9.3: Assessing Body Composition
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As noted previously, BMI is the measurement used as a screening tool to quickly and inexpensively identify whether a person is at a healthy weight, is underweight, overweight, or obese. BMI is not a measure of body composition, it will not tell you whether you are overfat, it will only tell you whether you are overweight. Understanding what the body is made up of (muscle, bone, water, and fat) is a better predictor of health, however the tools used to measure body composition can be expensive, intrusive, not widely available, or difficult to standardize across observers or machines. Due to the challenges with measuring body composition, BMI is considered the best method.
When evaluating body composition, there are two general types of evaluations, body fat distribution and body fat percentage.
Body Fat Distribution
Fat is stored as subcutaneous fat, which is fat stored just under the skin, and visceral fat, which is fat stored deeper in the body around the organs. We are all different and one difference we have is where we store body fat. Body fat distribution, meaning where a person stores their fat, has health implications. Visceral fat tends to be store around the abdomen and research indicates high amounts of visceral fat lead to negative health implications including all-cause mortality [5] .
One method for evaluating body composition is to look in the mirror at the outline of the body. People are often described as being either apple shaped, carrying body fat around the abdomen, or pear shaped, carry body fat around the hips. People who are apple-shaped, thus carrying excess visceral fat around the abdomen, are at higher risk of developing health issues.
Besides looking in the mirror, body fat distribution can be measured by calculating a person waist circumference, waist-to-hip ratio, and waist-to-height ratio.
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Waist Circumference
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Stand and place a tape measure around your middle, just above your hipbones. Measure your waist in inches just after you breathe out
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Your waistline may be telling you that you have a higher risk of developing obesity-related conditions if you are
- A man whose waist circumference is more than 40 inches
- A non-pregnant woman whose waist circumference is more than 35 inches
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Your waistline may be telling you that you have a higher risk of developing obesity-related conditions if you are
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Stand and place a tape measure around your middle, just above your hipbones. Measure your waist in inches just after you breathe out
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Waist-to-Hip Ratio
- You can calculate your waist-to-hip ratio by taking your waist circumference and dividing it by your hip circumference. The World Health Organization categorizes high risk as a ratio above 0.85 for women and more than 0.9 for men.
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Waist-to-Height Ratio (WHtR)
- You can calculate your waist-to-height ratio by dividing your waist circumference by your height. A waist-to-height ratio of more than 0.5 may put you at higher risk for heart disease and diabetes. Waist-to-height ratio has been shown to be a good predictor of both BF% and Visceral Fat mass in men and women [6] .
Body Fat Percentage
Body fat percentage is attempting to measure the percentage of your body that is fat-free mass versus fat mass. There are several measurement tools to estimate body fat percentage, each have pros and cons mostly related to difficulty, cost, and availability.
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Bioelectrical Impedance Analysis (BIA)
- Bioelectrical Impedance Analysis is likely the measuring method you are most familiar with. Have you ever used a scale that told you your body fat percentage? If so, you have used BIA. BIA devices emit a low-level electrical current through the body and measure the amount of resistance the current encounters. Based on the level of impedance, a pre-programed equation is used to estimate body fat percentage. Fat-tissue contains little water, making it a poor conductor of electricity; whereas, lean tissue contains mostly water and electrolytes, making it an excellent conductor. The most accurate BIA devices use electrodes on the feet and hands to administer the point-to-point electrical current. Because BIA devices primarily measure hydration, circumstances that may influence hydration status at the time of measurement must be taken into account. Recent exercise, bladder content, hydration habits, and meal timing can cause wide measurement variations and influence accuracy. However, this method is generally inexpensive, often portable, and requires limited training to use, making it a very practical option.
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Skinfold Calipers
- Skinfold analysis is a widely used method of assessing body composition because of its simplicity, portability, and affordability. Some call the skinfold test the “pinch test” because the skin and underlying subcutaneous fat is pinched with calipers to measure the thickness. The assumption of skinfold measurement is that the amount of subcutaneous fat is proportionate to overall body fat. These numbers are plugged into an equation to generate an estimate of body fat percentage. The skinfold test is fairly accurate when administered properly.
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Hydrostatic Weighing (underwater weighing)
- Hydrostatic weighing is a difficult process, but has long been considered the “gold standard” for assessing body composition. A person is weighed when dry and then enters a water tank and is weighed while underwater. Since fat is less dense than muscle tissue, a person with more body fat will weigh less in the water than a similar person with more lean mass. Using the measurements, the density can be determined and converted into body fat percentage. With a small margin of error (around 1-2%) this method is very accurate. Unfortunately, the expense and practicality of building and maintaining a water tank limits access for most. This method is also challenging for people who are afraid of water, don’t feel comfortable in bathing suits, or would be fearful of exhaling all air and sitting underwater without moving for a short time.
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Air Displacement (Plethysmography)
- Plethysmogography works similarly to hydrostatic weighing however measures the displacement of air instead of water. The Bod Pod is most commonly referenced machine. During the test, a person sits in a chamber that varies the air pressure allowing for body volume to be assessed. Air displacement provides a viable alternative for those with a fear of water. Like many other methods, the expense, availability, and training of personnel Air Displacement requires limit accessibility. Additionally, its accuracy is slightly less than underwater weighing.
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Dual X-ray Absorptiometry (DXA)
- Replacing underwater weighing as the new “gold standard,” DXA provides a quick and pain free method for measuring body fat by scanning the body. The patient lays down and is scanned by the DXA machine which takes about 6 minutes. Major disadvantages to this method are its high cost and the need for a welltrained professional to operate the equipment and analyze the results.