In actuality, there are certain factors that can affect fat-free mass composition and density. These include growth and maturation, physical training, aging, race, and sex. Some or all of these factors need to be taken into account when using densitometry for body composition assessment.

How does the BodPod measure body volume? The BodPod uses a technique called Air Displacement Plethysmography ADP. This technique relies on the physics of Boyle's Law, which states that pressure and volume vary inversely with one another.

In other words, as pressure goes up, volume goes down, and vice versa. Monitoring pressure changes in a closed chamber allows one to calculate volume.

The volume of the empty measurement chamber see schematic is determined first. The test is then performed with the subject inside, and that volume is measured.

In the second measurement, the chamber volume has been reduced by an amount equal to the subject's volume. So by subtraction, the volume of the subject can be calculated. To obtain an accurate volume measurement using ADP, lung volume must be taken into account.

The BodPod performs a separate test and measures the average lung volume, because the subject is breathing normally during the initial body volume test. Other densitometry techniques require the subject to empty their lungs during a test. back to densitometry. All Rights Reserved.

Contact Us. Densitometry Theory Densitometry: Estimating body composition from body density. Although the bone mineral is a small percentage of the fat-free mass, it contributes significantly to the density of the fat-free mass.

As a consequence, changes in the amount of bone mineral have a greater influence on the total density of the fat-free mass than might otherwise be expected.

This creates potential inaccuracies in the measurement of total body fat with UWW in which the amount of bone mineral is assumed, rather than measured. If the density of the fat-free mass is lower because of a decreased total body bone mineral content, the total body percent fat will be overestimated by UWW [ 5 ].

The converse is also true; if the density of the fat-free mass is higher because the total body bone mineral is greater than assumed, UWW will underestimate total body percent fat.

The technique calls for the complete submersion of the individual in a tank of water. The individual is asked to exhale as much as possible, and then, while holding their breath, they are submerged and weighed underwater. Under ideal circumstances, no clothing of any kind is allowed for the measurement.

This requirement is often waived, but this will affect the accuracy of the measurement to a small extent as well. The amount of air left in the lungs after completely exhaling, the residual lung volume, must be measured since this is expected to provide some buoyancy to the body while underwater.

In some institutions, the residual lung volume is estimated, rather than measured, using equations that are specific for age, height, and gender.

Consequently, the weight of the body underwater reflects the weight of the body minus the weight of the fluid that is displaced by the volume of the body.

When combined with knowledge of the residual lung volume, the volume and density of the body can be estimated. The classic equation used to calculate the density of the fat mass from the total body density is the equation from Siri [ 6 ] in which In recent years, however, equations have been developed for the calculation of fat density that is age and gender specific.

Skinfold Measurements Body fat can be estimated from multiple measurements of skinfold thickness. This is also considered a two-compartment method. In this method, it is assumed that the distribution of subcutaneous fat and internal fat is similar in everyone.

This is a major assumption that is not necessarily valid. Nevertheless, there are equations that allow the calculation of percent body fat from skinfold thickness for men and women. The technique requires that the skinfold thickness be measured at ­multiple sites.

There are calipers made specifically for this purpose such as the Lange Skinfold Caliper and the Harpenden Skinfold Caliper. The number of sites measured varies from 3 to 7. The 7-site method calls for measurements of skinfold thickness at the chest, triceps, subscapular region, axilla, suprailiac region, abdomen, and thigh.

The more commonly used 3-site method calls for measurements at the chest, abdomen, and thigh in men and at the triceps, thigh, and suprailiac region in women. The measurement of skinfold thickness at the various sites is then used in an equation to calculate the total body density.

Equations have been developed by Durnin and Womersley [ 7 ] as well as Jackson and Pollock [ 8 , 9 ] for this purpose, although Jackson and Pollock also combined gluteal or waist circumference with skinfold measurements to calculate body density.

The classic formula to calculate total body density from the 7-skinfold thickness measurement in men is In women, the corresponding formula is For men For women, the corresponding equation is Variations of these equations from other authors exist as well.

With any of these equations, once the total body density is known, an equation such as Today, these calculations are readily performed by computer programs. The accuracy and reproducibility of skinfold measurements are highly dependent on the skill of the individual making the measurements.

