Extreme endurance races such as ultra marathons and triathalons, as well as intense workouts such as CrossFit and P90X are attracting more and more participants, but a recent study warns that those who don’t prepare well for these extreme events risk blood poisoning.

Dr. Ricardo Costa of the Department of Nutrition and Dietetics at Monash University in Australia conducted a study published in the International Journal of Sports Medicine that tested the blood of 24-hour marathon participants before and during the race, and compared them to a control group.

The researchers found that extreme exercise causes disruptions in the walls of your intestines. As your push past your body’s limits, these gut disruptions can allow bacteria from your intestines to leak into your blood, potentially causing a dangerous and potentially fatal inflammatory response.