose of 570 IU.
  • Selenium: 20 to 200 ug, with a mean dose of 99 ug.

Beta-carotene was tested in 24 trials, vitamin A in 16 trials, vitamin C in 33, vitamin E in 54 and selenium in 21 trials.

Researchers found that 17,880 of 136,023 study participants who took the supplements died (13.1 per cent), compared to 10,136 of 96,527 participants who did not take the supplements (10.5 per cent). They did not track causes of death.

The antioxidants also appeared to offer no benefit to people with gastrointestinal, heart, neurological, eye, skin, rheumatoid, kidney and endocrine diseases, according to the authors.

Vitamin C also showed no health benefit, though in some of the studies, selenium did seem to reduce mortality.

The authors believe that although low levels of antioxidants consumed in fruits and vegetables may be beneficial for health, they may act as "double-edged" swords, adversely affecting "key physiological processes."

In the study, they link vitamin A to an increase in oxidative damage, while beta-carotene "may act as a co-carcinogen" or cancer-causing agent.

"The current evidence does not support the use of antioxidant supplements in the general population or in patients with certain diseases," reads the study.

However, the researchers suggest more study be done on Vitamin C and selenium. "Future trials could focus on Vitamin C and selenium and should assess both potential beneficial and harmful effects," reads the study.

The study was published online Wednesday in the Cochrane Review.