Deuterium Dilution (Total Body Water)
Criterion-Level Hydration & Fat-Free Mass Assessment
Research Service. Deuterium dilution runs within ongoing research studies or by customized collaborative agreement. Contact the lab to discuss protocol design and batch analysis.
Deuterium dilution is the scientific gold standard and criterion method for quantifying in vivo Total Body Water (TBW). By measuring the distribution volume of an ingested non-radioactive stable isotope tracer ($^2\text{H}_2\text{O}$), it eliminates the geometric and density assumptions inherent in field methods like bioimpedance or anthropometry.
How It Works
- Baseline Sample: A baseline saliva or urine sample is collected to establish the participant’s natural background deuterium level ($^2\text{H}$ naturally occurs in all terrestrial water at approximately 150 ppm).
- Tracer Administration: The participant drinks an accurately weighed dose of pharmaceutical-grade deuterium oxide ($^2\text{H}_2\text{O}$), followed by a rinse to ensure complete ingestion.
- Equilibration Period: Over 3 to 4 hours of rest, the tracer distributes evenly throughout all intracellular and extracellular fluid compartments.
- Post-Dose Sampling: A second sample is collected. Isotopic enrichment is quantified via high-precision isotope ratio mass spectrometry (IRMS) or cavity ring-down spectroscopy (CRDS).
What It Measures
- Total Body Water (TBW): Calculated directly from the dilution volume, adjusted by 4% to correct for non-aqueous exchangeable hydrogen atoms (e.g., in proteins and glycogen).
- Fat-Free Mass (FFM): Derived by dividing TBW by the biological hydration constant ($0.732$ in healthy adults, with pediatric/geriatric age-adjusted coefficients).
- Fat Mass (FM): Calculated by subtracting Fat-Free Mass from total scale body weight.
- Criterion Input for Multicompartment Models: Serves as the mandatory aqueous input for 3-compartment and 4-compartment criterion models.
What the Participant / Investigator Takes Home
- Detailed analytical laboratory report with measured baseline and equilibrated isotopic ppm/delta enrichment.
- Calibrated Total Body Water in kilograms and liters, with age- and sex-matched hydration fraction comparisons.
- Independent criterion 2-compartment body composition partition (Fat Mass vs. Fat-Free Mass).