Metabolism And Your Weight

It may seem logical to think that significant weight gain or being overweight is related to a low metabolism or possibly even a condition such as underactive thyroid gland (hypothyroidism). In reality, it's very uncommon for excess weight to be related to a low metabolism. And most people who are overweight don't have an underlying condition, such as hypothyroidism. However, a medical evaluation can determine whether a medical condition could be influencing your weight.

Weight gain is more likely due to an energy imbalance — consuming more calories than your body burns. To lose weight, then, you need to create an energy deficit by eating fewer calories, increasing the number of calories you burn through physical activity, or preferably both.

Influences on your calorie needs
If you and everyone else were physically and functionally identical, it would be easy to determine the standard energy needs. But many factors influence calorie requirements, including body size and composition, age, and sex.

Body size and composition. To function properly, a bigger body mass requires more energy (more calories) than does a smaller body mass. Also, muscle burns more calories than fat does. So the more muscle you have in relation to fat, the higher your basal metabolic rate.
Age. As you get older, the amount of muscle tends to decrease and fat accounts for more of your weight. Metabolism also slows naturally with age. Together these changes reduce your calorie needs.
Sex. Men usually have less body fat and more muscle than do women of the same age and weight. This is why men generally have a higher basal metabolic rate and burn more calories than women do.