BOSTON — Moderate exercise, particularly during the teen-age years, produces lifelong benefits to health, including a reduction in cancer risk, experts said here Sunday.
Women who played team sports in high school and college had a significantly reduced lifetime risk of breast cancer, colon cancer, cancers of the reproductive system and diabetes, according to epidemiologist Rose E. Frisch of the Harvard School of Public Health. Less extensive studies of men who exercise in their jobs, she said, show that they also have a lower risk of colon cancer.
But moderation is crucial, the experts said at a meeting of the American Assn. for the Advancement of Science. Both women and men who exercise intensively undergo temporary hormonal changes that reduce fertility. Those changes are reversed and fertility is restored when the heavy exercise is stopped.
The studies indicate that as little as 20 minutes of exercise three times per week can have beneficial effects on health, said surgeon Tenley E. Albright of the Advanced Medical Research Foundation in Boston. Albright, winner of a gold medal for figure skating at the 1956 Olympics, urged that employers establish a 6- or 7-minute program of daily exercise in the workplace to improve their employee's health.
And Frisch urged that young girls, beginning at age 10 to 12, have at least two hours of team sports per week at school. The benefits from that sport, she said, "will last them the rest of their lives."
Frisch also noted that active women who are having difficulty conceiving could increase their chances of success by cutting back on their exercise levels.
Researchers have known for many years that women athletes in training--as well as women who diet intensively--frequently do not have menstrual cycles. As the women become leaner, their reproductive system simply "shuts down" to conserve total energy, said endocrinologist David C. Cumming of the University of Alberta in Canada. A 5-foot, 5-inch woman of average build, for example, will not ovulate once she falls below 108 pounds.
"But as soon as she puts a layer of fat back on her body, her fertility is restored," Frisch said. She noted that a pregnancy normally consumes about 50,000 calories, and that the body acts to avert a pregnancy until it builds up a reserve of about that size.
Only within the last two years have researchers discovered that the same effects occur in male athletes. Ariel Barkan of the University of Michigan Medical Center reported in 1986 that she had observed reduced sperm production in marathoners and Olympic-class runners who trained very heavily.
The athletes reported no sexual problems, however, and fertility was restored when their training schedules were reduced.
The new results about the beneficial effects of exercise come from a study of 5,398 Harvard alumnae ranging in age from 21 to 80.
Nearly half of the alumnae, 2,622, had participated on college track, basketball, soccer, swimming, or tennis teams having at least two practice sessions of two hours each per week. If members of this group were not on an organized team, they ran at least two miles per day five days per week.
The remainder of the group, 2,776, had no formal exercise program.
Frisch and her colleagues found that the non-athletes had 2.5 times as many cancers of the reproductive system (uterus, ovaries, cervix and vagina) as the athletes and almost twice as many breast cancers. The non-athletes also had a higher incidence of colon cancer and diabetes, she said.
Frisch also found that the former athletes were typically taller and leaner than the non-athletes. But the two groups did not differ in their family history of cancer, in the number of children they had (an average of 2.1 in both groups) or in their pregnancy histories.
About 74% of the former athletes reported that they were still exercising regularly, compared to 57% of the non-athletes.
Onset of Menses
One key difference between the two groups was the age of menarche--the first menses. Among the non-athletes, the average age of menarche was 12.5 years, the national average in the United States.
Among the athlete group, in contrast, the average age of menarche was 15.5. Nearly 82% of these women reported that they had competed in organized sports before the age of 12, and such participation is known to delay menarche. Researchers also know that early menarche is a major risk factor for breast cancer, and Frisch speculated that late menarche in the athlete group was a major contributor to their reduced cancer risk.
The benefits of exercise for men are so far less clear-cut. But in November, 1987, Dr. Arthur S. Leon of the University of Minnesota reported that moderate physical activity reduced the risk of death from heart disease by more than one-third in middle-aged men who were at high risk for heart trouble.
"Even before we have all the answers, we must encourage programs for children and teen-aged students . . . (and for adults) in the workplace," Albright said.
She recommends three seven-minute exercise periods on the job every day. Such exercise, she said, would not only benefit health, but would also serve as "a stress reliever and an antidote for stress-related illness and decreased performance."