Obesity Epidemiology

Table 5.1 A Comparison of Commonly Used Methods for Measuring Body CompositionMethods Description Strengths and LimitationsReference methodsUnderwaterweighing(densitometry)Air-displacementplethysmography(ADP)Dilution method(hydrometry)Dualenergyx-rayabsorptiometry(DXA)Computedtomography(CT)/magneticresonanceimaging (MRI)This method is based on theprinciple that fat is less dense thanwater and that an individual withmore body fat will thus have alower body density. The techniqueinvolves measuring a subject’sweight in air and underwater.Percent body fat is calculated withprediction equations based on thetwo-compartment model.ADP is a recently developedtechnique using air rather thanwater displacement for measuringbody volume and density. Thenew BOD POD Body CompositionSystem has been validated againstdensitometry.This method measures total bodywater using isotopes (deuteriumis most commonly used) based onthe principle that water exists in arelatively stable proportion to fatfreemass (≈0.73). Total body wateris calculated with validated dilutionequations.This technique is based on theprinciple that two x-ray beamsof very low but different energypassing through the body areattenuated differentially by bonemineral tissue and soft tissue. Itprovides estimates for the threecomponents of the whole body(fat-free mass, fat mass, and bonemineral density) for specific regionssuch as the arms, legs, and trunk.Both CT and MRI provide highresolutioncross-sectional scansof selected tissue or organs andare considered the most accuratemethods for assessing bodycomposition and regional fatdistributions at tissue-organ levels.Densitometry has long beenconsidered the “gold standard”for measuring body composition.However, the procedure is timeconsumingand requires major activecooperation of the subjects. It is notsuitable for children and older adults.The procedure is relatively quick,more comfortable, and does notrequire the subject to be submergedin water. It is an attractive alternativeto the traditional densitometrymethod, especially for children.The procedure is simple, safe, andrelatively inexpensive. Measurementsof total body water have highprecision and accuracy. It can beused to measure body compositionof morbidly obese patients. However,the assumption of a stable ratio oftotal body water to fat-free massof 0.73 may not hold in patientswho are ill, are in the early phaseof weight loss, or have a differenthydration status.The procedure has very highreproducibility and accuracy inmeasuring body fat and lean bodymass and has gained increasingacceptance as a reference methodfor measuring body composition.Because x-ray exposure is extremelylow, it is safe for children, but it isnot suitable for pregnant women. Thedevice is expensive and not portable.It cannot accurately distinguishvisceral fat from subcutaneous fat.Both methods accurately quantifypercent body fat and visceral andsubcutaneous fat. A major advantageof MRI over CT is the lack ofradiation exposure. Both techniquesare expensive and not readilyavailable. They cannot usuallyaccommodate morbidly obese people.(continued)55