How Do We Estimate a Healthy Body Weight? 523
Dual-energy X-ray absorptiometry (DEXA) is the most accurate method of deter- mining body composition; its margin of error is only 1–4 percent. DEXA is a noninvasive method that can estimate three body compartments: fat mass, lean body mass, and bone mass. This noninvasive method uses two low-energy X-ray beams: one detects all tissues including fat mass and bone mass and the other detects only lean body mass. The com- puter then calculates the difference between lean body mass and fat mass to determine the percentage of body fat.
Bioelectrical impedance analysis (BIA) measures the resistance to a low-energy current as it travels through muscle and body fat. The current travels more quickly through lean tissue, which is high in body water and electrolytes, than through fat tissue. The resistance of the fat tissue is used to calculate body composition. BIA is not as accurate as body density tests and can be affected by age, hydration status, and consuming food and alcohol prior to the test.
Anthropometric (relating to body measurement) techniques are the simplest methods available and involve using a skinfold caliper to measure fat in various body locations. The metal calipers are used to pinch the subcutaneous fat at selected sites on the body. A trained technician grasps the skin and fat between the thumb and forefinger and pulls it gently away from the muscle. The caliper exerts a constant pressure while measuring the skinfold thickness in millimeters. These values are then used to calculate percent body fat. When conducted by a trained technician, skinfold caliper tests are fairly accurate.
Waist Circumference Because abdominal fat can be particularly detrimental to health, measuring a per- son’s waist circumference can quickly reveal whether he or she is at increased risk (Figure 14.9). A woman with a waist measurement of more than 35 inches or a man with a measurement of more than 40 inches is at a higher risk for disease than people with slim- mer middles. Carrying extra fat around the waist can increase health risks even if you are not overweight. In other words, a person who may be at a healthy weight based on their height, but who has excess fat around the middle, is at a higher risk for disease.
dual-energy X-ray absorptiometry (DEXA) Method that uses two low-energy X-rays to measure body density and bone mass.
bioelectrical impedance analysis (BIA) Method used to assess the percentage of body fat by using a low-level electrical cur- rent; body fat resists or impedes the current, whereas water and muscle mass conduct electricity.
skinfold caliper Tool used to measure the thickness of subcutaneous fat.
waist circumference Measurement taken at the top of the iliac crest or hip bone; used to determine the pattern of obesity.
▲ Figure 14.9 Measuring Waist Circumference The waist circumference measurement is taken at the top of the iliac crest (top of the hip bone), as shown by the dashed line.
LO 14.4: THE TAKE-HOME MESSAGE The body is composed of lean and fat tissue. Adipose tissue is classified as essential fat or fat that is stored as either subcutaneous or visceral fat. How much fat a person has and the placement of that fat can increase the risk of heart disease, diabetes, and hypertension, especially if the fat is distributed in the abdomen. Hydrostatic weighing, air displacement plethysmography, dual-energy X-ray absorptiometry (DEXA), bioelectrical impedance analysis (BIA), and skinfold measurements are all tech- niques used to determine body composition. Measuring waist circumference can determine whether an individual has excess abdominal (central) fat, which can increase the risk of several chronic diseases.
How Do We Estimate a Healthy Body Weight? LO 14.5 Explain the methods used to estimate a healthy body weight, and the
link between body weight and mortality.
The terms body weight and body composition are not synonymous. Body weight is defined as total mass expressed in either pounds (lb) or kilograms (kg). As you just learned, body composition is the percentage of body weight that is composed of fat and lean body mass. Even though the terms body weight and body composition do not measure the same component, they are often used interchangeably in the popular media.
524 Chapter 14 | Energy Balance and Body Composition
Two common methods used to help individuals estimate whether their own per- cent body fat falls within a healthy range are height–weight tables and body mass index (BMI). These reference standards are indirect estimates of body composition, and there- fore somewhat imprecise, but they can be used as a rough guide for most people.
Height and Weight Tables Are Problematic Height–weight tables have been used since the 1940s in large-scale studies that were designed to investigate the relationship between body weight and disease. The Metro- politan Life Insurance Company developed the most commonly used height–weight table. The company published the Desirable Weights for Men and Women table in 1959 based on data collected from millions of policyholders. The most recent version of the table was published in 1999 and provides a recommended desirable weight range for a given height based on gender and body frame size.
