Also, the body's production of growth hormone falls with age, and a fall in growth hormone appears to lead to a rise in weight. Menopause appears to cause some women to become heavier. In men, falling testosterone levels mean less muscle mass and more fat. Yes, and yes. The metabolic rate is the amount of calories a person burns in a day just to stay alive.
Scientists know that some people have higher metabolic rates than others.
If height, weight, muscle mass and activities are equal, then people with lower metabolic rates are considered more efficient. In other words, they can run their bodies on fewer calories than people with a higher metabolic rate. A good analogy is a car that gets good gas mileage - this car needs less fuel to travel the same distance as a car that gets worse gas mileage. Any excess food beyond what is needed to maintain body weight would be converted to fat in people with lower metabolic rates, unless they compensated for that with an increase in physical activity. The actual number depends on body size, age and activity level.