Much of that benefit was seen around the waist, with high-activity men gaining 3.1 fewer centimeters (1.2 inches) around the gut each year and women 3.8 fewer centimeters (1.5 inches) per year.
The researchers cautioned that higher levels of physical activity alone may not be entirely sufficient to keep off weight, however, noting that men and women at all activity levels gained weight over the 20-year period. Nonetheless, they noted, higher activity certainly helped hold down weight during the transition from youth to middle age.
The 20-year follow-up in this study was particularly impressive, Ory noted, especially given that most weight-focused studies these days are shorter term.
"You can track [weight] at key decision points -- what kinds of activities do they do in a reliable manner and what difference it makes," she said.
The gender difference (the magnitude of the benefit was double in women than in men) could be explained by physiological differences, the researchers suggested.
"The two physiological things that are associated with female gender that definitely play a role are having children and menopause," Hankinson said. "But there could be other physiologic differences we can't measure, and there may also be cultural differences."
"We know that for women who are going through menopause, there's this natural increase in weight gain," added Dr. Suzanne Steinbaum, director of women and heart disease at Lenox Hill Hospital in New York City. "My comment also is to train for menopause as if you're training for a marathon. If you start exercising before menopause hits and do that for 20 years, you don't have to gain weight. Health isn't about flipping a switch. It's about maintaining a lifestyle."
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