Nearly every exerciser knows the locker-room wisdom about workouts and sex: Getting fit improves your love life.

But as a seven-time world masters swimming champion, anthropologist Phillip Whitten decided to try to prove it. Five years ago, while teaching human behavioral biology at Harvard, Whitten studied 160 male and female swimmers, some in their forties and others in their sixties. The results showed, he says, "a positive relationship between regular exercise and sexuality in terms of both the frequency and enjoyment of intercourse.

"The swimmers in their sixties reported sex lives comparable to people in the general population in their forties. The swimmers in their forties had sex lives more like those of people in their twenties and thirties," Whitten said.

Common perceptions of sexual decline with age "may have more to do with cultural expectations and poor health than with years," says Whitten, who left Harvard in 1991 and is now editor-in-chief of Sports Publications Inc. in Pasadena. "The swimmers we studied were stronger and had more endurance than they did before they began training. They felt more attractive too, so it's no wonder they felt sexy."

Exercise enhances sexuality through various mechanisms that affect both body and mind. Physical boosts in endurance, muscle tone and blood flow all can improve sexual functioning. And the psychological benefits, such as stress reduction, mood elevation and self-confidence, also can enrich your love life.

"I'm thoroughly convinced it's a combination of psychological and physiological factors that makes exercise a powerful instrument for sexual enhancement," says James White, professor emeritus of physical education at the University of California, San Diego. He studied 78 sedentary but healthy middle-aged men who started exercising vigorously three to four days a week, for 60 minutes per session. The new exercisers reported more-frequent sexual activity and orgasms, more reliable function during sex and a higher percentage of satisfying orgasms.

"The degree of sexual enhancement correlated with the individual's fitness gain," says White, adding that a control group that began a moderate walking program reported no change in sexual activity, he says. Increased levels of the hormone testosterone resulting from vigorous exercise may account for some of the enhanced sexual behavior, White says. "And with exercise, men lost weight and looked better," he notes. "A man who feels better about himself may be more able to perform sexually."

Women also reported a similar sexual spark from exercise, according to a survey of more than 8,000 women ages 20 to 45 who responded to a questionnaire published in Shape magazine. "Forty percent said exercising made them more easily aroused, one third said exercising led to more frequent sexual activity, and 27 percent reported increased ability to climax," says Linda De Villers, a Santa Monica psychologist specializing in sexuality who analyzed the questionnaires. After the workouts, 89 percent reported heightened sexual confidence; 98 percent reported improved self-confidence.

While excessive exercise can sap sexual energy, De Villers notes, regular moderate exercise "seems to give people a sexual second wind. Aerobic activity exerts a calming effect, yet you've got all those endorphins going. There's a paradoxical feeling of being both energized and relaxed." This is an ideal mindset -- and bodyset -- for good sex.

This "sexercise" link is so strong that some sex therapists now prescribe exercise. "I encourage women whose sexual response isn't what they'd like to get regular exercise," says Jo Marie Kessler, a nurse practitioner and certified sex therapist in San Diego. "Even low levels of exercise tend to improve mood, and moderate exercise certainly helps you maintain the condition of the equipment you use for sex."

Both exercise and sex result in eu-stress, a name for the good stress that takes energy but also makes energy. And "sex is exercise," write Zev Wander and David Radell in "How Big Is Big: The Book of Sexual Measures." "A single act of sexual intercourse consumes (about) 150 calories." By comparison, they note, 18 holes of golf (riding a cart) uses just 118 calories.

Next week: Eating Right tells you how to lighten up on those rich winter comfort foods.