Define Isotope Dilution Method?

The measurement of total body water (TBW) is based on the principle of hydrometry. You must be aware that water is the most abundant substance in the body accounting for 67% to 74% of the body's weight, the amount of water contained in cells varies with different types of tissues. Adipose tissues contain less water than do muscle cells, if one assumes that the distribution of water in fat and fat free tissues is consistent among individuals. The fat% can be estimated by measuring volume of water contained in the body.

This method assumes that the distribution and exchange of the isotope by the body are similar to the distribution and exchange of water. The subjects are given to drink deuterium or tritium. Deuterium is a rare, non-radioactive isotope of hydrogen and ritium is a rare, radioactive isotope of hydrogen. After approximately 3 hours of ingestion of isotope, a urine or blood sample is taken. The amount of isotope contained
Sports Nutrition in the sample is inversely proportional to the amount of water in the body, total body water is measured. The concentration of hydrogen isotopes (deuterium or tritium) in biological fluids (saliva, plasma and urine) is measured and used to estimate TBW.

In this method, it is also assured that the hydration of the FFM is constant for all individuals (- 73% of FFM). Because TBW fluctuates widely within and among individuals depending on age, gender, level of obesity, and disease, large errors may result when hydrometry is used with the two-compartment model to derive reference measures of body composition. Thus, there are potential sources of error, it's expensive and again it is not very practical. Moreover, technical expertise is required.