Hormones change as people age, which causes their metabolic rate to change. Their metabolism slows as men lose testosterone and women lose estrogen, making the weight easier to put on and harder to take off. You burn less calories and differently break down foods. You lose lean muscles, too. The pounds will add up unless you exercise more and change your diet.
A chemical named leptin, which makes your brain tell you to stop eating, is one main player in age-related changes. Leptin signals do not function as well as you get older, so even though you’ve eaten, you may continue to feel hungry. Obesity makes it much less successful for leptin. For people in their 20s, weight control can become more complicated, but in your 30s, weight loss can become much trickier.