Factors that can be changed
The following known factors are amenable to individual control.
Dietary intake. Eating more calories than you expend is an important cause of obesity. Even small excesses in calorie intake can contribute to obesity over the long term. For example, a person who overeats by just 25 calories a day will consume 9,125 excess calories over the course of a year and so will gain 2 1/2 pounds (a pound of body fat is equivalent to 3,500 calories). A woman weighing 125 pounds who starts this pattern at age 20 will weigh 175 pounds by the time she is 40.
To point to overeating as the cause of obesity is overly simplistic, however. Numerous other factors contribute to weight gain, including RMR and physical activity. Nevertheless, a reduction in calorie intake is essential for weight loss.
Physical activity. Variations in physical activity can have a tremendous impact on total daily energy expenditure. A sedentary person may burn just a few hundred calories above RMR while going about daily activities (performing household chores or walking to the mailbox, for example), whereas an athlete can burn an additional 3,000 calories each day through vigorous exercise. Regular exercise not only burns calories but also builds lean muscle mass and raises RMR because muscle requires more energy for maintenance.
Behavioral and psychological issues. Several psychological factors affect weight control. The message to eat often comes from external cues rather than hunger--noon means it's time for lunch, for example. Food and emotions are closely linked; many people use food for comfort or to release tension. The amount of exercise a person engages in is also shaped by habit and attitudes toward physical activity. Some studies suggest that lean people may expend more energy than obese people in ordinary activities, as well as during formal exercise. For example, lean people may walk around (rather than sit) while on the phone, or they may take the stairs rather than an elevator or escalator.
Hormonal (endocrine) abnormalities. An underactive thyroid (hypothyroidism) is often a layperson's explanation for obesity, but even when present, hypothyroidism is rarely a primary cause. Other conditions that may affect weight include polycystic ovary disease, tumors of the pituitary or adrenal glands, an insufficient production of sex hormones, and insulin-producing tumors of the pancreas. Although they are uncommon, these disorders need to be ruled out by a thorough medical evaluation before determining the best course of action to achieve weight loss.
