Exercise is good for the immune system, right? Well, this type of training could have you feeling under the weather if you're not careful

By Caitlin Carlson
December 09, 2014

Training for a marathon? Signing up for your next endurance event? Better take extra stay-healthy measures, says new research from Bangor University, which found that long-duration, moderately intense workouts (over 120 minutes) actually temporarily weaken your immune system. "But I thought exercise boosted my immune system?!" you may be thinking. And you'd be right: Sweating it out helps you fend off sickness more easily. But it's believed that after 30 minutes, exercise increases stress hormones, such as cortisol, which suppress the immune system.

That's not to say you should cross that marathon off your bucket list and become a HIIT devotee (find out if you are overdoing your HIIT workouts), though. "In terms of building endurance fitness, managing body weight, and improving efficiency these sessions are important," confirms study author Neil Walsh, Ph.D. Instead, take these four steps to counteract any immune-suppressing side effects.

Smarten Up Post-Workout

Research shows that the immune system is most weak-and bugs can get a foothold-during the three to four hours following long-duration exercise (coined ‘the open window hypothesis'). "Take extra precautions during this timeframe," says Walsh, "avoid sick people and practice good hand and food hygiene." Find out if these weird habits can protect you from germs.

Swap Long Workouts Out

Swap some of your longer sessions for spike sessions (shorter and high intensity), says Walsh. Here are some interval sessions to try.

Follow the 5 Percent Rule

Don't increase your weekly training volume by more than five percent per week, advises Walsh. (So, if you ran 20 miles last week, you can run 21 the next.)


Make sure to take at least one rest day per week to allow your body to fully recover. Find out how to properly rest from your workout.