Every bodybuilder knows that there’s a down side to hard training: getting sick. Although training hard is needed to get strong and build serious muscle, overtraining can open you up to sickness. In a new study, researchers tried to figure out part of the reason why training can make you sick, and they think they’ve found part of the answer: it’s in your spit.

Researchers tested a team of world-class rowers to see how intense exercise affected levels of lactoferrin and lysozyme, two immune-system saliva proteins that kill microbes such as bacteria and viruses. High levels of these proteins helps keep you healthy by killing harmful invaders before they can get into your body. After examining saliva samples from these athletes taken before and after an intense workout, the scientists discovered that an intense workout lowers the total count of these protective proteins, so your body kills off fewer microbes, making it easier for you to get sick.

FUSION FACT

It’s not clear why training has this effect, but to combat your risk of getting sick, researchers recommend drinking plenty of water during exercise to flush out potentially harmful bacteria. We also recommend supplementing with vitamin C for oxidant protection and eating plenty of protein so you heal faster.