“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.
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. 

"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.
“Has something happened — a change in relationship or job? Has that affected the times that you’re eating, how and what you’re eating, and who you’re eating with? Those changes in eating nutritionally can affect your weight,” Weiner said. “If you used to eat with other people, you maybe had more vegetables. Alone, you may be eating less healthy foods.”
Unfortunately, you can't just focus on one area of your body. "Women should focus on all muscle groups in order to lose weight and build muscle because there's no such thing as spot training," Diamond reminds. "With that said, women have a tendency to gain weight in the midsection as we age and focusing on the core and a well-balanced workout will help to reshape body composition."
Your muscles are important for bone density, strength, and overall health. However, your body starts to lose muscle as you age. Building muscle can increase your metabolism and help you burn more calories at rest. Muscle also takes up less space than fat, so you can feel leaner and toned as you increase your muscle mass. Good options to increase your muscle mass include light weightlifting, body weight exercises, and Pilates.

It’s usually not just due to losing muscle mass over time (more on that later). "It's multifactorial," says Fatima Stanford, MD, MPH, obesity medicine physician scientist at Massachusetts General Hospital and Harvard Medical School. Numerous factors can sneak up on you and help you gain or hold onto weight, including your lifestyle, your food, your biology, and your sleep habits. The good news in that is that there are also numerous ways to tackle getting your weight to where you want it—you don't have to force yourself into one approach, and you get to choose what works for you.
One of the main culprits for weight gain is, of course, our hormones, which start to change right around the mid-30s and into the 40s. This change in hormones, less estrogen for women and less testosterone for men, cause the fat in our bodies fat to shift to the middle of the body while abandoning other areas of the body you could care less about. That's one reason you may get a little fluffier around the middle while other parts of you actually get smaller.
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