For years, scientists and active people have debated whether and how exercise affects sexual desire and human reproduction.
But most past studies have centered on women. Typically, this research has found that when some female athletes, such as marathon runners, train intensely for many hours a week, they can develop menstrual dysfunctions. These problems seem caused by hormonal imbalances occuring because of physical stress and frequently affect a woman’s interest in sex and her ability to conceive.
But such dysfunctions are rare and usually resolve after the athlete lightens her training load.
Less is known about the effects of exercise, especially heavy exercise, on men’s libidos and fertility. There have been hints that, in moderate amounts, physical activity increases the male body’s production of the hormone testosterone, which theoretically should ramp up sex drive. Other small studies, on the other hand, have suggested that lengthy and grueling training may blunt the levels of testosterone in a man’s bloodstream both immediately and over the long term.
But those studies examined only hormone changes related to exercise, which can be measured easily, and not differences in sexual emotions and behavior, which are tougher to quantify.
So for the new study, which was published this month in Medicine & Science in Sports & Exercise, researchers at the University of North Carolina in Chapel Hill decided to ask active men about their sex lives.
They began by developing a questionnaire based on earlier psychological research into men’s sexual behavior that asked, for instance, how often they thought about and engaged in sex. The scientists also created a separate questionnaire with detailed queries about exercise habits, including how often and intensely the men worked out each week.
A final set of questions asked about general health and medical histories.
Then the researchers contacted running, cycling and triathlon training groups, university athletic departments, and publications targeted at endurance athletes and asked them to alert members and readers to the questionnaires, which were available online.
Almost 1,100 physically active adult men completed all of the questions. Most were experienced athletes who had participated for years in training and competitions.
The scientists used their responses to stratify the men based on both the extent and intensity of their workouts. They wound up with groups whose weekly exercise was short, moderately lengthy or quite prolonged, and separately whose weekly exercise was light, moderate or extremely intense.
It was possible, of course, for someone to be in the top or bottom of both of these categories, meaning that their workouts were both long and intense or light and short. But the scientists wanted to examine each of those aspects of a workout separately, so did not track such overlaps.
They also categorized the men according to their answers about their sex lives, creating groups with relatively high, moderate or low libidos.
Finally, they compared the men’s exercise habits to their reported interest and engagement in sex.
And there were clear patterns. The men whose exercise routines were moderate or light in intensity or duration were far more likely to report moderate or high libidos than were the men whose workouts were especially prolonged or intense, even after the researchers controlled for age. (Older men tend to report less interest in sex, although not by much.)
In effect, strenuous exercise “was associated with lower libido,” says Anthony Hackney, a professor of exercise physiology and nutrition at the University of North Carolina who led the study.
Of course, this was a small sample of men who voluntarily chose to complete a personally intrusive survey. It is impossible to know whether they were truthful or representative of the rest of their gender.
This type of study also cannot tell us whether too much exercise actually causes low libido, only that the two are linked.
And it did not examine why strenuous exercise might dampen libidos. But Dr. Hackney speculates that both physical fatigue and lower testosterone levels after exhausting exercise likely play a role.
He and his colleagues hope to soon mount experiments that directly track exercise, hormone levels and libidos to learn more about their interactions. They also aim to learn more about whether the intensity of the workouts or duration has the greater impact on male sex drive.
Perhaps most important, he hopes eventually to pin down at what point exercise might start to lower some men’s libidos. Both moderate and light physical activity were associated in this study with relatively high libidos, he points out. “But there does seem be a potential tipping point,” after which more exercise may blunt desire. The necessary studies likely will require years and many cooperative men to complete.
In the meantime, he suggests that if someone is worried about whether his training is affecting his sex life, he might try exercising a little less, to see if his libido changes.
This advice could be especially important for couples trying to conceive, he says. “Fertility specialists will often ask a woman about whether and how much she exercises,” he says. “Based on our data, we think they should also be asking the man.”
