Remember how we said that the more muscle you have on your body, the better your metabolism works? Having more muscle increases your resting metabolic rate (RMR). This helps you burn more calories at rest—which is what you want if you’re aiming for weight loss after 40. So if you’ve been avoiding those weight machines or dumbbells, now’s the time to dedicate yourself to strength training. It is never too late to start strength training! If it is new to you, take advantage of any free personal training sessions that might be offered when you join a local gym or health club. This is often to familiarize you with the equipment and the layout of the facility, but can be a useful tool to get you started on a routine. Of course, if you have the finances to hire a trainer for a few sessions that’s another way to go.
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.
In your forties, your body may have an increasingly difficult time digesting carbohydrates, which may lead to weight gain. Living a stressful life can lead to an increase in the stress-hormone called cortisol, which can make you more prone to gaining belly fat. Estrogen loss may also lead to fat redistribution in your body and cause belly fat gain.11
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.
Your metabolic rate naturally continues to decline 5 percent each decade once you hit 40.3 Your resting metabolic rate (RMR) determines how many calories you just to survive. It is determined by a variety of chemical reactions inside your body. These reactions rely on your mitochondrial function and sodium-potassium pumps. Your mitochondria help to create energy, while your sodium-potassium pumps generate muscle contractions and nerve impulses.4,5,6,7,8 Both of these processes become less effective over the years and slow your metabolism.9,10 If you eat more than your RMR yet lead a sedentary life, you set yourself up for weight gain.
It’s no secret that losing weight after 40 can be difficult for some women. As you age, you begin to lose muscle mass, causing your metabolism to slow down. Add to the slow metabolism a dose of wildly fluctuating hormones and you have the perfect recipe for weight gain. Weight gain after 40 no longer translates into only a tight-fitting pair of pants. You are now at increased risks for a wide range of diseases, including depression, high blood pressure, and diabetes.

People naturally lose muscle after 40, especially women after menopause. Because muscle burns more calories than fat, this can slow down your metabolism and make it harder to shake those stubborn pounds. Strength-training exercises -- lifting weights or doing body-weight exercises, like push-ups and squats -- at least twice a week can help you keep those muscles.

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