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Background: Little information is available on the assessment of changes in body composition as a function of weight change with the use of the fan beam of dual-energy X-ray absorptiometry (DXA).

Objective: The objective was to determine the accuracy of the fan beam of the QDR 4500A densitometer and the pencil beam of the QDR 2000 densitometer in estimating changes in whole-body lean soft tissue mass (LSTMDXA) and fat mass (FM) with weight change.

Design: Thirty-seven subjects who lost 5.7 +/- 4.5 kg were measured before and after weight change. Using total body water and computed tomography (CT) of the midthigh, we compared changes in FFMTBW and LSTMCT with changes in LSTMDXA.

Results: Overall, compared with TBW, the fan beam gave a larger estimate of change ((x) over bar +/- SD) in LSTM (fan beam - TBW: -0.7 +/- 1.6 kg) than did the pencil beam (pencil beam - TBW: -0.1 +/- 1.6 kg)When the change in LSTM obtained with the fan beam and pencil beam was regressed against the change in FFMTBW, the slope of the line for the fan beam was 0.97 (r(2=) 0.61) and that for the pencil beam was 0.86 (r(2=) 0.61). Regression analysis showed that the results between the 2 units were not interchangeable. For the midthigh region, the change in LSTMCT. was moderately correlated with the change in LSTMDXA, with the fan beam arid pencil beam.

Conclusions: The measurement of change in LSTM with the fan and pencil beams provides the same relation to changes in FFM assessed by TBW, but the 2 systems are not interchangeable.