100 apples a day may indeed keep the doctor away

The equivalent of 100 apples can stop people under extreme physical or mental stress from getting sick.

It's a common complaint of marathon runners and soldiers alike: Overdo the exercise and you fall victim to illness, particularly chest infections.

Now. it seems that quercetin -- a flavonoid found in fruits, berries and tea -- may offer protection.

For the past few years, DARPA -- the U.S. Pentagon's research arm -- has been sponsoring studies of quercetin in the hope that it could protect U.S. troops. "During missions, soldiers are running around for two or three days with heavy packs on. They don't eat or sleep, and infections are as much of a problem, if not a more serious issue than injuries," says David Nieman of Appalachian State University in North Carolina, who led the research.

He gave 40 male cyclists either 1 gram of quercetin a day -- equivalent to eating 100 apples -- or a placebo, for three weeks. During this time, the cyclists spent a three-day period training at maximum intensity for three hours each day. "By the time they were done, they were just wasted," Nieman says.

Two weeks later, nine of the cyclists in the placebo group had suffered chest infections, compared with just one in the quercetin group.

Tests showed that the cyclists taking the supplement had high levels of quercetin in their blood. Lab studies have previously shown that quercetin can bind to viruses and bacteria and stop them replicating; this is what Nieman believes was happening in the cyclists to stop them from getting sick.

Nieman also found the cyclists had reduced levels of IL-8, a chemical that helps mediate the immune response to antigens, suggesting that quercetin may also be influencing the immune system in some way.

He's now looking at whether quercetin could benefit people suffering from high mental stress, who are also at greater risk of infection. He also hopes to establish the minimum amount of quercetin needed to achieve a protective effect. The average American typically eats around 107 milligrams of flavonoids -- which are polyphenols -- per day. There are no apparent side effects of boosting the intake, says Nieman.

Nieman's studies "provide important new evidence regarding the health benefits of polyphenols in general and quercetin in particular," says Holden MacRae of Pepperdine University in California. Last year, he showed that quercetin improved cycling time-trial performance by around 3 percent, when given in combination with antioxidants, in a small study of elite male athletes.