Laboratory-based body-composition techniques include hydrostatic weighing (HW), dual-energy X-ray absorptiometry (DXA), measurement of total body water (TBW) by isotope dilution, measurement of total body potassium, and multicompartment models. Although these reference methods are used routinely, each has inherent practical limitations. Whole-body air-displacement plethysmography is a new practical alternative to these more traditional body-composition methods. We reviewed the principal findings from studies published between December 1995 and August 2001 that compared the BOD POD method (Life Measurement, Inc, Concord, CA) with reference methods and summarized factors contributing to the different study findings. The average of the study means indicates that the BOD POD and HW agree within 1% body fat (BF) for adults and children, whereas the BOD POD and DXA agree within 1% BF for adults and 2% BF for children. Few studies have compared the BOD POD with multicompartment models; those that have suggest a similar average underestimation of approximate 2-3% BF by both the BOD POD and HW. Individual variations between 2-compartment models compared with DXA and 4 -compartment models are partly attributable to deviations from the assumed chemical composition of the body. Wide variations among study means, -4.0% to 1.9% BF for BOD POD - HW and -3.0% to 1.7% BF for BOD POD - DXA, are likely due in part to differences in laboratory equipment, study design, and subject characteristics and in some cases to failure to follow the manufacturer's recommended protocol. Wide intersubject variations between methods are partly attributed to technical precision and biological error but to a large extent remain unexplained. On the basis of this review, future research goals are suggested.

OBJECTIVE: The objective of this study was to examine the in¯uence of body weight and body composition on aspects of aerobic ®tness. Our hypothesis was that increased body weight, speci®cally increased fat mass (FM), would not limit VO 2max relative to fat-free mass (FFM), but would reduce maximal and sub-maximal VO 2max relative to body weight. DESIGN: We used data from two ongoing studies. In Study 1 a cross-sectional analysis of 129 children across a wide spectrum of body composition was performed. In Study 2 we examined data from 31 overweight women before and after weight loss. METHODS: VO 2max was measured using a treadmill test. Sub-maximal aerobic capacity was evaluated with respiratory exchange ratio (RER), heart-rate (HR), and oxygen uptake relative to VO 2max at a given workload (%VO 2max ). Body composition was assessed using dual energy X-ray absorptiometry (DXA) (Study 1) and a fourcompartment model (Study 2). RESULTS: In Study 1, FFM was the strongest determinant of VO 2max (r 0.87; P`0.0001). After adjusting for FFM, there was no signi®cant in¯uence of FM on VO 2max . After separating children into lean and obese sub-groups, absolute VO 2max was signi®cantly higher in the obese (1.24 AE 0.27 vs 1.56 AE 0.40) and VO 2max relative to body weight was signi®cantly lower (44.2 AE 3.2 vs 32.0 AE 4.1 mla(kg-min)), whereas there was no signi®cant difference when expressed relative to FFM (57.9 AE 5.8 vs 59.2 AE 4.9 mla(kgFFM-min)). Sub-maximal aerobic capacity was signi®cantly lower in the obese children, as indicated by a higher HR and %VO 2max ; time to exhaustion was signi®cantly lower in the obese children (15.3 AE 2.9 vs 11.1 AE 2.1 min). In Study 2, FFM was also the strongest determinant of VO 2max before and after weight loss. The relationship between VO 2max and FFM was identical before and after weight loss so that VO 2max relative to FFM was identical before and after weight loss (43.8 AE 4.9 vs 45.5 AE 6.4 mla(kgFFM-min)). However, sub-maximal aerobic capacity was lower in the obese state, as indicated by a signi®cantly higher RER (0.85 AE 0.06 vs 0.79 AE 0.05), HR (124 AE 14 vs 102 AE 11 bpm), and %VO 2max (44% vs 36%). CONCLUSION: The major in¯uence of body weight on VO 2max is explained by FFM; FM does not have any effect on VO 2max . Fatness and excess body weight do not necessarily imply a reduced ability to maximally consume oxygen, but excess fatness does have a detrimental effect on submaximal aerobic capacity. Thus, fatness and VO 2max should be considered independent entities.