Must women step it up as they age?

For nine years, 47-year-old Detroiter Elizabeth Schneider has exercised for 40 minutes, three times a week. Her workouts have held her weight at a steady 145 pounds, which is in the ideal range for her 5-foot-9 frame.

While studies show most American women her age are overweight, Schneider is fit.

But one new report suggests she might not be for long.

Schneider's workouts aren't likely to be enough to maintain her weight as she ages, according to findings reported late last month in the Journal of the American Medical Association.

The report built on research conducted at Harvard's Brigham and Women's Hospital, which followed 34,079 non-dieting, middle-age women for 13 years, and found that women with a body mass index less than 25 needed 60 minutes of daily moderate activity to ward off weight gain.

"It seems like that is asking a lot," Schneider said before a floor aerobics class at the South Oakland Family YMCA in Royal Oak, Mich.

She says she doesn't diet, but looking after her 15-year-old son and 13-year-old daughter helps keep her active.

"I eat dessert," Schneider says, "pretty much every day."

Since 2008, the U.S. Centers for Disease Control and Prevention has recommended adults get 150 minutes — 30 minutes, five days a week — of moderate activity. The CDC also recommends twice-weekly strength-training sessions.

Caroline Richardson, an associate professor of family medicine at the University of Michigan, says that even though the study suggests more activity is needed, the recommendation has not changed.

"We should be doing 150 minutes a week, that's a given," says Richardson, who reviewed the study's findings.

"There is nothing in this report that says we shouldn't be doing that."

Richardson says she knows the 150-minute weekly mandate is already hard for many busy women to meet. She hopes the new study won't scare people into doing nothing because it sounds like too tall a challenge.

"I think the data is interesting and the study well-done," Richardson said. "But it has some built-in weaknesses."

For example, the women in the study self-reported their exercise, and that method has pitfalls, as people both over- and underreport their actions.

Women also reported their own eating habits, a problem because most people tend to underreport how much they eat.

Sue Vian , 60, of Royal Oak says she's the same size she was in high school and hasn't increased her activity level much in the past decade. She has taken a floor aerobics class twice a week for four years. She also walks and rides a stationary bike.

"I think an hour a day is a lot to ask people to do. People are busy. It's not realistic, and it's going to turn people off," Vian said.