An apple a day may really keep the doctor away.

A natural antioxidant called quercetin, found in red apples as well as broccoli and green tea, may give an immunity boost to individuals under stress, says a report on health website WebMD.

In a study conducted by DrDavid Nieman, professor at Appalachian State University, only 5 per cent of cyclists who took 1,000mg of quercetin every day for five weeks reported upper respiratory illness during a two-week period following extreme exercise, compared to 45 per cent of the cyclists who took a placebo having reported illness following extreme exercise. There were no significant differences in measures of immune system function in the two groups.

The researchers also found that athletes taking the quercetin supplement maintained better mental alertness and reaction time over the placebo group.

So take advantage of this fruit that's available all year round. Crunch on one at least once a day and you may just be able to keep the doctor at bay.

This article was first published in Mind Your Body, The Straits Times on Dec 18, 2008.