If you've gone on and off strict diets in the past, then your body has been programmed to react in a certain way. The body naturally responds to severe calorie restrictions as though it's starving. Rather than losing fat, the body holds on to it, because fat is where fuel is stored. The body begins to break down muscle instead, which sets the stage for an unfortunate cycle of weight gain, not loss. Fat has a slower metabolism (burning fewer calories) than muscle. With less muscle, the body needs even fewer calories, so losing fat becomes harder.

Highly restrictive diets often make people feel deprived, which may lead to binge eating. Because these diets are hard to maintain, dieters simply drop restrictive routines altogether after a while and return to their previous high-fat diet. But now their bodies need fewer calories than ever. In the end, they carry more fat than they ever did before dieting.

There are two keys to breaking out of this cycle:

• Adopt a healthy pattern of low-fat eating that consists of neither starving yourself nor bingeing.
• Exercise regularly. Over time, your body will begin to restore muscle. This will increase your need for calories and you'll start to lose body fat.

It's also important that you think of these positive changes as a lifelong commitment to a healthier lifestyle. This is different from going on and off a diet or fitness plan. Instead, you'll eliminate a certain number of surplus calories from your daily food intake for good. Also, you'll make physical activity—and all the benefits it brings—an important and permanent part of your daily life.

Family history and genetics
Not all aspects of your metabolism are under your control. If your parents tend to be overweight, you may have a tendency for being overweight too. That's because some people inherit characteristics that make them gain weight more easily than others do.

But even though you may not be able to change your basic body shape, your genes are not necessarily your destiny. Studies show that, despite characteristics people may inherit, environmental factors account for most family-shared obesity. In other words, children of overweight parents often become overweight themselves by following their parents' unhealthy eating patterns and inactive lifestyle. Those who grow up in a house filled with high-fat foods are likely to eat those foods and may become overweight. Through exercise and adopting a healthy diet, however, children of obese parents can achieve and maintain a healthy weight.

Your age
Most people lose muscle mass and gain fat as they grow older. This is not necessarily a result of aging: Many simply become less active or eat more food in their later years. While you can't turn back the clock, you can stay fit and healthy as you age. Studies show that older people who exercise regularly and eat a healthy diet do not gain weight with age. Even after age 50, people can build muscle tissue back up, achieving a better balance between muscle and fat, and losing weight as a result.