"One of the reasons that it's difficult to lose weight in your 40s is that you are beginning to lose muscle mass, so the composition of your body tissue changes," explains Keri Peterson, MD, Women's Health advisor. "Having higher muscle mass raises your metabolism, so your body burns more calories." So when you're dealing with the opposite—less muscle mass—that means a slower metabolism. Argh.
“There are many eating patterns that can be used to lose weight,” says William Samuel Yancy, M.D., director of the Duke Diet and Fitness Center and associate professor of medicine at Duke University. Many have evidence behind them, whether that’s keto, Paleo, Mediterranean, vegan, or anything else. Interestingly, there’s not as much research on what works for men as there is on women, but “for men, sometimes it’s as simple as shortening the time in which you are eating to an 8- to 10-hour window a day,” says Kristin Kirkpatrick, R.D.N., consultant for Integrative and Lifestyle Medicine for the Cleveland Clinic. Even something as straightforward as not eating after 6 PM can make a big difference—one of her male clients who lost 150 pounds found that to be especially helpful, she says.
There appears to be a connection between estrogen and body weight regulation.  With lower estrogen levels, lab animals tend to eat more and be less physically active.  Levels that are too high or too low appear to lead to fat storage.  And, lower estrogen levels may also slow down your metabolic rate (the speed  at your body converts stored energy into working energy).
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.
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