Bioelectrical Impedance Analysis Bioelectrical impedance analysis BIA is also considered a 2-compartment method. In this technique, the individual commonly stands barefoot on a metal foot plate from which an extremely low-voltage electric current is sent up one leg and then down the other. The individual may also be asked to lie on a table with electrodes connected to both legs after which an undetectable low-voltage electric current is transmitted through the body from the electrodes.

Because fat is a very poor conductor of electricity and water, a component of the fat-free mass, is a very good conductor of electricity, the resistance to the electric current can be used to estimate the percent body fat.

BIA is a popular method, often found in health clubs because of its ease of use and short measurement time less than 1 min. It is generally considered to overestimate body fat in lean individuals and underestimate body fat in obese individuals.

Because BIA results are highly dependent on total body water, the hydration status of the individual can profoundly affect the results. As a consequence, individuals are advised to abstain from eating or drinking 4 h prior to the test, avoid exercise within 12 h of the test, and abstain from alcohol for 48 h prior to test.

Immediately prior to testing, the individual is asked to empty their bladder. Diuretic use can also affect the results obtained with BIA. Air Displacement Plethysmography Air displacement plethysmography can be considered analogous to underwater weighing, except that it is the displacement of air instead of the displacement of water that is used to measure the density and volume of the body.

One technique using air displacement is called the BOD POD ® COSMED USA, Inc. This is an enclosed, egg-shaped, capsule-like structure.

An individual undergoing the measurement sits very still within the capsule and breathes quietly during the 5—8 min measurement. A swim cap and tight fitting clothing are generally worn. As in underwater weighing, it is necessary to determine the residual lung volume, by either measurement or estimation.

The equipment is relatively expensive which limits its availability, although less expensive models have become available. Measurements of body fat with the BOD POD ® have been reported to be highly correlated with those made by underwater weighing [ 10 ].

A computerized spectrophotometer is used in this method. A probe which emits an infrared light is placed on the body. The infrared light passes through the fat and muscle and is then reflected back to the probe. The reflected light is quantified and used to calculate body density.

The method is considered quite safe and is fast, convenient, and inexpensive. It requires no particular patient preparation. The NIR unit itself is generally small and portable.

The Futrex ® and Futrex ® Futrex Inc. The accuracy of NIR has been compared to skinfolds and BIA using UWW as the reference technique [ 12 — 14 ]. In general, the results obtained from skinfold measurements, BIA, and NIR were all highly correlated with the results from UWW.

At the extremes of weight, skinfold measurements appeared to be more accurate than BIA or NIR when UWW was used as the reference measurement, but all three performed reasonably well in normal weight individuals.

Dual-Energy X-Ray Absorptiometry Dual-energy X-ray absorptiometry DXA is a 3-compartment method for the assessment of body composition because bone mineral is measured as well as the fat and fat-free mass.

Unlike UWW or air displacement plethysmography, no assumption needs to be made regarding the amount of bone mineral because it is measured with the technique.

Values are also not affected by the residual lung volume, making it unnecessary to either measure or estimate this value.

Body volume was measured using either the pressure-differential method Dell et al. As discussed earlier, the overestimation of TBW by tritium, particularly in the infant, may introduce error to the four-compartment approach of estimating fat.

Recently, Lewis et al. NMR's potential for the analysis of body composition appears promising; also with NMR imaging, regional distribution of body fat can be analyzed Fuller et al.

Further developments of this technique may result in the measurement of total body fat without the use of assumptions as in the compartmental approaches. Progress has been made in the development of methods to measure composition at various regions of the body.

Body fat is calculated from these measurements using equations that establish a relationship between these measurements and body fat estimated by another indirect method. Such methods, which will not be discussed in detail, range from simple anthropometric measures used primarily for population studies to sophisticated computerized methods in a research setting.

For all these methods, validation has been primarily with another indirect method; that is, the values obtained were compared with reported body fat values in the literature or compared against values obtained by other indirect methods performed on the same individual.

The least expensive and most frequently used method uses calipers to measure skin-fold thicknesses at specific sites. Other methods include soft-tissue radiography Garn, and ultrasonography Borkan et al.

More recently, infrared interactance has been proposed as a rapid, safe, and noninvasive method to measure subcutaneous fat in both research and field settings Conway et al. Numerous studies have attempted to validate the extrapolation of subcutaneous fat thickness measured at a number of sites on the body to total body fat and to establish subcutaneous fat thickness as a "standard" for the assessment of total body fat Durnin and Rahaman, Although the thickness of subcutaneous fat is roughly proportional to the total weight of body fat, body fat calculated by this method may be inaccurate and misleading because of the variation among population norms.

