A flavonoid found in fruit, such as apples and berries, could help reduce illnesses in people who have carried out extensive exercise.
Researchers writing in the Medicine and Science in Sports and Exercise journal found that supplementation of quercetin reduced incidence of upper respiratory tract infections (URTI).
According to the New Scientist, studies of quercetin have been supported by DARPA (the Defense Advanced Research Projects Agency) - the Pentagon's research arm - in the hope it could be used to protect US troops.
Lead author David Nieman from Appalachian State University in Boone, North Carolina, said in the New Scientist: "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,"
Researchers gave 40 male cyclists one gram of quercetin a day, which is the 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.
They found that URTI incidence during the two week post-exercise period "differed significantly" in the quercetin's group favor.