The "sex glow." Carrie Bradshaw and her Sex and the City trio may be the champions of detecting it, getting it and keeping it, but you don't need a closetful of Prada to appreciate the rosy radiance that follows a pleasant sexual encounter. The fact is, sex leaves its mark, not just on the mind but on the body as well. Researchers have begun to explore its effects on almost every part of the body, from the brain to the heart to the immune system.
Studies are showing that arousal and an active sex life may lead to a longer life, better heart health, an improved ability to ward off pain, a more robust immune system and even protection against certain cancers, not to mention lower rates of depression.
But finding mechanisms for these benefits and proving cause and effect are no easy matter. "The associations are out there, so there has to be an explanation for it," says Dr. Ronald Glaser, director of the Institute of Behavioral Medicine Research at Ohio State University. Thanks to a better understanding of the biochemistry of arousal, as well as advances in imaging techniques, doctors are closing in on some possibilities. Their efforts are leading them to the hormone oxytocin, which may be the key lubricant for the machinery of sex. Known for controlling the muscles of the uterus during childbirth, oxytocin surges up to five times as high as its normal blood level during orgasm. Studies in animals have also revealed oxytocin's softer side. It is responsible for helping individuals forge strong emotional bonds, earning its moniker as the cuddle hormone. Released in the brain, oxytocin works in the blood, where it travels to tissues as distant as the uterus, as well as along nerve fibers, where it regulates body temperature, blood pressure, wound healing and even relief from pain.
While it is unlikely that oxytocin alone is responsible for sex's wide-ranging effects on the body, researchers hope that by tracking the hormone they can expose the network of body systems affected by sexual activity and identify other biochemical players along the way. Here's what they have learned so far:
--THE HEART OF THE MATTER
The strongest case that can be made for the benefits of sex come from studies of aerobic fitness. The act of intercourse burns about 200 calories, the equivalent of running vigorously for 30 minutes. During orgasm, both heart rate and blood pressure typically double, all under the influence of oxytocin. It would be logical to conclude that sex, like other aerobic workouts, can protect against heart disease, but studies in support of this link have yet to be done. "Can we make the claim that having sex is equal to walking a mile or bicycling? We don't know," says Robert Friar, a biologist at Michigan's Ferris State University. "The data don't really exist."
At least not yet. A study conducted in Wales in the 1980s showed that men who had sex twice a week or more often experienced half as many heart attacks after 10 years as men who had intercourse less than once a month. The trial, however, did not include a parallel group of randomly chosen control subjects, the scientific gold standard. So it's unclear whether frequent intercourse was responsible for the lower rate of heart attacks or whether, for example, the men who were sexually active were healthier or less prone to heart disease to begin with.