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The International Atomic Energy Agency International Doubly Labelled Water Database: Aims, Scope and Procedures

Background: The doubly labelled water (DLW) method is an isotope-based technique that quantifies total energy expenditure (TEE) over periods of 1–3 weeks from the differential elimination of stable isotopes of oxygen and hydrogen. The method was invented in the 1950s, but limited ability to measure low isotope enrichments combined with the high cost of isotopes meant it only became feasible to use in humans in the 1980s. It is still relatively expensive to use, and alone small samples are unable to tackle some of the important questions surrounding energy balance such as how have expenditures changed over time and how do expenditures differ with age, between sexes and in different environments? Summary: By combining information across studies, answers to such questions may be possible. The International Atomic Energy Agency (IAEA) DLW database was established to pool DLW data across multiple studies. It was initiated by the main labs currently using the method and is hosted by the IAEA. At present, the database contains 6,621 measures of TEE by DLW from individuals in 23 countries, along with various additional data on the study participants. Key Messages: The IAEA DLW database is a key resource enabling future studies of energy demands.


The article is part of the Proceedings of the International Symposium on Understanding the Double Burden of Malnutrition for Effective Interventions organized by the International Atomic Energy Agency (IAEA) in cooperation with United Nations Children's Fund (UNICEF) and World
Health Organization (WHO) (10-13 December 2018, Vienna, Austria).
All rights reserved. This is an Open Access article licensed under the terms of the Creative Commons Attribution 3.0 IGO License (CC BY 3.0 IGO)https://creativecommons.org/licenses/by/3.0/igo/#). Usage, distribution and reproduction in any medium or format, even for commercial purposes, is permitted, provided the original work is properly cited.

Introduction
Many key questions in the study of modern nutrition revolve around energy. These include whether the supply of energy is sufficient to meet demands, particularly during the period of growth, and the converse problem of whether supply is too great leading to excess energy intake and obesity and its associated comorbidities. These seemingly contrasting problems often occur side by side and are widely recognized as the double burden of malnutrition. Central to understanding these issues is the need to have accurate measurements of energy demands of free-living individuals. The gold standard method for free-living energy expenditure is the doubly labelled water (DLW) method [1], an isotope-based technique for the assessment of energy expenditure.
The DLW technique is based on the difference between the turnover rates of the hydrogen and oxygen of body water that are a function of carbon dioxide (CO 2 ) production. The method allows the measurement of CO 2 production over a period when the subject is completely free living. As CO 2 production can be directly related to energy expenditure, DLW gives a reflection of free-living energy expenditure. The method involves dosing subjects with water that contains artificially elevated levels of 2 stable isotopic tracers, deuterium ( 2 H) and oxygen ( 18 O), hence doubly labelled. These are both naturally occurring stable isotopes that have background levels of around 150 ppm for deuterium and around 2,000 ppm for 18 O. When individuals drink a dose of DLW, the levels of these isotopes in their bodies rise to 225-260 and 2,150-2,200 ppm, respectively. These levels are not known to produce any toxic reactions or physiological effects, and the method is sufficiently safe that it is routinely used in potentially vulnerable groups like infants, children and pregnant mothers.
The method was invented in the 1950s by Nathan Lifson and colleagues at the University of Minnesota in the USA [2,3] but was only used on small mammals and birds until the early 1980s mostly because of the cost of the isotopes. The first human application was made by Schoeller and van Santen [4], followed closely by several other groups in Texas [5], the UK [6] and the Netherlands [7]. Subsequently, the use of the method has increased enormously. Since 2000 there have been between 94 and 142 papers published annually that have used the DLW method to measure free-living energy requirements based on a literature search of the ISI Web of Science core collection, generated using the search string ("DLW" OR "DLW" OR "H2O18" OR "field metabolic rate"; Fig. 1).
An example application of the method was the study by Rosetta et al. [8] to measure the energy demands of chronically malnourished lactating females living in Bangladesh. The individuals involved in this study were approximately 12 months post-partum and were all breastfeeding their infants. They had an average BMI of only 17.4, indicating severe food shortage, and based on recognized cut-offs, 79% had chronic energy deficiency. Measured daily energy demands were dependent on if the individuals were workers on tea plantations or nonworkers -with the workers expending around 1.6 MJ per day more than non-workers -a difference of about 23% on their total demands. Surprisingly, however, milk output assessed simultaneously by isotope transfer to their babies did not differ between the 2 groups. These data provide useful information on the energy requirements for successful lactation in communities under food shortage, some of whom are engaged in strenuous activities. Such accurate information can be obtained in no other way.

