This is an excerpt from Exercise Testing and Prescription Lab Manual-2nd Edition by Edmund O. Acevedo & Michael A. Starks.
We know that a high amount of fat weight (obesity) is a risk factor for heart disease, diabetes, cancer, and other health problems (Howley and Franks 2007). Excess fat weight makes movement inefficient and difficult. A high lean body weight allows the body to accomplish work efficiently and expend more calories even at rest.
The term body composition refers to the percentage of body weight that is fat (% body fat) compared with total lean mass. Its measurement is based on the assumption that body weight can be divided into lean body weight and fat weight. Lean body mass encompasses all of the body's nonfat tissues, including the skeleton, water, muscle, connective tissue, organ tissues, and teeth. The fat component includes both the essential and nonessential fat stores. Essential fat includes fat incorporated into organs and tissues; nonessential fat is primarily within adipose tissue. Evaluation of body composition typically is included as part of a health screen or physical fitness assessment.
Body composition can be measured in many ways, including both laboratory and field techniques. Although they are not as accurate, anthropometric methods provide a more practical and less expensive alternative than hydrostatic weighing to estimate body composition. Thus, this lab will focus on formulas and equations involving skinfolds, circumference, waist-to-hip ratio, and body mass index (BMI).
Interpreting Body Composition Data
It is important to distinguish between being overweight and being overfat (obese). Overweight is defined as exceeding the normal or standard weight for a specific height and skeletal frame size, when grouped by gender. Being overweight is not necessarily undesirable, especially when the lean body mass is high. Measuring body density and calculating fat weight and lean body weight allow a more accurate method of estimating desired weight rather than using height-and-weight tables, which do not determine fat and lean weight. Overfat, or obesity, is defined as the state of having excess body fat. Although no universal agreement exists on the specific percentage of body fat that constitutes obesity, it is recommended that men and women maintain body fat percentages of 10 to 22% and 20 to 32%, respectively.
Interpretation of body composition must be individualized to each person. As a fitness professional you should be aware of the wide range of normal values and should not encourage all participants to achieve the same particular value. The principle of variability is important. Humans vary widely on any trait that can be measured and body composition is no exception. You must recognize this and take it into account in interpreting body composition data.
- Body mass index. The body mass index (BMI), or Quetelet index, examines body weight relative to height. It is calculated by dividing body weight (in kg) by height (in m) squared (wt/ht2). BMI is a good indicator of total body composition in population-based studies and is related to health outcomes. As the BMI increases, mortality from heart disease, cancer, and diabetes also increases. Significant increases in risk begin at a BMI of about 30.0 kg/m2 for men and women.
- Circumference. Measurement of body girths may be reasonably accurate in estimating body fat (from prediction equations) in unfit subjects. These measurements, however, do not detect changes in body composition over time when lean tissue increases and fat mass decreases. Thus, for consistent evaluation of body fat percentage, they appear to be impractical.
- Waist-to-hip ratio. As mentioned earlier, excessive body fat is a health hazard. The distribution of body fat also affects a person's health. Levels of subcutaneous body fat levels in the upper body (waist measurement) and lower body (hip measurement) are distributed differently by sex, age, body type, and activity level. Fat in the abdomen (upper body) is associated with greater morbidity and mortality than is fat distributed below the abdomen (lower body). Ideally, waist circumference should be smaller than hip circumference. Waist-to-hip ratios above .95 for men and .85 for women are considered to place the person at significantly increased risk for obesity-related health problems.
- Skinfold. Results from the skinfold method of estimating body composition correlate fairly well with hydrostatic weighing. Measuring the thickness of skinfolds involves grasping a fold of skin and fat and holding it away from the underlying muscle. The reliability of skinfold measurements depends on meticulous attention to detail in the techniques. Practice the techniques extensively and be precise in measuring the exact anatomical locations indicated.
You can use skinfold measurement values in several ways: You can total the values from several sites to arrive at the sum of skinfolds. You can use the sum of skinfolds to rate individuals within a given group. You can also use the sum of skinfolds to evaluate changes in body fat following dietary restriction, exercise conditioning programs, or a combination of them. You can also use skinfold measurements with mathematical equations to predict percentage of body fat. Keep in mind that equations derived from one segment of the population do a poor job of predicting percent fat for other populations; the equations are applicable only to groups similar in age and activity level to those from which the equations are derived. When well-trained, experienced technicians perform the measurements, skinfold-based estimates of body fat percentages are generally within 3.5% of measurements obtained using the underwater weighing technique.