Antioxidants reduce asthma risk in children
along with the antioxidant trace mineral selenium, were associated
with a lower risk of asthma in a large study on young Americans,
published this week.
The antioxidants had even stronger protection against asthma in subgroups of children exposed to passive smoke, said the researchers from Cornell University, New York.
But scientists are still wary about recommending these antioxidants, with new advice reiterating previous suggestions that foods rich in the protective nutrients are better than taking supplements.
Fruit and vegetables, and vitamin C alone, have previously been linked to better respiratory health and reduced risk of asthma.
The Cornell team studied data from 6,153 people aged from four to 16 years old who were a part of the Third National Health and Nutrition Survey (NHAMES III). A household youth questionnaire, answered usually by the mother, was used for participants less than 17 years old and the study included a comprehensive health examination. Serum cotinine was used to identify youth with no cigarette smoke exposure and passive exposure. Active smokers were too few to be studied further.
Separate antioxidant models found that blood levels of vitamin E had little or no association with asthma. However, a standard deviation increase in beta-carotene was associated with a 10 per cent reduction in asthma prevalence in those not exposed to smoke and a 40 per cent reduction in young persons who had passive smoke exposure, shows the study, published in the 1 February issue of the American Journal of Respiratory and Critical Care Medicine (vol 169, pp 393-398).
The pattern for vitamin C was similar to beta-carotene results, according to the researchers.
An increase in selenium was associated with a 10 to 20 per cent decrease in asthma prevalence, and in youth with passive smoke exposure, investigators found a 50 per cent reduction in asthma prevalence associated with selenium.
The researchers however add a warning about the potential risk from excessive selenium supplements, which have been found to be toxic. Dietary selenium comes from cereal grains, fish, meat, and poultry.
Tufts university also published a warning on beta-carotene this week, suggesting that carrots are a better source than supplements.
Research has shown that beta-carotene and other carotenoids have anti-cancer activity, especially on lung cancer, but two major trials caused concern about the danger of taking 'mega doses?of beta-carotene. They both show that very high doses increase the risk of lung cancer, by 18 per cent in one trial and 28 per cent in another.
Neither the US Food and Nutrition Board nor the EU Scientific Committee on Food have yet set safe intake levels for the antioxidant but it is thought that people would only get these 'mega?doses from supplement intake.
"Beta-carotene's paradoxical effects on lung cancer appear to be related to dose. I'm concerned that the public perception of something as 'natural' is that it is safe, even at very high doses," says Dr Robert Russell, director of the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts.
In a recent article in the Journal of Nutrition, Dr Russell explains that laboratory models that simulate human metabolism should be used to thoroughly test nutrient doses that greatly exceed normal dietary levels before embarking on large-scale trials. Without these results, the effect on people may be unexpected and even harmful, as was the case with using unnaturally high doses of beta-carotene.
He added that while beta-carotene is still promising as an anti-cancer nutrient, the protective dose seems to be best obtained from dietary sources such as fruits and vegetables at natural levels.
"Experiments have taught us that simply taking a natural product at any dose is not wise. Dose does count," said Russell.
