"These data suggest that the 2008 federal recommendation for 150 minutes per week, while clearly sufficient to lower the risks of chronic diseases, is insufficient for weight gain prevention absent caloric restriction," wrote Dr. Lee.

Expert speak

According to Suzanne Phelan, PhD, assistant professor of kinesiology at California Polytechnic State University in San Luis Obispo and adjunct assistant professor of research at Brown Medical School in Providence, R.I., who has researched the topic.

"To keep a normal body weight is hard work," she says. As you age, she says, "you have to exercise a lot whether you are normal weight or maintaining a weight loss." She says 10 minute bouts of moderate activity can be a solution if 30 or 60 minutes seem too long.

Peter Galier, MD, a staff physician at Santa Monica-UCLA & Orthopaedic Hospital in California and an associate professor of medicine at the University of California, David Geffen School of Medicine says,

''As you get older, your basal metabolic rate [calories burned at rest] goes down," he says. Thus he emphasizes the need to exercise regularly to keep the same weight.