supplements taken by millions of people every day for their health
could be increasing their risk of death a new Danish-led study suggests.
The study is published in the Journal of the American Medical Association (read full study here).
The international research team reviewed the published evidence on beta carotene, vitamin A, vitamin E, Vitamin C and selenium. The team was led by Dr Goran Bjelakovic, from Copenhagen University Hospital, Denmark.
These dietary supplements are marketed as antioxidants and people take them in the hope they will improve health and guard against diseases like cancer and heart disease by eliminating the free radicals that cause "oxidative stress" and damage and kill off cells.
Antioxidants are also marketed as anti-aging products because they are thought to slow down the aging process.
Some studies have suggested that antioxidants are beneficial to health, while others, mostly larger clinical trials, have concluded they have no effect on health and say in some cases that taking too many of them can be harmful.
In this study, Dr Bjelakovic and colleagues did a meta-analysis on research published before October 2005.
Meta-analysis is a way of sifting through published studies against a quality standard that looks at a number of reliability criteria such as potential for bias. Then the ones that pass the standard are taken through a statistical process to find out if they are saying something consistently reliable. There can be several levels of "sifting", each producing a more reliable and robust set of evidence.
In this case the researchers followed a method established by the Cochrane Collaboration, a group of 6,000 health care specialists who review biomedical trials and other research projects.
They started with 815 clinical trials of which 68 passed the first level of quality standard. At this level the results were inconclusive. The supplements were found to have no effect on death risk one way or the other.
They then went back and eliminated 21 of the trials, leaving only the "low-bias" ones. This was the next level of quality standard.
At this level of meta-analysis the results were different.
When looked at separately they found that Vitamin A increased death risk by 16 per cent, beta carotene by 7 per cent and Vitamin E by 4 per cent. The results for Vitamin C were not so clear, but by looking at the best quality trials there was a suggestion that it increased death risk by 6 per cent, either on its own or in combination with other supplements.
The figures from the best quality trials on selenium however showed that it might reduce death risk by 10 per cent, either on its own or in combination with other supplements, but this was not found to be statistically significant.
The overall conclusion of the study was that on balance, the best quality research shows that beta carotene, vitamin A and vitamin E may increase mortality risk, but vitamin C and selenium need further study.
A spokesperson for the supplements industry said the research was "flawed" because it included studies conducted on people who were already very sick.
The researchers say there are several potential reasons for these results. One is that the free radicals that are thought to cause the oxidative stress are the byproduct rather than the cause of disease. Another is that they may play an important role in the immune system and eliminating them could be counterproductive.
The researchers pointed out that the studies they examined only used synthetic supplements, and therefore their observations and conclusions do not apply to natural antioxidants such as those found in fruit and vegetables.
They added however that this study is important for public health reasons because between 10 and 20 per cent of people in Europe and North America take dietary supplements.
Nutritionists say that instead of taking supplements the best way to protect your health is to eat a balanced diet and to get all the vitamins you need from your food.
"Mortality in Randomized Trials of Antioxidant Supplements for Primary and Secondary Prevention; Systematic Review and Meta-analysis."
Goran Bjelakovic, Dimitrinka Nikolova, Lise Lotte Gluud, Rosa G. Simonetti, and Christian Gluud.
Vol. 297 No. 8, February 28, 2007