Hormonal changes are one of the main reasons that women tend to lose more muscle as they approach menopause. Diet and exercise also play a role.12 On average, adults lose 3 to 8 percent muscle every decade after age 30.13,14 By the time you reach 80, you may have about 30 percent less muscle than you had at age 20.15,16 Muscle loss over time can lead to slower RMR, increased fat, weight gain, weakness, and fractures.17
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.
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.
When you’re busy with work, kids, and life, you can be tempted to grab food on-the-go or multitask through a meal. But you’re more likely to overeat -- and be hungry again soon after -- if you don’t focus on your food. Sit down for meals and tune in to what’s on your plate (not what’s on your TV or computer screen). That helps your brain realize when you’ve had enough.