A review of body composition studies with emphasis on total body water and fat

Tritiated water meausres a volume 4 to 15% body weight larger than that by desiccation, and, at present, only 0.5 to 2.0% of the overestimation can be explained by the exchange of hydrogen of tritiated water with those of the proteins and carbohydrates of the body. The remainder of the error is unexplained. Water in the lumen of the gut is an appreciable percentage of total body water (TBW) in many mammalian species, with the pig and the human as possible exceptions, and it should be considered an integral part of TBW. Consequently, the exclusion or inclusion of this transcellular water as part of TBW significantly affects the final TBW volume. As tritiated water exchanges with water in the gut, a comparison of the data from the indirect method with the data from the direct method can only be made when water in the gut is included in the desiccation method. Conceptually, the amount of water in lean body mass is a reflection of the actively metabolizing cell mass of the body. However, water in the gut is outside this cell mass, and if included, it significantly overestimates the water associated with the lean body mass compartment. The percentage of water in fat-free wet weight for most mature animals is estimated at 73.2%, although the mean values in the literature range from 63% for the beagle to 80% for the mouse, with the mean for the majority of species between 70 and 76%. If the percentage of water in fat-free wet weight lies between 70 and 76% for most species, then the error in calculating fat using the figure 73.2% in the equation (% fat = 100 - % TBW/0.732) is significant. In the application of this equation, the largest potential error lies in the estimation of TBW with tritiated water.