Exercising Too Long Can Weaken Your Immune System

Exercise is vital for good health and a strong immune system, but researchers recently found exercise sessions lasting too long can weaken your immune system.

They followed four groups:
30 minutes of moderate running on a treadmill
30 minutes of intense running (80 percent of peak oxygen uptake) on a treadmill
2 hours of moderate running
The last group did not exercise

The researchers found 30 minutes of intense running did not affect the subjects’ immune response, but two hours at a moderate pace reduced their immunity. This might be because prolonged exercise increases the levels of stress hormones like cortisol, which temporarily decreases immune function.

A previous study found people who exercise at least five days a week are 43 percent less likely to catch an upper respiratory tract infection than people who exercise once a week or not at all.

The best exercise strategy may be to do shorter, more frequent workouts and keep your workout sessions between 30 and 90 minutes.

If you are training for a marathon, it is important to eat enough (good) carbs. Carbohydrates minimize your energy deficit (“hitting the wall”) when exercising longer than an hour and a half. That will help reduce the rise of immune system–suppressing stress hormones.