Obesity Epidemiology

MEASUREMENTS OF ADIPOSITY AND BODY COMPOSITION 57estimates of percent fat and those provided by DXA (R 2 = .78 to .91). 10 Ginde et al. 11demon strated high accuracy in measuring body fat even in severely obese patients (bodymass index, BMI ≥ 40). A high validity of BOD POD measurements of body fat has alsobeen observed among children. 12 Because it has high validity and is less burdensome forparticipants, BOD POD has become an excellent alternative to traditional densitometry,especially in children, pregnant women, and morbidly obese persons.HydrometryHydrometry (or the dilution method) uses isotopes [deuterium ( 2 H 2O), tritium ( 3 H 2O),and oxygen-18-labeled water (H 218O)] to measure TBW. 13 Calculation of FFM is basedon the dilution principle (the proportion of water to FFM is relatively stable: 0.73). Thismethod is widely used as a reference method to estimate FFM and total body fat invivo. Typically, an accurately weighed oral dose of isotope-labeled water is carefullyadministered to a subject after an overnight fast. Biological samples such as serum, urine,or saliva are collected before the dose is administered and at the end of equilibration,which usually takes 3 to 4 hours. TBW is calculated using validated dilution equationsbased on the degree of the dilution of the isotopes by total body fluid. 13 FFM is thencalculated as TBW/0.73 and total body fat is calculated as the difference between bodyweight and FFM. The validity of this method relies on several reasonable assumptions:the presence of tracers in body water only; equal and rapid distribution of the tracer inall anatomical water compartments; and the absence of metabolism of both the tracerand body water during the course of equilibration after oral dosing. 13 The procedureis relatively straightforward, but achieving accurate and precise measurements requirescareful attention to preparation of the subject, dosage, sample collection (urine, saliva, orblood), and isotopic analyses. 13Deuterium is the most widely used isotope because of its safety and relatively lowcost. Oxygen-18-labeled water is also safe for children and pregnant women, but is moreexpensive than deuterium. The concentrations of both isotopes in biological samples aremeasured with high precision by a mass spectrometry technique. If the standard procedureis carefully followed, TBW measurements are highly accurate and precise, withtechnical errors in the range of 1% to 2%. 13 Overall, the use of hydrometry to estimateFFM and total body fat is simple and safe. It is one of the few methods that can be usedto measure body composition in morbidly obese subjects; however, the assumption ofa stable ratio of TBW to FFM of 0.73 may not be accurate in these subjects. 13 Disease,early phase of weight loss, or hydration status may also alter the ratio and affect thevalidity of estimates of FFM and body fat. Modified equations have been developed forestimating TBW and FFM in pregnant women according to different stages of pregnancyand the presence or absence of edema. 14Whole-Body Potassium CountingWhole-body potassium counting is a classic reference method for estimating total body fat.It is based on the principle that the naturally occurring radioactive potassium isotope 40 K inhuman tissues represents total-body potassium (TBK); that 40 K accounts for 0.0118% of TBK;and that TBK is evenly distributed in intracellular components of FFM. 15 TBK can thus bequantified by using special equipment to detect γ-rays that emanate from 40 K. By assuming aconstant proportion of FFM as potassium, one can estimate FFM by the following method:FFM = TBK/63.3,