Can You Be Too Fit to Be Fertile?

Exercise and Infertility
It’s well known that women athletes can stop having regular periods if they exercise too much. Termed “exercise-induced amenorrhea,” this prevents pregnancy during times of extreme physiological stress. Think of it as Darwin would: Why would you want to bear children as a cavewoman running for your life?

Survival Comes First

The mechanism for this cycle-stoppage involves the same hormones that stimulate puberty, estrogens and leptins and all. The body’s reaction to high stress is to channel all available energy to maintaining basic bodily functions, which means that no energy can be devoted to activities that draw extra fuel.
But does strenuous exercise show a similar effect on body balance and fertility in men? Given that men don’t have periods and menstrual cycles, it’s a bit difficult to compare directly. But men do make sperm and that’s a pretty good a “biomarker” of fertility. And guess what. . .it’s been studied!

Birds of a Feather

In what is probably the best study ever done on the topic, 286 very healthy, 20- to 40-year old men with normal semen quality were asked to do 1 year of frequent, regular, high-intensity exercise. Their hormones and semen quality were measured before and after the year-long challenge. Let me explain what I mean by high-intensity exercise: 2 hours a day, 5 days a week of continuous, inclined-treadmill running; each man was required to reach 80% of “volitional exhaustion.” That’s a lot of hard running. Gotta believe that cavemen running this hard were either in search of scarce and fast-moving food or escaping predators. Here’s what they found:

  • Sperm counts fell by almost 50% (from 65 million/mL to 35 million/mL)
  • Testosterone levels fell by almost 40%.
  • There was full recovery of both sperm quality and testosterone levels during a more restful recovery phase.

Tell me that this is not an obvious, clear-as-day demonstration of the effects of physical stress on male fertility potential! To boot, the mechanism of semen impairment appears to be similar to that of women and amenorrhea: alternations in hormonal signaling. This makes great sense to me given what we know about the profound effect of acute physical stress (“operational field duties”) on testosterone levels from an older military study. And, it also harkens back to the 2016 announcement by America’s top men’s marathon runner Ryan Hall of his retirement from the sport due to fatigue, weakness and depression from having chronically low testosterone levels. A cautionary tale, to be sure.

Balance and Health Matters Most

So, despite the obvious health benefits of exercise, relevant fertility markers in both sexes suggest that too much is definitely not good for fertility. How much is too much is still very much unknown as exercise tolerance varies widely among individuals. Some can take a lot and keep on ticking and others not so much. My advice for optimal fertility is to take all things in moderation, including exercise. When it comes to fertility and fitness, one could say exactly what the jazz-loving, Trappist monk and poet Thomas Merton opined about happiness: “it’s not a matter of intensity but of balance, order, rhythm and harmony.”