Gloria Hale rose at 5:30 a.m. Tuesday, as usual, to swim laps before work. Active though she is, however, the 55-year-old Orange County woman was a bit stunned to learn the latest advice from researchers regarding exercise -- that women should work out 60 minutes a day, seven days a week, to maintain a normal weight over their lifetime.

“Most people are going to say, ‘No way. I don’t have time for that,’ ” said Hale, a trim 5-foot-5 and 138 pounds.

The 60-minute-a-day recommendation, released online Tuesday in the Journal of the American Medical Assn., is aimed at women of normal weight who don’t want to diet but do want to avoid gaining weight over time. Most Americans gain about 1.5 pounds a year between age 25 and 55.

But the issue of how much exercise is required to maintain a normal weight is far from settled. Other exercise experts say that an average of 35 minutes a day, seven days a week, is probably sufficient. But that’s still a lot of exercise.

“We wanted to see in regular folks -- people not on any particular diet -- what level of physical activity do you need to prevent weight gain over time,” said the lead author of the study, Dr. I-Min Lee, an epidemiologist at Brigham and Women’s Hospital in Boston and an associate professor of medicine at Harvard University. “It’s a large amount of activity. If you’re not willing to do a high amount of activity, you need to curtail your calories a lot.”

The study was based on surveys of more than 34,000 U.S. women who were, on average, age 54 at the start of it. They reported their physical activity and weight, as well as health factors such as smoking and menopausal status, over 13 years. On average, the women gained 5.7 pounds during the study.

Only those women who were normal weight at the start of the study and engaged in moderate-intensity activity an average of 60 minutes per day, seven days a week, maintained a normal body weight, defined as a body mass index of less than 25. That amount of exercise is three times higher than the amount recommended by the federal government -- 150 minutes per week -- to lower the risk of chronic ailments such as heart disease.

“You can still do much for your health with a lower level of exercise,” Lee said. “But if you want to exercise for weight control, it’s 60 minutes a day.”

Moderate-intensity activity was defined in the study as walking or hiking, jogging, running, bicycling, aerobic exercise or dance, use of exercise machines, yoga, tennis, squash, racquetball and swimming. Housework and gardening were not included in the analysis.

It’s unclear what level of physical activity younger women need to maintain weight. Studies in men have consistently shown they require less exercise to maintain body weight.

The study does not prove that this level of exercise is the only factor in maintaining a normal weight. It’s possible that other factors contribute, said Dr. Samuel Klein, director of the Center for Human Nutrition at Washington University School of Medicine. Klein was not involved in the new study.

“Women who decide to be physically active may also decide to eat healthier and do other things to maintain a normal body weight,” he said. “This study doesn’t tell you it’s the physical activity.”

Some people who exercise a lot, he said, will eat more as a result. Others will be so tired from exercise that they don’t do much activity the rest of the day.

“It’s complex,” Klein said. “It’s all about energy balance. It takes a very small imbalance to gain a significant amount of weight over many years.”

Although few randomized, controlled studies -- the gold standard in research -- have looked at the issue of weight maintenance over time, they tend to point to about 200 to 250 minutes of exercise a week, said Dr. Joseph E. Donnelly, director of the Energy Balance Laboratory at the University of Kansas.

“But there is huge individual variation,” he said. “It’s very difficult to filter out what it takes for the average person not to gain weight when you don’t do randomized studies.”

Given the new advice, however, Hale says she’s happy she took up swimming. “It’s easy on my joints,” she said, “and it’s something I’ll be able to do the rest of my life.”

shari.roan@latimes.com