Total body water - Variation with age

The accuracy with which total body water can be measured depends on the uniformity of distribution of the deterium oxide throughout the water compartments of the body and the precision with which the concentration of deuterium can be measured in a given body fluid. As early as 1959 it was demonstrated that salivary glands concentrated deuterium above the level in serum, following the administration of label to normal pregnant women [236]. This fact has been repeatedly questioned by various workers [237] on the grounds of possible contamination of the salivary distillate, required for deuterium analysis by the falling drop method employed. More recently the use of isotope mass spectrometry has provided unequivocal evidence that in both the rat [238] and man 

A variety of analytical methods have been employed for the estimation of deuterium in biological fluids in the context of total body water measurements. These include the falling drop method [241], freezing point elevation [242], infrared spectroscopy [243], gas chromatography [237] and mass spectrometry [240,244]. A range of accuracy between 0.5-5.0% is claimed from these various techniques in the overall protocol of body water measurement. 

Validation of body water measurements, using dilution principles and either of the hydrogen isotopes, can only be achieved from complementary desiccation studies. In fact it has been repeatedly demonstrated in animals that the volume of dilution of deuterium oxide corresponds closely to the total body water space [249—252]. A single report has suggested that the difference between isotope dilution using tritium and desiccation, was statistically significant in the rat and that the former method overestimated body water by 12% of the desiccation value [253]. 

As a generalisation, it is true to state that, when water content is expressed as a percentage of body weight, a continual dehydration of the body occurs from developing foetus to late adulthood. At a gestational age of one month, the foetus is more than 90% water, but at birth published figures estimate a water 

Age Sex No. of subjects Condition Body water as % body weight Reference 

---