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
These guidelines are very important for people at risk for diabetes, heart disease, or certain cancers. It is also used to determine treatment approaches such as when surgery may be appropriate. The higher the BMI, the greater the risk for significant health problems.
Calculating Body Mass Index. A person's body mass index is calculated as follows:
- Multiply one's weight (in pounds) by 703.
- Divide that answer by height in inches.
- Divide that answer again by height in inches.
For example, a woman who weighs 150 pounds and is 5 feet 8 inches (or 68 inches) tall has a BMI of 22.8.
You can check your BMI at the Centers for Disease Control and Prevention BMI calculator.
Waist Circumference and Waist-Hip Ratio. The extent of abdominal fat can also be used in assessing risk of disease. Some studies suggest that:
- Women whose waistlines are over 31.5 inches and men whose waists measure over 37 inches should watch their weight.
- A waist size greater than 35 inches in women and 40 inches in men is associated with a higher risk for heart disease, diabetes, and impaired health.
Evidence strongly suggests that more body fat around the abdomen and hips (the apple-shape) is a more consistent predictor of heart problems and health risks than BMI.
The distribution of fat can be evaluated by dividing waist size by hip size. For example, a woman with a 30-inch waist and 40-inch hip circumference would have a ratio of 0.75; one with a 41-inch waist and 39-inch hips would have a ratio of 1.05. The lower the ratio the better. The risk of heart disease rises sharply for women with ratios above 0.8 and for men with ratios above 1.0.Click the icon to see a depiction of the waist-to-hip ratio.
Anthropometry. Anthropometry is the measurement of skin fold thickness in different areas, particularly around the triceps, shoulder blades, and hips. This measurement is useful in determining how much weight is due to muscle or fat.