But gaining as little as five pounds had the reverse effect in normal-weight and overweight women, researchers reported in today's Journal of the American Medical Association.
The four-year study of more than 40,000 nurses, ages 46 to 71, is one of few to show that small changes in weight can have a big impact on quality of life, researchers said. Most research into overweight has focused on dire long-term risks diabetes, high blood pressure, heart disease, stroke and some cancers, they said.
"The vast majority of overweight people don't end up getting those diseases. But when we're talking about things like being able to walk up stairs, we're talking about something that potentially affects everyone," said the senior author, Dr. Ichiro Kawachi, an assistant professor of medicine at Harvard-affiliated Brigham and Women's Hospital in Boston.
A separate study in JAMA reported that obese women were more than six times as likely as lean women to have a silent, smoldering inflammation in their arteries that may boost their already elevated risk of developing heart disease.
Results of that study, of more than 16,000 U.S. adults, were reported in April at a meeting in Washington by obesity researcher Marjolein Visser of Vrije University in Amsterdam, the Netherlands.
Obese men were twice as likely to have the low-grade inflammation as normal-weight men, a gender difference that researchers can't yet explain.
The findings suggest that low-dose aspirin therapy, used to prevent heart attacks and strokes in patients with cardiovascular disease, might also benefit the overweight.
But more research is needed before such treatment could be justified, according to an editorial by an expert not involved in the work, Dr. John Danesh of the University of Oxford in Oxford, England.
The inflammation study involved adults of all ages, while the quality-of-life study's findings were probably limited because it involved only older women, said another expert not involved in the research.
"Younger women, I think, probably experience more social and psychological ramifications of weight," said the expert, Dr. Michael Steelman, past president of the American Society of Bariatric Physicians, specialists in medical issues related to weight.
Weight change was more strongly associated with physical rather than mental health among the nurses. Steelman praised the research but said some patients clearly benefit greatly in mental and emotional functioning with modest weight loss.
"It gives them the kind of self-confidence they need to move ahead with their life, perhaps to earn promotions, to change jobs, to improve relationships," he said in a telephone interview from Oklahoma City, where he practice.
In the nurses' study, 38 percent of subjects gained 5 to 20 pounds over four years, 17 percent lost 5 to 20 pounds and 39 percent maintained their weight. Of the remainder, 5 percent gained more than 20 pounds and 2 percent lost more than 20 pounds. The numbers add to 101 percent due to rounding.
Based on questionnaire answers, physical impairments were significantly more likely to develop in the 5- to 20-pound gainers compared with maintainers. And the leanest women who gained 20 pounds or more were twice as likely to develop physical limitations as maintainers, researchers said.
Weight loss in overweight women was associated with improved physical function and vitality as well as decreased bodily pain, the researchers said.