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Certain nutritional supplements may be of no benefit to life expectancy, and in some cases, may even raise the risk of premature death, a study review claimed on Wednesday.

Researchers in Denmark analyzed data from 67 clinical trials, totaling more than 200,000 participants. They found that antioxidant supplements did not reduce the subjects' risk of death.

Overall, 17,880 of 136,023 participants (13.1 per cent) who took supplements died, while 10,136 of 96,527 participants (10.5 per cent) taking a placebo died.

The report included data from clinical trials on beta-carotene, vitamin A, vitamin C, vitamin E and selenium supplements.

The report's authors found:

• Vitamin A was linked to a 14 per cent greater risk of death;
• Beta-carotene was linked to an increased risk of 7 per cent; and
• Vitamin E was linked to a 4 per cent increased risk.

The review was conducted by the Cochrane Collection and published in The Cochrane Library.

Vitamin C and selenium were not associated with any detrimental health effects.

The findings suggest that high doses of some vitamins may interfere with the body's natural defence mechanisms.

One of the review's authors, Dr. Christian Gluud of Copenhagen University Hospital, admitted that the information seems to call into question the supposed health benefits of taking antioxidants.

"For some people this will come as a shock and a surprise," Gluud said.

The researchers found no difference between the effects of antioxidant supplements in healthy subjects versus those who had health problems.

In a summary, the review's authors wrote: "The current evidence does not support the use of antioxidant supplements in the general population, or in patients with certain diseases.

The combined evidence suggests that additional research on antioxidant supplements is needed."

The findings trouble some who speak for the vitamin industry. Penelope Merritt of the Canadian Health Food Association says that the authors excluded hundreds more studies on antioxidants and supplements in their research.

"We're concerned consumers will start to question the validity of taking vitamins and supplements," Merritt said.

"And studies have shown there is value in taking supplements."

Merritt pointed out that it's not easy for everyone to eat balanced meals all the time, and supplements can help fill in dietary holes.

The researchers also noted that this review does not assess the role that antioxidant supplements may play in the treatment of disease. It also doesn't analyze the impact of antioxidants contained in foods.

Click for more information on Health Canada's recommended daily vitamin doses.

Abstract:

Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases (Review)

Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C

Background: Animal and physiological research as well as observational studies suggest that antioxidant supplements may improve survival.

Objectives: To assess the effect of antioxidant supplements on mortality in primary or secondary prevention randomised clinical trials.

Search strategy: We searched The Cochrane Library (Issue 3, 2005), MEDLINE (1966 to October 2005), EMBASE (1985 to October 2005), and the ScienceCitation IndexExpanded (1945 toOctober 2005).We scanned bibliographies of relevant publications andwrote to pharmaceutical companies for additional trials.

Selection criteria: We included all primary and secondary prevention randomised clinical trials on antioxidant supplements (beta-carotene, vitamin A, vitamin C, vitamin E, and selenium) versus placebo or no intervention. Included participants were either healthy (primary prevention trials) or had any disease (secondary prevention trials).

Data collection and analysis: Three authors extracted data. Trials with adequate randomisation, blinding, and follow-up were classified as having a low risk of bias. Random-effects and fixed-effect meta-analyses were performed. Random-effects meta-regression analyses were performed to assess sources of intertrial heterogeneity.

Main results: Sixty-seven randomised trials with 232,550 participants were included. Forty-seven trials including 180,938 participants had low risk of bias. Twenty-one trials included 164,439 healthy participants. Forty-six trials included 68111 participants with various diseases (gastrointestinal, cardiovascular, neurological, ocular, dermatological, rheumatoid, renal, endocrinological, or unspecified). Overall, the antioxidant supplements had no significant effect on mortality in a random-effects meta-analysis (relative risk [RR] 1.02, 95% confidence interval [CI] 0.99 to 1.06), but significantly increased mortality in a fixed effect model (RR 1.04, 95% CI 1.02 to 1.06). In meta-regression analysis, the risk of bias and type of antioxidant supplement were the only significant predictors of intertribal heterogeneity. In the trials with a low risk of bias, the antioxidant supplements significantly increased mortality (RR 1.05, 95% CI 1.02 to 1.08). When the different antioxidants were assessed separately, analyses including trials with a low risk of bias and excluding selenium trials found significantly increased mortality by vitamin A (RR 1.16, 95% CI 1.10 to 1.24), beta-carotene (RR 1.07, 95% CI 1.02 to 1.11), and vitamin E (RR 1.04, 95% CI 1.01 to 1.07), but no significant detrimental effect of vitamin C (RR 1.06, 95% CI 0.94 to 1.20). Low-bias risk trials on selenium found no significant effect on mortality (RR 0.91, 95% CI 0.76 to 1.09).

Authors' conclusions: We found no evidence to support antioxidant supplements for primary or secondary prevention. Vitamin A, beta-carotene, and vitamin E may increase mortality. Future randomised trials could evaluate the potential effects of vitamin C and selenium for primary and secondary prevention. Such trials should be closely monitored for potential harmful effects. Antioxidant supplements need to be considered medicinal products and should undergo sufficient evaluation before marketing.