Starting in your 30s, you can lose three to five percent of your muscle mass each decade if you don’t stay active. Note the last part of that point: “If you don’t stay active.” As you get older, "there's a lot of competition for your time and energy," says Dr. Yancy. For some men, that can push exercise out of the picture. But it doesn't take heroic efforts to put it back into your life.
Additionally, attempts to lose weight on low-calorie diets can lead to even more lost muscle. Studies have found that regular resistance or strength training may be a better alternative than your daily runs to preserve and gain muscle — even when coupled with a low-calorie diet. Aerobic exercise is still important, just don’t make it your only form of activity.
Between desk jobs, commutes, and family activities, many 40-somethings don’t have a lot of free time to work out. But it’s important -- for your weight and your overall health -- to fit in at least 2 1/2 hours of moderate physical activity (like brisk walking or light yard work) every week. Pencil times in to your calendar, and make them a priority.