Several factors make the data used in these tables problematic. For example, the data does not represent the American population as a whole. The tables were originally designed with data from 25- to 59-year-olds, which means they may underrepresent older adults and individuals younger than 25 years of age. The researchers did not standardize the original data. For instance, subjects self-reported their height and weight; the weights were measured at different times of the year; and there was no standard procedure regarding wearing shoes or clothing when taking the height and weight measurements. Lastly, the tables were constructed with the assumption that weight is associated with body fat. Today, mostly insurance companies use height–weight tables to determine mortality rates. Most health experts use body mass index rather than height–weight tables to determine healthy weight.
Body Mass Index Is a Useful Indicator of Healthy Weight for Most People Body mass index (BMI) (Figure 14.10) is a convenient method of calculating body weight in relationship to height, and is a useful screening tool to determine an individual’s risk of disease. It is calculated using either of the following formulas:
BMI = body weight (in kilograms)
height2 (in meters)
BMI = body weight (in pounds) * 703
height2 (in inches)
A BMI of 18.5924.9 kg/m2 is considered a healthy weight based on height. A BMI below 18.5 kg/m2 is considered underweight or having a body weight that is below normal, average, or considered healthy. A BMI between 25 and 29.9 kg/m2 is considered overweight, and a BMI between 30 and 39.9 kg/m2 is considered obese. Severe obesity is a BMI at or above 40. Table 14.6 summarizes the BMI criteria for defining these terms in adults, and Table 14.7 compares three ways to classify obesity specifically.
As the BMI decreases below 18.5, or increases above 25, the risk of premature death increases. That risk is modest for those with a BMI between 25 and 29.9;11 however, obese individuals have a 50–100 percent higher risk of dying prematurely than those at a healthy weight. Again, underweight is also risky. Adults with a BMI below 18.5 have a greater risk of premature death than those with a BMI of 18.5–24.9.12 BMI has been shown to cor- relate with health risks associated with excess body fat, and an equation can be used to estimate body fat percentage from BMI (see the Calculation Corner).
While BMI can be useful in determining disease risks, it is important to note that BMI is not a direct measure of the percentage of body fat, and it doesn’t specify if body weight is predominantly muscle or fat.13 Therefore, athletes and people with a high
body mass index (BMI) Calculation of body weight in relationship to height.
healthy weight Body weight in relationship to height that doesn’t increase the risk of developing any weight-related health problems or diseases. A BMI between 18.5 and 24.9 is considered healthy.
underweight Weighing too little for your height; defined as a BMI less than 18.5.
overweight Body weight that increases risk of developing weight-related health problems; defined as having a BMI between 25 and 29.9.
obese Condition of excess body weight due to an abnormal accumulation of stored body fat; a BMI of 30 or more is considered obese.
severe obesity Defined as a BMI greater than 40 or more than 100 pounds over ideal body weight.
How Do We Estimate a Healthy Body Weight? 525
◀ Figure 14.10 What’s Your BMI? A BMI between 18.5 and 24.9 is considered healthy. A BMI over 25 is considered over- weight, and a BMI over 30 is obese. A BMI under 18.5 is considered underweight, and can also be unhealthy.
6’3″ 6’2″ 6’1″
5’0″ 4’11” 4’10”
50 75 100 125
18.5 25 BMI (Body Mass Index)
Weight (pounds) †
* The height is without shoes. † The weight is without clothing.
175 200 225
Classification BMI (kg/m2)
Underweight 6 18.5 Normal weight 18.5–24.9
Overweight 25–29.9 (also defined as being 10–15 pounds above a healthy weight)
Severe obesity 7 40
TABLE 14.6 Definitions of Underweight, Overweight, and Obesity in Adults
Obesity Is Classified by . . .
Percent of body fat Women: 7 32, Men: 7 25,
Distribution of body fat Excess subcutaneous and visceral fat stored in the upper body (abdomen and waist); referred to as central or android obesity
• Waist circumference 7 35 inches in women • Waist circumference 7 40 inches in men Excess subcutaneous fat stored in the lower body (hips, buttocks, thighs); referred to as gynoid obesity
• Waist-to-hip ratio 6 0.8 in women • Waist-to-hip ratio 6 0.95 in men
Body mass index (BMI) Women: 7 30 kg/m2
Men: 7 25 kg/m2
Source: Adapted from A. G. Kazaks and J. Stern, Nutrition and Obesity. Assessment, Management, and Prevention (Burlington, MA: Jones & Bartlett Publishing, 2013).
TABLE 14.7 Ways to Classify Obesity in Adults