Stable Isotope Dilution
Deuterium dilution · Doubly labeled water
Two criterion measurements for research studies, from the same family of methods: deuterium dilution gives total body water — the foundation of fat-free mass and of multicompartment body-composition models — and doubly labeled water gives total energy expenditure in free-living people, with no laboratory confinement and no wearable to remember.
What is stable isotope dilution?
Every method on this page rests on one idea: give the body a precisely weighed dose of water labeled with a stable, non-radioactive isotope, wait for it to mix evenly through the body’s water, then measure how much the label has been diluted. The more body water there is, the more dilute the label becomes — so the enrichment measured in a urine or saliva sample gives the size of the compartment the tracer distributed into.
The isotopes used are deuterium (²H), a naturally occurring heavy form of hydrogen, and oxygen-18 (¹⁸O), a naturally occurring heavy form of oxygen. Both are already present in every glass of water you drink; a dose simply raises that concentration for a few days. Neither is radioactive, and both are used routinely in newborns, children, and pregnant women.
Because dilution measures a physical quantity directly rather than inferring it from a signal, isotope dilution is the criterion method against which DXA, ADP, and BIA are validated — including in this lab’s own methods work. Cataldi D, Bennett JP, Quon BK, Liu YE, Heymsfield SB, Kelly T, Shepherd JA. Agreement and precision of deuterium dilution for total body water and multicompartment body composition assessment in collegiate athletes. J Nutr. 2022;152:2048–2059. doi:10.1093/jn/nxac116
Deuterium dilution — total body water and body composition
A weighed dose of deuterium-labeled water (²H₂O) is consumed after a baseline sample is collected. Over the next three to four hours the deuterium equilibrates with body water; a post-dose urine or saliva sample then quantifies the dilution and, with it, the size of the total body water pool.
What it measures
Total Body Water (TBW). The single largest component of the human body — roughly 50–60% of body weight in adults, and higher in infants. TBW is a clinical measure in its own right for hydration status, fluid shifts, and nutritional assessment, and it is the input on which everything below depends.
Fat-Free Mass (FFM) and Fat Mass (FM). Body water resides almost entirely in fat-free tissue, at a hydration fraction that is stable and well characterized (about 73.2% in healthy adults, with established age- and sex-specific values for children and infants). Dividing TBW by that fraction gives fat-free mass; subtracting fat-free mass from body weight gives fat mass. This two-compartment model rests on different assumptions than DXA or ADP, which is why it serves as an independent reference. Wang Z, Deurenberg P, Wang W, Pietrobelli A, Baumgartner RN, Heymsfield SB. Hydration of fat-free body mass: review and critique of a classic body-composition constant. Am J Clin Nutr. 1999;69:833–841. doi:10.1093/ajcn/69.5.833
The water term in multicompartment models. Deuterium-derived TBW is the water compartment of the four-compartment (4C) criterion model — fat, water, mineral, and protein — which combines body volume from air displacement plethysmography, bone mineral from DXA, and TBW from dilution. The 4C model removes the population-level assumptions that limit any single method, and it is the standard against which new techniques are judged. Adding a dilution measurement to a DXA and Bod Pod visit upgrades the whole assessment from an estimate to a criterion result.
Validation of faster methods. The lab’s own work uses deuterium dilution to characterize how well accessible methods track the truth — including BIA-derived total body water and the 3DO + BIA multicompartment model developed here. Studies that need a reference arm for a device evaluation are the most common use of this service.
Doubly labeled water — total energy expenditure
Doubly labeled water (DLW) is the only method that measures total energy expenditure in free-living people over days to weeks. Participants take a single dose of water labeled with both deuterium and oxygen-18, then go about their ordinary lives, providing timed urine samples over the following one to two weeks.
The method exploits a difference in how the two labels leave the body. Deuterium leaves as water only. Oxygen-18 leaves as water and as the oxygen in exhaled carbon dioxide, because CO₂ and body water exchange oxygen rapidly. The gap between the two elimination rates is therefore a direct measure of CO₂ production — and CO₂ production, with an estimate of the food quotient, converts to energy expenditure by the same calorimetry principles used in a metabolic cart, without anyone having to sit under a hood. Schoeller DA, van Santen E. Measurement of energy expenditure in humans by doubly labeled water method. J Appl Physiol. 1982;53:955–959. doi:10.1152/jappl.1982.53.4.955
What it measures
Total Energy Expenditure (TEE). Total calories burned per day under ordinary living conditions — work, exercise, fidgeting, digestion, sleep, and everything else — averaged over the measurement window. Accuracy is on the order of a few percent against whole-room calorimetry, and unlike accelerometers, questionnaires, or prediction equations, DLW does not depend on the participant reporting or remembering anything.
