Not surprisingly, several researchers have looked into the effect exercise has on sexual arousal and function. The verdict: yes, sex is physiologically better for people who exercise—in moderation.
One such study found that women who exercised on a treadmill for 20 minutes, then watched an erotic film, showed “a significant increase in physiological sexual arousal with exercise.” Another showed that men under 40 who burned 200 or more calories per day, or more than 1,400 calories per week, had better erectile function than men who burned fewer calories. (As long as those calories weren’t all burned cycling, which can—major bummer here—increase your risk for sexual dysfunction.)
Unfortunately, the longer you exercise, the less research there is. One study published in 2009, however, examined male endurance athletes' reproductive systems, and the results weren’t pretty.
In the study, 286 men ran two hours on a treadmill, five times per week for 60 weeks. Half of the men ran at moderate intensity (60 percent of VO2 max), while the other half ran at high intensity (80 percent of VO2 max). Throughout the study, researchers analyzed the participants’ semen quality and reproductive hormones, including testosterone.
The result most pertinent to your sex life: After 12 weeks, testosterone levels decreased in both groups, but more so for the high-intensity athletes. (Low testosterone levels in men can decrease sex drive, and cause erectile dysfunction.) The high-intensity athletes also experienced significantly decreased sperm counts.
The good news is those effects aren’t permanent. Testosterone and semen levels returned to normal during a 36-week recovery period. But if you’re an endurance athlete, you’re probably not going to take an entire season off. (Unless you’re trying to make a mini-you, in which case easing up on training might help.)
Endurance training can have similar deleterious effects on a woman’s reproductive system as well, potentially decreasing estrogen levels. Estrogen has been shown to play a major role in female genital blood flow and lubrication.
Of course, confidence plays a major role in sexual satisfaction, and none of this science can accurately quantify how sexy your endurance-chiseled body makes you feel.