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Understanding Obesity -- the Basics

What Is Obesity?

The medical definition of obesity is based on the amount of body fat a person has. A person can weigh more than what is thought to be healthy without being obese. For example:

  • A person may weigh too much because his or her body retains too much water, called edema. Though this is not a healthy situation, the person is not obese.
  • Some athletes, such as football players and body builders, may weigh more than what is normally considered a healthy weight, but their excess weight is muscle, not fat.

Until recently, a person was thought to be obese if he or she weighed at least 20% more than his or her ideal body weight. However, ideal body weight tables have been replaced by the body mass index (BMI). The BMI is based on your height and weight, and it gives a better -- but not perfect -- estimate of body fat.

BMI is calculated by dividing a person's weight (in kilograms) by his or her height (in meters, squared). BMI can also be calculated by multiplying weight (in pounds) by 705, then dividing by height (in inches) twice.

A BMI of 30 or more is considered obese and a BMI between 25 to 29.9 is considered overweight.

Attitudes about the causes of obesity are changing as we learn more about the condition. People who are obese were once thought to lack self-control and have poor eating habits. Though it is true that many Americans eat too much and exercise too little, recent research has shown that genetic factors also play a role in obesity.

Because of this research, obesity is now thought to be a chronic disease. This means dieting once in a while is not enough to take care of the problem. Obesity must be treated for the rest of the person's life if it is to be controlled.

What Are the Risks of Obesity?

People who are obese are much more likely to have certain health problems, such diabetes, high blood pressure, heart disease, and stroke. This is even more likely if the person also has high cholesterol or triglyceride (fat) levels in the blood, as obese people often do.

A person who is very obese may have breathing problems including sleep apnea. Women who are obese have a greater risk of uterine, breast, and gallbladder cancer. Men and women who are obese have an increased risk for colorectal cancer and gallstones. Men who are obese have an increased risk for prostate cancer. Extra weight puts more stress on the joints than normal, especially in the legs and lower back. Because of this, people who are obese are more likely to develop arthritis, especially in the back, knees and hips.

WebMD Medical Reference

Reviewed by Kimball Johnson, MD on September 21, 2012

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