by NSCA’s Guide to Sport and Exercise Nutrition
Kinetic Select May 2017
The following is an exclusive excerpt from the book NSCA’s Guide to Sport and Exercise Nutrition, published by Human Kinetics. All text and images provided by Human Kinetics.
Field methods for assessing body composition are those that are portable and easy to use for assessment of several people in a short time period. How do these measures compare, and what are the major differences between them?
First, it is important to take a look at the most common field measures that sport nutritionist professionals use. Body mass index is a simple calculation: weight in kilograms/height in meters2. The resulting number categorizes the person as underweight, normal weight, overweight, or obese. Body mass index is a very convenient way to measure obesity rates in a population but should not be used by itself to categorize an individual. Table 11.1 shows the BMI categories used by the World Health Organization.
The National Health and Nutrition Examination Survey (NHANES), a survey research program started in 1959 and directed by the Department of Health and Human Services (a federal government agency), measures BMI in the physical examination component of the survey (both physical exams and interviews are used). \
The NHANES assesses the health and nutrition status of adults and children in the United States and tracks changes over time to determine the prevalence of disease and risk factors for disease (United States Department of Health and Human Services and Centers for Disease Control and Prevention 2009, 2010).
Measuring BMI is noninvasive and requires only an accurate scale and stadiometer (for measuring height). The downside to BMI is that it does not assess actual body composition or distinguish between fat and muscle tissue. Since muscle has greater density than fat and weighs more than fat per volume of tissue, BMI tends to overestimate body fat levels in muscular individuals (Witt and Bush 2005).
It can also overestimate body fat in individuals with large body frames (Ortiz-Hernández et al. 2008). For example, a professional running back who is 5 feet 9 inches (1.75 m), weighs 210 pounds (95.5 kg), and has 8% body fat would have a BMI of 31.2. This BMI classifies him as obese. However, at 8% body fat, the athlete is not obese or overfat. The example illustrates a limitation of using BMI in athletes.
Though BMI may overestimate body fat in muscular individuals, it may underestimate body fat in other populations (Chang et al. 2003; Jones, Legge, and Goulding 2003). A study conducted using NHANES data showed that BMI cannot accurately diagnose obesity, especially in men and the elderly as well as people with intermediate BMI ranges, and therefore, BMI should only be used to assess population-based rates of obesity and not an individual’s status (Romero-Corral et al. 2008).
Body mass index is a tool best used to estimate population-based rates of weight correlated to height and not a tool designed to assess obesity or underweight in single individuals in a clinical setting in the absence of other clinical measures (Piers et al. 2000).
Trainers and coaches commonly turn to skinfold calipers for assessing body composition. Skinfold calipers measure skinfold thickness at various sites on the body. The technician takes measures by grasping a fold of skin and subcutaneous fat with the thumb and forefingers and pulling the fold away from the underlying muscle, then pinching it with the caliper and taking the reading within 2 seconds.
• Skinfold calipers—A tool for measuring skinfold thickness, which can then be used to estimate body fat.
A three-site skinfold is commonly done and includes the chest, abdomen, and thigh on men and triceps, suprailiac, and thigh on women (figure 11.1).
The five most common sites assessed include triceps, subscapular, suprailiac, abdomen, thigh (McArdle, Katch, and Katch 2005). Chest and biceps are additional sites that are sometimes used. The skinfold measures (which should be taken two or three times at each site and then averaged) are then incorporated into equations to predict percent body fat. These are the main advantages of skinfold calipers:
• They are easy to use (once the person is well trained in the technique).
• They do not require much time per person.
• They are noninvasive and inexpensive.
However, there are also several disadvantages. These include interperson variability (if body fat is measured by one person and then months later by another person) and less accuracy when less expensive calipers are used. In addition, over 100 different equations are used to estimate body fat from calipers, and people measure different sites among the seven. All of these factors can lead to errors in reliability, validity, or both. An accurate measure of body composition using skinfold calipers is within ±3% to 5% error of hydrostatic weighing (McArdle, Katch, and Katch 2005).
• Hydrostatic weighing (underwater weighing) —A method of measuring body composition whereby the subject is submerged into a tank of water and body composition is determined based on total body density using Archimedes’ principle of displacement (the weight of displaced fluid can be found mathematically). Underwater weighing assumes that the densities of fat mass and fat-free mass are constant, lean tissue is more dense than water, and fat tissue is less dense than water.
Bioelectrical impedance measures the impedance to the flow and distribution of a radiofrequency, alternating current (Lukaski et al. 1985). Both water and electrolytes influence the impedance of the applied current; therefore BIA measures total body water and then indirectly determines fat-free mass from this measure (Lukaski et al. 1985).
Bioelectrical impedance is convenient, cost-effective, and quick; and operation requires little knowledge. However, it cannot accurately measure short-term changes in body composition nor can it accurately assess body composition in obese individuals (in whom it may underestimate body fat) and very lean individuals (in whom it may overestimate body fat) (Sun et al. 2005). Finally, small changes in fluid balance can affect the measurements (Saunders, Blevins, and Broeder 1998).
• Bioelectrical impedance—A way of assessing body composition by measuring the flow of a small electrical current through the body. This measures total body water, which can be used to determine total fat free mass.
NSCA’s Guide to Sport and Exercise Nutrition will lead you through the key concepts of sport and exercise nutrition so that you can assess an individual’s nutrition status and—if it falls within your scope of practice—develop customized nutrition plans. The book is available in bookstores everywhere, as well as online at the NSCA Store.