Двойно меченая вода: метод измерения метаболизма организма.

Water made of uncommon hydrogen and oxygen isotopes
Doubly labeled water is water in which both the hydrogen and the oxygen have been partly or completely replaced (i. e. labeled) with an uncommon isotope of these elements for tracing purposes. In practice, for both practical and safety reasons, almost all recent applications of the "doubly labeled water" method use water labeled with heavy but non radioactive forms of each element (deuterium and oxygen 18). In theory, radioactive heavy isotopes of the elements could be used for such labeling; this was the case in many early applications of the method. In particular, doubly labeled water (DLW) can be used for a method to measure the average daily metabolic rate of an organism over a period of time (often also called the Field metabolic rate, or FMR, in non human animals). This is done by administering a dose of DLW, then measuring the elimination rates of deuterium and oxygen 18 in the subject over time (through regular sampling of heavy isotope concentrations in body water, by sampling saliva, urine, or blood). At least two samples are required: an initial sample (after the isotopes have reached equilibrium in the body), and a second sample some time later. The time between these samples depends on the size of the animal. In small animals, the period may be as short as 24 hours; in larger animals (such as adult humans), the period may be as long as 14 days. The method was invented in the 1950s by Nathan Lifson and colleagues at the University of Minnesota. However, its use was restricted to small animals until the 1980s because of the high cost of the oxygen 18 isotope. Advances in mass spectrometry during the 1970s and early 1980s reduced the amount of isotope required, which made it feasible to apply the method to larger animals, including humans. The first application to humans was in 1982, by Dale Schoeller, over 25 years after the method was initially discovered. A complete summary of the technique is provided in a book by British biologist John Speakman.

Doubly labeled water may be administered by injection, or orally (the usual route in humans). Since the isotopes will be diluted in body water, there is no need to administer them in a state of high isotopic purity, no need to employ water in which all or even most atoms are heavy atoms, or even to begin with water which is doubly labeled. It is also unnecessary to administer exactly one atom of 18O for every two atoms of deuterium. This matter in practice is governed by the economics of buying 18O enriched water, and the sensitivity of the mass spectrographic equipment available. In practice, doses of doubly labeled water for metabolic work are prepared by simply mixing a dose of deuterium oxide (heavy water) (90 to 99%) with a second dose of H218O, which is water which has been separately enriched with 18O (though usually not to a high level, since doing this would be expensive, and unnecessary for this use), but otherwise contains normal hydrogen. The mixed water sample then contains both types of heavy atoms, in a far higher degree than normal water, and is now "doubly labeled." The free interchange of hydrogens between water molecules (via normal ionization) in liquid water ensures that the pools of oxygen and hydrogen in any sample of water (including the body's pool of water) will be separately equilibrated in a short time with any dose of added heavy isotope(s).

The doubly labeled water method is particularly useful for measuring average metabolic rate (field metabolic rate) over relatively long periods of time (a few days or weeks), in subjects for which other types of direct or indirect calorimetric measurements of metabolic rate would be difficult or impossible. For example, the technique can measure the metabolism of animals in the wild state, with the technical problems being related mainly to how to administer the dose of isotope, and collect several samples of body water at later times to check for differential isotope elimination. Most animal studies involve capturing the subject animals and injecting them, then holding them for a variable period before the first blood sample has been collected. This period depends on the size of the animal involved and varies between 30 minutes for very small animals to 6 hours for much larger animals. In both animals and humans, the test is made more accurate if a single determination of respiratory quotient has been made for the organism eating the standard diet at the time of measurement, since this value changes relatively little (and more slowly) compared with the much larger metabolic rate changes related to thermoregulation and activity. Because the heavy hydrogen and oxygen isotopes used in the standard doubly labeled water measurement are non radioactive, and also non toxic in the doses used (see heavy water), the doubly labeled water measurement of mean metabolic rate has been used extensively in human volunteers, and even in infants and pregnant women. The technique has been used on over 200 species of wild animals (mostly birds, mammals and some reptiles). Applications of the method to animals have been reviewed. A paper by Pontzer, Yamada, Sagayama and colleagues in 2021 summarized the results of over 6400 measurements using the technique in humans aged between 8 days and 96 years old. Doubly labeled water (2H218O) can also be used for unusually warm ice and unusually dense water, as it has a higher melting point than and is denser than either light water or what is normally meant by "heavy water" (2H216O). 2H218O melts at 4.00~4.04 °C (39.2 °F~39.27 °F) and the liquid reaches its maximum density of 1.21684~1.21699 g/cm3 at 11.43~11.49 °C (52.57 °F~52.68 °F).