Even after adjusting for smoking, intake of carotene (alpha- and beta-carotene were measured together) was directly associated with lower risk of death from cancer, said the researchers from Wageningen University in the Netherlands, supported by Swiss and French colleagues.
The findings were surprising as both supplemental trials and observational studies have found the carotenoid to raise the risk of cancer in smokers.
In addition, findings from large-scale supplemental trials "make it very unlikely that pharmacologic doses of beta-carotene are effective in preventing cardiovascular disease and cancer," write the authors in this month's issue of the American Journal of Clinical Nutrition (vol 82, no 4, pp879-886).
But their work is supported by a recent French observational study linking a diet rich in the carotenoid to a lower risk of cancer in non-smokers.
Brian Buijsse, lead author on the new paper, says the confusing evidence so far may suggest that beta-carotene needs to be taken 'in concert with other antioxidants' to have an effect on health.
"Plasma carotene may be a marker for fruit and vegetable intake," he told NutraIngredients.com, noting that most people participating in the study did not have carotene levels high enough to be consuming it in supplement form.
"The question is, are there any other nutrients in fruit and vegetables that we haven't discovered yet, or is beta-carotene working alongside other antioxidants," Buijsse explained.
The study analysed data on plasma concentrations of carotene and alpha-tocopherol among 1168 elderly men and women in Belgium, Switzerland, France, Denmark, Italy, the Netherlands, Portugal and Spain.
Previous studies relating intake of these nutrients to heart disease and cancer risk have been done on middle-aged populations. However oxidative damage is higher in the elderly, prompting the researchers to look for possible benefits in an older population.
Using data from several different countries gave the researchers a wide range of plasma concentrations, added Buijsse. Both antioxidants were lowest among Belgians and highest among the Swiss, but only Denmark had levels that suggested significant supplement use.
After a follow-up period of 10 years, the researchers looked at causes of mortality in the study population.
Alpha-tocopherol, a form of vitamin E, was not associated with all-cause or cause-specific mortality.
However carotene concentrations were associated with a lower mortality risk for both cancer and cardiovascular disease, although the lower risk of cardiovascular death was confined to those with a body mass index of less than 25.
These results were confirmed by a meta-analysis of five observational studies on an elderly population, including the current one, report the scientists.
The association between beta-carotene and higher cancer risk among smokers has raised questions about its role in supplements yet it is also a commonly consumed antioxidant in the diet, being found naturally in carrots, cantaloupes, and other fruit and vegetables with red, orange or yellow pigment.
Buijsse said that it is now believed that beta-carotene may act as both an antioxidant and an oxidant, when in higher doses.
A new trial using lower dose supplements than those tested in previous supplementation trials, and in a population at high risk of oxidative damage like the elderly, may confirm a beneficial role, he suggests.