The water content of fat (i.e. pure triacylglycerol, not adipose tissue) is zero, while the fat-free mass of the body is 73% water. The total amount of water in the body can be determined by giving a dose of water labelled with 2H or 18O, and then measuring the dilution of the label in urine or saliva.
An alternative approach is to measure the total body content of potassium; again, fat contains no potassium, which occurs only in the fat-free mass of the body. There is a gender difference here: in males the fat-free mass contains 60 mmol potassium/kg, and in females 66 mmol/kg. The radioactive isotope of potassium, 40K, occurs naturally. It is a weak y-emitter, and therefore total body potassium can be determined by measuring the y-radiation of the appropriate wavelength emitted by the body. This requires total enclosure in a shielded whole-body counter for about 15 minutes to achieve adequate precision, and because of this, and the cost of the equipment required, this is again purely a research technique.
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