Application of conservation principle in estimating body volume in rats

Obesity is a risk factor in diving and aviation during acute reduction of pressure, because N2 is five times more soluble in fat than in water. In addition to concerns of decompression applications, body fat content (BFC) bears importance in the disposition of drugs, the evaluation of dietary regimens and exercise programs, and in long-term health status. Most of the currently available methods for quantifying BFC are impractical for small animals. We devised a method that does not require the cooperation of the animal. Our method uses a simple gas-dilution technique to measure the body volume. The body density was determined as the ratio of body weight (g) to body volume (ml). After the whole body density was determined, the fraction of body fat was calculated according to established formulas. Calibration with aluminum blocks showed a high correlation (r = 0.99) between measured volumes and the known block volumes. Furthermore, comparing body volumes of shaved and unshaved rats showed that the volume of air between hairs did not affect the measurement of body volume with the dilution method. In four groups, each consisting of 10 male Wistar rats [body wt 81 +/- 1 (SE), 152 +/- 3,297 +/- 5, and 496 +/- 4 g], BFC was 7.0 +/- 2.8, 14.6 +/- 1.9, 22.0 +/- 1.8, and 32.3 +/- 1.3%, respectively. These values compared favorably with chemically determined BFC values for rats in the literature. Our results suggest that the gas-dilution technique is a feasible and simple method for the determination of BFC in small animals.