Equations are being developed to overcome this difficulty; specific formulas for body fat estimation are suggested for specific population groups Lohman, Interest in adapting complex diagnostic tools to estimate body fat is increasing. Images depicting fat and muscle of body regions can be obtained with computerized axial tomography Borkan et al.

Sophisticated software allows total body fat to be computed from a series of cross-sectional fat areas along the length of the body.

Although all these techniques show great potential in the estimation of body fat, they are expensive and relatively unavailable for routine measurements. Furthermore, a degree of radiation exposure is involved with both computerized axial to-mography and dual-photon absorptiometry methods.

Many indirect methods of varying degrees of complexity are available for estimation of body fat. Most of the methods have been validated for predictability and precision using other indirect methods.

The reference method most commonly used is that based on densitometry to estimate body fat from a two-compartment approach Sheng et al. The accuracy of most of these methods has been validated only in a few instances by direct carcass analysis. The final choice of an indirect method ultimately depends on its cost, the objective of the experiment, and the physical conditions under which it is to be used.

This work is a publication of the U. This project has been partially funded by the U. Department of Agriculture, Agricultural Research Service, under Cooperative Agreement MN Turn recording back on. National Library of Medicine Rockville Pike Bethesda, MD Web Policies FOIA HHS Vulnerability Disclosure.

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Show details National Research Council US Committee on Technological Options to Improve the Nutritional Attributes of Animal Products. Contents Hardcopy Version at National Academies Press. Search term. Methodologies for Measuring Body Composition in Humans.

HWAI-PING SHENG ''Nothing is measured with greater error than the human body. Whole-Body Measurements Fat-Soluble Gases Because most anesthetic gases are rare inert gases for example, cyclopropane, xenone, and krypton and are highly soluble in fat but not in water, it is theoretically possible to use the dilution principle to calculate total body fat by measuring the absorption of these gases.

Body Compartmentalization into Two Components Most indirect methods compartmentalize the body in numerous ways, depending on the purpose of the study and the requirements of the investigators. Determination of Fat by Densitometry Densitometric determination of body fat is considered by many investigators as the "reference method" or the "standard" against which other indirect methods are compared.

Determination of FFm by Hydrometry A value for body fat may be derived simply from the total body water TBW measurement based on the assumption that FFM has a constant water content of Determination of Fat from Potassium FFM can be estimated from potassium K by the following equation:.

Total Body Electrical Conductivity and Impedance Total body electrical conductivity TOBEC and bioelectrical impedance analysis BIA have recently been used to assess adiposity Cochran et al. Multicompartmentalization of the Body The above methods to estimate body fat from FFM assume that the chemical composition of FFM is constant, an assumption that undoubtedly introduces an error whose boundaries are not well defined.

Regional Fat Measurement Progress has been made in the development of methods to measure composition at various regions of the body. Summary Many indirect methods of varying degrees of complexity are available for estimation of body fat.

Acknowledgments This work is a publication of the U. References Anderson, E. Three-component body composition on analysis based upon potassium and water determinations. Behnke, A. Feen, and W. The specific gravity of healthy men: Body weight and volume as an index to obesity.

Borkan, G. Hults, J. Cardarelli, and B. Comparison of ultrasound and skin-fold measurements in assessment of subcutaneous and total fatness. Hults, S. Gerzof, B. Burrows, and A. Relationships between computed tomography tissue areas, thicknesses and total body composition. Human Biol.

Brozek, J. Body composition, Parts I and II. Human Body Composition: Approaches and Applications. New York. Henschel, editor.

Techniques for Measuring Body Composition. Washington, D. Grande, J. Anderson, and A. Densitometric analysis of body composition: Revision of some quantitative assumptions. Cochran, W. Klish, W. Wong, and P. Total body electrical conductivity used to determine body composition in infants.

Cohn, S. How valid are bioelectric impedance measurements in body composition studies? Vaswani, S. Yasumura, K. Yuen, and K. Improved models for determination of body fat by in vivo neutron activation.

J Clin. Assessment of cellular mass and lean body mass by noninvasive nuclear techniques. Conway, J. Norris, and C. A new approach for the estimation of body composition: Infrared interactance. Corsa, J. Olney, Jr. Steenburg, M. Ball, and F.

