- How calories are used (energy metabolism). Some people use calories efficiently-they need fewer calories to fuel the body, which can result in "leftover" calories being stored as fat. Other people use calories less efficiently-they need more calories to fuel the body, so there are fewer leftover calories to store as fat.
- Basal metabolic rate (BMR)Basal metabolic rate (BMR), which is how much energy you burn when you are at rest. If you have a lower BMR, it is easier to gain weight. Your BMR can change slightly in response to certain conditions. For example, starvation or very low-calorie diets decrease your BMR, because you lose muscle as well as fat. Muscle increases your resting metabolic rate, so losing too much muscle reduces metabolism. Fever and severe physical stress, such as recovery from surgery or from extensive burns, increases your BMR.
Body signals. Hunger, fullness (satiety), and appetite are body signals that tell you how much to eat. These signals also can be influenced by the environment and can be ignored for short periods of time.
- Hunger is a normal sensation (growling in your stomach, feeling hunger pangs) that makes you want to eat. It is partially controlled by a region of your brain called the hypothalamus, your blood sugar (glucose) level, how empty your stomach and intestines are, and certain hormone levels in your body.
- Satiety is a feeling of fullness and satisfaction. Stretch receptors in the stomach send signals to the brain that the stomach is filled. Increased blood sugar (glucose), the activity of the hypothalamus, and the presence of food in the intestines all contribute to satiety.
- Appetite is a desire for or an interest in food that is associated with the sight, smell, or thought of food. Appetite can override hunger and satiety, such as when you continue to eat even after you feel full. You can also have no appetite for food even though you are hungry, such as in a stressful situation or during an illness.
- Set point. This theory suggests that your body tries to keep your weight within a specific range, called your set point. The range seems to be influenced by your genetic makeup. But your actual weight within that range is influenced by your lifestyle or environment. Your set point adjusts to a new level when it is maintained over time and can be altered by overeating, exercise, some medicines, and some brain conditions.
- Fat distribution. Typically, men store fat in the abdomen while women store more in the hips and thighs. As women age, more fat is stored in the abdomen. Both men and women lose muscle weight as they age.