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Body Composition Overview

Your body composition is a barometer of health and fitness. Tracking it, along with factors like strength, mood, productivity, and sleep, can help you gauge the effectiveness of your training and nutrition program. You may even be able to quantify an ideal body composition range at which you feel and perform best.

Body Mass Index

You may recall reading a few years back a shocking news headline reporting the high level of obesity in the NFL. These headlines were based on a study published in the Journal of the American Medical Association, which evaluated the body mass index (BMI) of more than 2,000 NFL players.

This particular study found that 56 percent of the players evaluated were obese. How can this be? These are some of the best athletes in the world, competing at the highest of levels. If more than half of these athletes are obese how do they achieve such amazing feats of athleticism? The answer to both of these questions lies in the tool used to categorize these footballers as obese.

BMI is simply a relationship between weight and height. It does not differentiate between lean body mass (muscle) and fat mass. While this is a useful tool for quickly evaluating the average individual who sits in front of his computer and is never active, it’s not necessarily an ideal measurement of body fat for those of us who work out or play sports.

Body Composition Analysis

Body composition analysis is a more appropriate tool to evaluate the functional weight of an athletic population. The origins of body composition analysis can be traced back to the 19th century. Justus Von Liebig, a biochemist, first recognized that the human body is composed of various substances similar to those found in the food we eat. He also discovered that body fluids contain more sodium and less potassium than the body’s tissues. These findings would lay the groundwork for the research of Lawes and Gilbert.

In 1859, these two scientists showed the inverse relationship between body water and fat content. Pfeiffer would later, in 1887, show that the variation in animal body water could be decreased when fat-free mass was used to express the data. These early scientists essentially introduced the concept fat-free mass and fat mass, through the use of body water content. They showed that body fat has relatively low water content and fat-free mass has relatively high water content.

Albert Behnke applied this relationship to Archimedes’ principle (“An object is buoyed up by a force equal to the weight of the water displaced.”) and developed a body-composition assessment based on body density. Then, in 1953, Keys and Brozek elaborated on the densitometry technique with a more detailed study. These early body-density tests led to the use of underwater hydrostatic weighing to determine body-fat percentage.

How to Calculate Body Composition

The modern era of science has led to the development of numerous body composition assessments such as skinfold measurements, bioelectrical impedance, and Dual Energy X-ray Absorptiometry (DEXA). Which technique is best for you?

Hydrostatic Weighing

This technique measures a subject’s weight on land and then, again, when totally submerged in water. Then, based on Archimedes’ principle, the volume and density of the subject are calculated. Density is then plugged into a proven equation to determine body fat percentage. The density of muscle and bone is more than that of water while the density of fat is less. Therefore, a person with more lean mass will weigh more under water than a person with more fat mass.

  • Pros: This technique is relatively reliable and accurate, provided all criteria are met and was once considered the “gold standard” of body composition analysis.
  • Cons: Despite its popularity, hydrostatic weighing has limitations and possible sources of error. The most obvious limitation is the willingness of the subject to remain underwater while performing a forced maximal exhalation. The subject must feel very comfortable in the water. If this is not the case, the hydrostatic weighing test will not be accurate. In addition, expensive equipment and a large space are required. Bone and muscle density as well as residual lung volume must be estimated, serving as additional sources of potential error.


This technique is conducted by measuring the thickness of skinfolds at predetermined sites on the body, using a special device called a caliper. There are several variations of this technique. The two most common variations are the 3-site and the 7-site. In general, the more sites evaluated, the more accurate the analysis. The sum of these measurements is then inserted into an equation (e.g., Jackson-Pollock) along with sex, age and weight to determine the subject’s body- fat percentage.

  • Pros: Skinfold analysis is inexpensive, portable and requires minimal equipment. It is also relatively accurate, provided the same trained technician is used, when compared to hydrostatic weighing (+/- 3%).
  • Cons: Skinfold analysis only measures subcutaneous fat (fat directly underneath the skin). Visceral and intra-abdominal fat are not reflected. Therefore, measurements may not reflect true body fat. To ensure reliability, the same trained technician must administer the analysis. This poses a problem for subjects on the move. In addition, skinfold measurements are difficult to obtain from overly obese and overly lean subjects.

Bioelectrical Impedance (BEI)

BEI measures the body’s resistance to a painless electrical current. There are several equipment choices when utilizing BEI, ranging from a bathroom scale, which is relatively inexpensive, to a more clinical electrode model, which can be quite pricey.

The bathroom-scale model simply requires the subject to stand on two electrodes that send a small current through their body, measuring the impedance to this resistance current. The more expensive clinical models require the subject to place electrodes on one hand and one foot, which then send a small current through the subject’s body, again measuring impedance to this current. As previously stated, lean body mass contains a greater percentage of water than does fat mass, and water is a great conductor of electricity. Therefore, a leaner person will impede the current less than an over fat person.

  • Pros: BEI is a quick, portable, and minimally invasive technique. Depending on the equipment used, it can also be relatively inexpensive.
  • Cons: This test is sensitive to the hydration level of the subject. Consequently, changes in hydration status will affect the accuracy of this technique. Skin temperature will also affect this analysis. The warmer the environment, the less impedance, and the colder the environment, vice versa. Even when both of these variables are at normal levels, BEI tends to overestimate body fat percentage when compared to hydrostatic weighing.

Dual Energy X-ray Absorptiometry (DEXA or DXA)

This technique requires a person to lay motionless on a table similar to that of an x-ray or CAT scan. Then a slow moving arm scans the length of the subject’s body, using several low-level x-rays. These x-rays are able to assess the lean body mass, fat mass, and bone mineral content of the subject. The entire test is complete in approximately 10-15 minutes. A visual image of the subject’s body and composition is provided along with bone-mineral content and density. Whole-body scans as well as segmental scans are available.

  • Pros: This is one of the most accurate and reliable of all the reviewed techniques. The ability to measure bone mineral content is what sets this technique apart from hydrostatic weighing. The procedures of this technique are also favorable. The subject simply has to lay motionless on a table. There is no need to remove any clothing and water is not involved.
  • Cons: Because of this technique’s accuracy and reliability, it is very expensive. These machines cost thousands of dollars. Consequently, they are generally only found in research or clinical settings. A single scan can cost upwards of $300.

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