Residual volume on land and when immersed in water: effect on percent body fat

There is a large residual volume (RV) error when assessing percent body fat by means of hydrostatic weighing. It has generally been measured before hydrostatic weighing. However, an individual's maximal exhalations on land and in the water may not be identical. The aims of this study were to compare residual volumes and vital capacities on land and when immersed to the neck in water, and to examine the influence of the measurement error on percent body fat. The participants were 20 healthy Japanese males and 20 healthy Japanese females. To assess the influence of the RV error on percent body fat in both conditions and to evaluate the cross-validity of the prediction equation, another 20 males and 20 females were measured using hydrostatic weighing. Residual volume was measured on land and in the water using a nitrogen wash-out technique based on an open-circuit approach. In water, residual volume was measured with the participant sitting on a chair while the whole body, except the head, was submerged . The trial-to-trial reliabilities of residual volume in both conditions were very good (intraclass correlation coefficient > 0.98). Although residual volume measured under the two conditions did not agree completely, they showed a high correlation (males: 0.880; females: 0.853; P < 0.05). The limits of agreement for residual volumes in both conditions using Bland-Altman plots were -0.430 to 0.508 litres. This range was larger than the trial-to-trial error of residual volume on land (-0.260 to 0.304 litres). Moreover, the relationship between percent body fat computed using residual volume measured in both conditions was very good for both sexes (males: r = 0.902; females: r = 0.869, P < 0.0001), and the errors were approximately -6 to 4% (limits of agreement for percent body fat: -3.4 to 2.2% for males; -6.3 to 4.4% for females). We conclude that if these errors are of no importance, residual volume measured on land can be used when assessing body composition.