A stable weight depends on a balance between the energy you get from food and the energy you use. You use energy during the day in three ways:
- Energy expended during rest (basal metabolism)
- Energy used to break down food (thermogenesis)
- Energy used during physical activity
Basal metabolism accounts for about two-thirds of spent energy. Your body generally uses this energy to keep your temperature steady and the muscles of your heart and intestine working. Thermogenesis accounts for about 10% of spent energy.
When a person consumes more calories than the energy they use, the body stores the extra calories in fat cells (lipocytes). Fat cells function as energy reservoirs. They grow or shrink depending on how people use energy. If people do not balance energy input and output by eating right and exercising, fat can build up. This can lead to weight gain.
Measurement of Obesity
Obesity is determined by measuring body fat, not just body weight. People might be over the weight limit for normal standards, but if they are very muscular with low body fat, they are not obese. Others might be at normal weight or even underweight, but still have excessive body fat. The following measurements and factors are used to determine whether or not a person is overweight to a degree that threatens their health:
- Body mass index (BMI) -- a measure of body fat
- Waist circumference (size around the waist)
- Waist-hip ratio
- Skin fold measurement (anthropometry)
A person's disease risk factors and their BMI may be the most important components in determining health risks with weight.
The Body Mass Index (BMI). The current standard measurement for obesity is the body mass index (BMI). In general, a BMI of 25 - 29.9 means you are overweight. Obesity is a BMI of 30 and above. Obesity is then classified into three categories:
- Class 1: BMI of 30 - 34.9
- Class II: BMI 35 - 39.9
- Class III: BMI of 40 and greater
Review Date: 04/14/2010
Reviewed By: A.D.A.M. Editorial Team: David Zieve, MD, MHA, and David R. Eltz. Previously reviewed by Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital (4/14/2010).