Physical Activity Level (PAL) and activity energy expenditure. Pairing TEE with a resting metabolic rate — measured, or estimated from fat-free mass — separates the resting and non-resting halves of the energy budget and yields PAL, the ratio of total to resting expenditure. This is the reference measure used to calibrate physical-activity questionnaires and wearable devices.
Energy intake, by balance. Over a period of stable weight, energy intake equals energy expenditure. DLW is therefore also the criterion method for validating self-reported dietary intake — the basis for the well-documented finding that food records systematically under-report, and a standard quality-control arm in nutrition trials.
Total body water, included. The deuterium half of a DLW dose is a deuterium dilution measurement. Every DLW protocol therefore returns TBW and fat-free mass alongside energy expenditure, which is why the two services on this page are usually scoped together.
DLW underpins the international reference data on human energy requirements across the lifespan, including the IAEA DLW database analyses of daily energy expenditure from infancy through old age. Pontzer H, Yamada Y, Sagayama H, et al. Daily energy expenditure through the human life course. Science. 2021;373:808–812. doi:10.1126/science.abe5017
What a session looks like
| Deuterium dilution (TBW) | Doubly labeled water (TEE) | |
|---|---|---|
| Dose | ²H₂O, weighed to the gram | ²H₂O + H₂¹⁸O, weighed to the gram |
| Samples | Baseline, then a post-dose sample at ~3–4 h | Baseline, then timed samples over 7–14 days |
| Sample type | Urine or saliva | Urine, typically collected at home |
| Time on site | One visit of about four hours, or two short visits | One dosing visit; the rest collected at home |
| Results | TBW, fat-free mass, fat mass | TEE, PAL, plus TBW and body composition |
Samples are analyzed off site by isotope-ratio mass spectrometry (IRMS) or FTIR spectrometry through the lab’s analytical collaborators, so results are returned after analysis rather than at the visit. Turnaround depends on batch scheduling — the lab will give you a date when the protocol is scoped.
How to prepare
- Fast overnight before the dosing visit (water is fine), so the baseline sample reflects a stable state.
- Provide a baseline sample before drinking the dose — the measurement cannot be made without it.
- Nothing to eat or drink for the first hour after the dose. The dose container is rinsed and the rinse consumed so that the full dose is accounted for.
- Avoid unusual fluid loads — no sauna, no extreme exercise, and no deliberate over- or under-hydration during the equilibration period.
- For doubly labeled water: keep to your normal routine for the whole collection window. The point of the method is to capture ordinary life, so do not change your activity because you are being measured. Collect each sample at the scheduled time and record the exact time.
Is it safe?
Yes. Deuterium and oxygen-18 are stable isotopes — they are not radioactive and they emit nothing. Both occur naturally in body water and in everything you drink; a dose raises that natural background for a few days before it washes out. The technique has been used for decades in newborns, infants, children, pregnant and lactating women, and older adults, and international guidance documents the protocols in detail. International Atomic Energy Agency. Assessment of Body Composition and Total Energy Expenditure in Humans Using Stable Isotope Techniques. IAEA Human Health Series No. 3. Vienna; 2009. International Atomic Energy Agency. Introduction to Body Composition Assessment Using the Deuterium Dilution Technique with Analysis of Saliva Samples by Fourier Transform Infrared Spectrometry. IAEA Human Health Series No. 12. Vienna; 2010.
Who it is for
Device and method validation. Any study that needs a criterion arm for a body-composition or activity-monitoring device — the role dilution plays in this lab’s BIA, 3DO, and multicompartment-model work.
Nutrition and weight-management trials. Energy requirements, intervention adherence, and the validation of self-reported intake.
Pediatric and infant research. Body composition and energy requirements at ages where few other methods apply, complementing the Pea Pod and Tod Pod.
Clinical populations. Hydration and fluid-shift assessment, and energy requirements in cancer, cachexia, and metabolic disease.