Why People Become Overweight.

Everyone knows some people who can eat ice cream, cake, and whatever else they want and still not gain weight. At the other extreme are people who seem to gain weight no matter how little they eat. Why? What allows one person to remain thin without effort but demands that another struggle to avoid gaining weight or regaining the pounds he or she has lost?

The simple answer is that your weight depends on the number of calories you consume, how many of those calories you store, and how many you burn up. The balance of calories stored and burned depends on your genetic make-up, your level of physical activity, and your resting energy expenditure (the number of calories your body burns while at rest). If you consistently burn all of the calories that you consume in the course of a day, you will maintain your weight. If you consume more energy (calories) than you expend, you will gain weight.

Excess calories are stored throughout your body as fat. Your body stores this fat within specialized fat cells (adipose tissue) — either by enlarging fat cells, which are always present in the body, or by creating more of them. If you decrease your food intake and consume fewer calories than you burn up, or if you exercise more and burn up more calories, your body will reduce some of your fat stores. When this happens, fat cells shrink, along with your waistline.

The more complex issue is why some people store more calories than they burn. Is it a matter of will power? Do overweight people simply eat too much and exercise too little? Until recently, even weight-loss experts could not agree on the answers.

Some experts argued that the explanation was genetic: that some people are genetically destined to be thin, others to be overweight. They suggested that these people have thrifty metabolisms, which conserve energy and predispose them toward fat storage. Other experts pointed out that many features of modern life promote weight gain. For example, we don’t get enough physical activity because we drive to work and spend much of our day sitting at computer terminals. We have trouble finding the time to go to the gym, play a sport, or exercise in other ways because we work long hours. With the majority of mothers now in the work force, working parents have the extra pressure of coming home and caring for children after a long workday. Perhaps we overeat because practically everywhere we go — shopping centers, sports stadiums, movie theaters, and so on — fast-food restaurants and snack bars tempt us with oversized portions of high-calorie foods and drinks. In short, these environmental factors lead us to eat too much and get too little physical activity, with the result that many people are overweight or obese.

Still other experts stressed that people’s eating habits and weight are related to how they handle stress and cope with difficult situations. Many people who are overweight also suffer from depression and other emotional conditions that may exacerbate their tendency to overeat.

Now, weight-loss researchers recognize that all three of these factors — genetics, environment, and psychological issues — can contribute to weight disorders. Most overweight people are genetically predisposed to being overweight. But many of these people wouldn’t be overweight in an environment that promoted healthier food choices and encouraged physical activity. Because of these genetic and environmental influences, most people who become overweight have trouble losing weight and keeping it off. This difficulty understandably leads to frustration that can cause depression, anxiety, binge eating disorder, and even bulimia. These psychological complications may interfere with healthy eating and exercise habits, leading to further weight gain and a vicious cycle that only magnifies the problem. Many people have the experience of losing weight only to gain back even more and then repeat this cycle many times over the years.

These three factors — genetics, environment, and psychology — do not affect all overweight people to the same degree. For some people, the cause is so strongly genetic that they would be overweight regardless of other factors. For others, the genetic predisposition to be overweight is less powerful, and environmental factors have the dominant role. The foods these people eat; how much, how often, and when they eat; and the lack of time for physical activity conspire to make them gain weight. For some people, emotions play a dominant role, either because they tend to overeat in response to stressful situations or because they use food to diminish the effects of psychological trauma.

To optimize your chances for maintaining a healthy weight, think about which of these factors contributes to your weight problem. Understanding the role of these factors can help you determine the best approach to achieving and maintaining a healthy weight.