12 Body Composition
Body Mass Index (BMI)
BMI is a useful measure of overweight and obesity. It is calculated from your height and weight. BMI is an estimate of body fat and a good gauge of your risk for diseases that can occur with more body fat. The higher your BMI, the higher your risk for certain diseases such as heart disease, high blood pressure, type 2 diabetes, gallstones, breathing problems, and certain cancers.
Although BMI can be used for most men and women, it does have some limits:
- It may overestimate body fat in athletes and others who have a muscular build.
- It may underestimate body fat in older persons and others who have lost muscle.
How is obesity measured?
Measuring the exact amount of a person’s body fat is not easy. The most accurate measures are to weigh a person underwater or in a chamber that uses air displacement to measure body volume, or to use an X-ray test called Dual Energy X-ray Absorptiometry, also known as DEXA. These methods are not practical for the average person, and are done only in research centers with special equipment.
There are simpler methods to estimate body fat. One is to measure the thickness of the layer of fat just under the skin in several parts of the body. Another involves sending a harmless amount of electricity through a person’s body. Results from these methods, however, can be inaccurate if done by an inexperienced person or on someone with extreme obesity.
Because measuring a person’s body fat is difficult, health care professionals often rely on other means to diagnose obesity. Weight-for-height tables, used for decades, have a range of acceptable weights for a person of a given height. One problem with these tables is that there are many versions, all with different weight ranges. Another problem is that they do not distinguish between excess fat and muscle. According to the tables, a very muscular person may be classified obese when he or she is not. The Body Mass Index (BMI) is less likely to misidentify a person’s appropriate weight-for-height range.
Body Mass Index
The BMI is a tool used to assess overweight and obesity and monitor changes in body weight. Like the weight-for-height tables, BMI has its limitations because it does not measure body fat or muscle directly. It is calculated by dividing a person’s weight in pounds by height in inches squared and multiplied by 703.
Men and women can have the same BMI but different body fat percentages. As a rule, women usually have more body fat than men. A bodybuilder with a large muscle mass and low percentage of body fat may have the same BMI as a person who has more body fat. However, a BMI of 30 or higher usually indicates excess body fat.
A BMI of 25 to 29.9 is considered overweight. A person with a BMI of 30 or higher is considered obese. Please review your findings with your health care provider if your BMI is outside of the normal range.
Waist Circumference
Measuring waist circumference helps screen for possible health risks that come with overweight and obesity. If most of your fat is around your waist rather than at your hips, you’re at a higher risk for heart disease and type 2 diabetes. This risk goes up with a waist size that is greater than 35 inches for women or greater than 40 inches for men. To correctly measure your waist, stand and place a tape measure around your middle, just above your hipbones. Measure your waist just after you breathe out.
The table Risks of Obesity-Associated Diseases by BMI and Waist Circumference provides you with an idea of whether your BMI combined with your waist circumference increases your risk for developing obesity-associated diseases or conditions.
Body Fat Distribution
Health care professionals are concerned not only with how much fat a person has, but also where the fat is located on the body. Women typically collect fat in their hips and buttocks, giving them a “pear” shape. Men usually build up fat around their bellies, giving them more of an “apple” shape. Of course, some men are pear-shaped and some women become apple-shaped, especially after menopause.
Excess abdominal fat is an important, independent risk factor for disease. Research has shown that waist circumference is directly associated with abdominal fat and can be used in the assessment of the risks associated with obesity or overweight. If you carry fat mainly around your waist, you are more likely to develop obesity-related health problems.
Women with a waist measurement of more than 35 inches and men with a waist measurement of more than 40 inches may have more health risks than people with lower waist measurements because of their body fat distribution.
Learning Activity
The BMI Calculator is an easy-to-use online tool to help you estimate body fat. The higher your BMI, the higher your risk of obesity-related disease.
Use the BMI Calculator or BMI Tables to estimate your body fat. The BMI score means the following:
| BMI Score | |
|---|---|
| Underweight | Below 18.5 |
| Normal | 18.5–24.9 |
| Overweight | 25.0–29.9 |
| Obesity | 30.0 and Above |
Show Sources
Sources
Body Mass Index (BMI): Assessing Your Weight and Health Risk, NHIBI, NIH, http://www.nhlbi.nih.gov/health/public/heart/obesity/lose_wt/risk.htm
How is obesity measured?: Understanding Adult Obesity, WIN, NIDDK, NIH, http://win.niddk.nih.gov/publications/understanding.htm