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
Oxidative stress occurs when the balance between free radicals in the body and our ability to fight against them is uneven, with free radicals prevailing. Free radicals can cause disease and there is an association with an increased risk of formation of free radicals as we age. That's why after a certain age, building up our defenses (through having lots of antioxidants in plants) can help reduce this imbalance and stack the cards in our defense system instead.
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.