Millions of people around the world run long distance to burn calories or stay fit, yet both groups seem to overlook the risk involved. Those who run to specifically burn calories often end up with side effects associated with suppressed immune systems and a diminished capacity to resist stress. Competitive runners also suffer the same consequences. According to a recent review, competitive long-distance runners generally considered super-fit die at the same rates as sedentary people.

How is this possible? The answer is all about the biological function of hormesis, which conditions our bodies to better survive and live longer. Hormesis is a process of adaptation. It makes stress work for and not against you.

Engagements in extreme endurance events such as marathons, triathlons, cross-country skiing, and long distance cycling increase the risk of cardiovascular damage and overall mortality. The magnitude of cardiovascular damage is associated with the duration and intensity of the aerobic activity, possibly due to the cumulative impact of mechanical stress on the heart. Beyond a certain level, every stressor turns harmful. Endurance athletes that cross that threshold erode the health benefits that would have otherwise been obtained a by shorter workout.

Hormesis is a tricky phenomenon. You can either benefit from stress or end up damaged by it. To make stress work for you it must be intermittent and not chronic. All types of training have the ability to reach an area of chronic stress, which makes me doubt the viability of endurance conditioning that pushes athletes to their limits. What is the point of having training programs that produce super-athletes if it shortens their lives?

This question is not just theoretical. There is accumulating evidence indicating that there is an inverse relationship between extreme aerobics and longevity. Based on one report, the risk of atrial fibrillation (irregular heartbeat) is higher in athletes than in the general population. A wide-scale Swedish study found a higher rate of atrial fibrillation in participants completing cross-country ski races and in those with faster finishing times. The relationship between physical activity and cardiovascular risk seems to follow a U-shaped curve: optimal amounts of exercise decrease health risk as opposed to excessive volumes of exercise, which increase that risk. Runners who have a pace faster than an eight-minute mile for more than 2.5 hours per week – about the level of a competitive runner – are destined to lower their life expectancy to the level of sedentary people.

Does this mean that intense competitive endurance sports make people prone to live less? Not necessarily. Smart running routines can help improve insulin sensitivity and lower overall mortality. For example, moderate volumes of intense aerobic training and sprint intervals have been shown to aid cardiovascular health. To be beneficial, all types of training must comply with the concept of hormesis.

It’s about creating hormesis and avoiding chronic stress. Runners should allow themselves sufficient time to recuperate between running sessions. They must give their bodies the chance to adapt and improve in accordance with the distance and duration of their workout. Counter to conventional wisdom, properly designed intermittent bouts of endurance drills–along with a viable diet regimen–can help improve biological fitness, and decrease the risk of chronic stress and premature death. 

Excerpted from The 7 Principles of Stress: Extend Life, Stay Fit, and Ward Off Fat. Copyright © 2017 by Ori Hofmekler by permission of North Atlantic Books. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.