Metabolism is the process through which the body converts what a person eats into energy. Catabolism is the breakdown of molecules to obtain energy and anabolism is the synthesis of all compounds needed by the cells. After 30, gaining weight becomes easier and losing it harder. The foods you enjoy suddenly seem to stick to your waistline. This is because losing muscle slows down the body’s natural calorie-burning ability. Adjusting to new routines, such as consuming fewer calories because the metabolism is slowly declining and lifting weights to build muscle as we get older are facts of life.