The measurement of exchangeable potassium in many by isotope dilution. Dell, R. Aksoy, S. Kashyap, M. Forsythe, R. Ramakrishnan, C. Zucker, and W.

Relationship between density and body weight in prematurely born infants receiving different diets. Ellis, editor; , S. Yasamura, editor; , and W. Morgan, editor.

London: The Institute of Physical Medicine. Deskins, W. Winter, H. Sheng, and C. Use of a resonating cavity to measure body volume. Diethelm, R. Garrow, and S. An apparatus for measuring the density of obese subjects.

Durnin, J. The assessment of the amount of fat in the human body from measurements of skinfold thickness. Faulkner, F. An air displacement method for measuring body volume in babies. A preliminary communication. Fiorotto, M. Cochran, R. Funk, H.

Sheng, and W. Total body electrical conductivity measurements: Effects of body composition and geometry. Fomon, S. Jensen, and G. Determination of body volume of infants by a method of helium displacement. Haschke, E. Ziegler, and S. Body composition of reference children from birth to age 10 years.

Forbes, G. Methods for determining composition of the human body. Pediatrics Age and sex trends in lean body mass calculated from K 40 measurements: With a note on the theoretical basis for the procedure. Fuller, M. Foster, and J. Estimation of body fat by nuclear magnetic resonance imaging.

Garn, S. Roentgenogrammetric determinations of body compositions. Nolan, Jr. A tank to measure body volume by water displacement BOVOTA. Garrow, J. New approaches to body composition. Stalley, R. Diethelm, P.

Pittet, R. Hesp, and D. A new method for measuring the body density of obese adults. Gnaedinger, R. Reineke, A. Pearson, W. Van Huss, J. Wessel, and H. Determination of body density by air displacement, helium dilution, and underwater weighing. Gotfredsen, A, J. Jensen, J. Borg, and C. Measurement of lean body mass and total body fat using dual photon absorptiometry.

Metabolism Harrison, G. Van Itallie Estimation of body composition: A new approach based on electromagnetic principles. Heymsfield, S. Radiographic analysis of body composition by computerized axial tomography.

Newell, editor; and N. Ellison, editor. New York: Raven. Kirton, A. Relationships between potassium content and body composition.

Knight, G. Beddoe, S. Streat, and G. Body composition of two human cadavers by neutron activation and chemical analysis. Lesser, G. Measurement of total body fat in man by the simultaneous absorption of two inert gases.

Lewis, D. Rollwitz, H. Bertrand, and E. Use of NMR for measurement of total body water and estimation of body fat. Critical evaluation of the pneumatic method for determining body volume. Its history and technique. Lobman, T. G Skinfolds and body density and their relation to body fatness: A review.

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McManus, W. Prichard, C. Remember, body composition is the ratio of Fat Mass and Fat-Free Mass used to help determine health risks.

Of the other methods already mentioned waist, waist-to-hip ratio, and BMI , none provide estimates of body composition but do provide measurements of other weight-related health markers, such as abdominal fat. Experts have designed several methods to estimate body composition.

While they are not flawless, they do provide a fairly accurate representation of body composition. The most common are:.

At one time, hydrostatic weighing also and maybe more accurately called hydrodensitometry was considered the criterion for measuring body composition. Many other methods are founded on this model, in one form or another. The mass and volume components are measured by using dry weight and then weight while being submerged in a water tank.

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. Unfortunately, the expense and practicality of building and maintaining a water tank limits access for most.

Also, for those with a fear of water, this would obviously not be the preferred method. While underwater weighing accurately compartmentalizes FM and FFM, DEXA adds a third compartment by using low-radiation X-rays to distinguish bone mineral. This addition slightly increases the accuracy of DEXA by eliminating some of the guess work associated with individual differences, such as total body water and bone mineral density.

Originally, DEXA scanners were designed to determine and help diagnose bone density diseases. However, a full body scan, which takes only a few minutes, is all that is needed to also determine body fat percentage.

Major disadvantages to this method are its high cost and the need for a well-trained professional to operate the equipment and analyze the results.

A good alternative to more expensive methods, air displacement determines body density using the same principle as underwater weighing, by measuring mass and volume.

Clearly, the main difference is that mass and volume are being determined by air displacement rather than water displacement. Using a commercial device the Bod Pod is most commonly referenced , 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.

BIA takes a slightly different approach to measuring FFM. The premise behind BIA is that FFM will be proportional to the electrical conductivity of the body.