Obesity, is the condition of having an excessive amount of body fat. Obesity increases a person's risk for many illnesses, including diabetes, heart disease, hypertension (high blood pressure), arthritis, and cancer. However, it is difficult to measure body fat accurately. Physicians use a measurement called body mass index (BMI) to determine if a person is obese. The body mass index represents the relationship between a person's weight and height.

You can calculate your BMI by dividing your weight in kilograms by the square of your height in meters. For example, a 130-pound (59-kilogram) woman who is 5 feet 4 inches (1.6 meters) tall has a BMI of 59 divided by 2.56 (that is, 1.62), or about 23. Another way to calculate BMI is to multiply your weight in pounds by 703 and then divide that figure by the square of your height in inches.

In children, the normal range of BMI changes with age and differs between boys and girls. Physicians use BMI-for-age growth charts to determine obesity in children. In adults, a BMI between 18.5 and 24.9 is considered normal. A BMI of 25 to 29.9 is considered overweight and a BMI of 30 and above is considered obese.

People in industrialized countries tend to be heavier than people who live in underdeveloped countries. A high percentage of the United States population is overweight or obese. Approximately 65 percent of adults and about 15 percent of children and adolescents in the United States are considered overweight or obese.