Deuterium Dilution (Hydrometry)
Deuterium dilution measures the amount of water in the body after drinking a safe tracer, and from that calculates fat-free mass - it is the reference method for total body water.
Plain English
Patient-Friendly Explanation
Deuterium dilution is how researchers measure body water with high accuracy. The person drinks a small, harmless amount of deuterium oxide - water in which the hydrogen has been replaced by its heavier isotope, sometimes called heavy water - and waits about three to four hours while it mixes evenly through every fluid compartment. A urine or saliva sample then goes to a laboratory that measures how much the tracer was diluted, and the dilution tells the lab exactly how many litres of water the body contains. That matters because lean tissue is roughly 73% water while fat holds almost none, so dividing total body water by about 0.73 gives fat-free mass, and subtracting it from body weight gives fat mass. It is a gold standard in research and is mainly used to check that faster clinical tools such as bioelectrical impedance scales or DXA scans are reading correctly. It has limits: the 73% figure is an average, so results drift when someone is dehydrated, retaining fluid, or pregnant, and the test is not done during routine clinic appointments.
Medical
Clinical Definition
Isotope dilution for total body water was introduced by Hevesy in 1934 using deuterium oxide and remains the reference method. A known oral dose of deuterium oxide is given, the subject waits about three to four hours for equilibration, and enrichment in a body fluid is read by isotope-ratio mass spectrometry or Fourier-transform infrared spectroscopy. Fat-free mass is then calculated as total body water divided by the hydration constant of fat-free mass, classically 0.732 - approximately 0.73 in mammals, an assumption reviewed and critiqued by Wang and colleagues (American Journal of Clinical Nutrition 1999;69:833-41), who showed it varies with age, adiposity, growth and disease and so limits two-component accuracy. Measurement conditions matter: Cataldi and colleagues (2022) examined the agreement and precision of deuterium dilution for total body water and how hydration status shifts the derived body composition. Dilution space is systematically larger than body water because deuterium exchanges with non-aqueous hydrogen, so a correction factor (on the order of 1.04) is applied. The method performs poorly or is invalid in dehydration, over-hydration, oedema, ascites, chronic kidney disease and pregnancy, and it is the total-body-water component of the four-compartment reference model alongside a body-volume measurement and DXA bone mineral content, which is why it is used to validate the devices used in everyday practice rather than as a routine clinical test.
Key Insight
Why It Matters
Fat-free mass is defined through the assumption that muscle is about 73% water, which is exactly why an impedance scale can read badly after dehydration, a salty meal or a hard training session.
Sources
Reviewed against standard medical references.
Medically reviewed by
Family Physician & Registered Acupuncturist, Accord Medical Clinic. MBBS (NUS), Graduate Diploma in Family Medicine (NUS), Graduate Diploma in Acupuncture (Singapore College of TCM).
Last reviewed: September 2026
This page is general health education, not medical advice. Whether any treatment is appropriate for you is a decision made in consultation after assessment.
