Many factors determine how easy -- or how difficult -- it will be for an individual to lose weight. These include how much you weigh, when you gained the weight, your genetic makeup, how much you eat and how much you exercise.

For most people who are overweight, cutting down on portion size, eating nutritiously, and increasing physical activity will result in weight loss, says Benjamin T. Burton, associate director for disease prevention at the National Institute of Arthritis, Diabetes, and Digestive and Kidney Diseases.

Three groups of people often find it particularly difficult to lose weight, says Dr. C. Wayne Callaway, director of the Center for Clinical Nutrition at George Washington University Medical Center:

Those who chronically starve themselves.

Those who have been fat all their lives.

Those who have a family history of obesity.

Weight loss should be assessed not daily or weekly but at six-week intervals, he says. For sustained weight loss, aim to lose two to four pounds every six weeks. Says Callaway, "All evidence suggests that rapid weight loss is not really sustained." This is true for all but morbidly obese people who exceed their ideal weight by 100 percent or more. For these people, a medically supervised weight-loss program is advised.

For most people, experts say, patient- or client-centered programs, including commercial weight-loss programs or hospital affiliated obesity clinics, are fine.

When selecting a weight-loss program, experts suggest you look for:

Some type of behavior therapy, psychological support and exercise program.

Nutrition counseling and an eating plan that provides adequate nutrition, including foods from the four basic food groups -- meat, poultry, fish, beans; fruits and vegetables; breads and cereals; and milk and cheese.

Emphasis on maintenance after weight loss.

Avoid programs that promise rapid weight loss, use appetite-suppressing or other types of drugs as the focus of treatment, or emphasize products that you are expected to buy.