The IAEA DLW Database
The International Atomic Energy Agency (IAEA) database had its genesis in 2014 at a DLW training workshop in Tokyo, Japan, following the "Recent Advances in Calorimetry and Measurement of Energy Metabolism: RACMEM" meeting. The idea of compiling together all the published DLW measurements on humans was proposed, and there were several immediately obvious benefits of compiling such a database. First, the technique is still relatively expensive meaning most studies have a sample size of < 30 individuals. Alone these samples are unable to tackle some of the big questions in nutritional science. For example, how has total energy expenditure (TEE) changed over time? How do TEEs differ between different lifestyles -for example, hunter gatherers, subsistence farming and modern urban living? How do expenditures differ with age, between sexes and in different socio-economic or physical environments? Combining information across studies in a database makes answering such questions possible. The second big advantage is that different studies have used different calculations to generate the estimates of CO 2 production and hence TEE. The variation between calculations is not always trivial. Having the raw data underpinning the calculations would allow DOI: 10.1159/000503668 all the data to be recalculated using a single common equation.
The IAEA agreed to host the database, and a total of 6,621 measurements (1 record = 1 measurement on 1 person) have been initially uploaded to the website. The database was formally launched on December 11, 2018 at the International Symposium on the Double Burden of Malnutrition for Effective Interventions, organized by the IAEA in cooperation with WHO and UNICEF in Vienna/Austria. The URL of the website is https://doubly-labelled-water-database.iaea.org/ home.
With over 2,500 published studies applying DLW to humans since the early 1980s, we estimate the number of valid data points that could be included into the database of un-manipulated free-living subjects to be around 20-25,000. If this estimate is correct, then at present we have captured only about 20% of the available data. It is the objective of the IAEA database to include as many as possible measurements, and investigators are highly encouraged to submit their records for inclusion, in particular from pre-2000. Individuals submitting data will be acknowledged as authors on any publications that use the database. Additional details regarding data submission is available on the IAEA database website.
If individuals wish to perform an analysis of the data in the database, they should examine the list of currently planned analyses that is available on the website under the "data analysis" tab. A short description of the types of data available in the database can be found on the website under the "data overview tab". The site is interactive and contains the number and type of measurements made in any target country. The detailed steps to apply for performing additional data analysis are described on the IAEA database website.
We would like the findings from all analyses using the database to be published. Manuscripts using the database must adhere to a number of rules that have been agreed by the management group, including draft manuscripts must be approved by the management group, and all contributing authors to the database will be co-authors on papers derived from it. Members of the management group will be co-corresponding authors. This will generate a large authorship list but follows the common practice in many multi-laboratory collaborations. Potential users of the database need to agree to this publication policy before access to perform analyses will be granted. The detailed instructions for potential authors can be viewed on the IAEA database website.
As a flavour of the size of the database and the variability in the data it contains, we show here a simple example analysis; Figure 2 shows the relationship between TEE by DLW (in MJ/day) and body weight (kg) in adults divided by sex. The database includes 3,220 measurements of adult females and 1,810 measurements of adult   Fig. 2b). If these are excluded, the average male expenditure falls to 12.34 MJ/day (equivalent to 2,950 kcal/day). A key goal of the future analyses of the database is to try and understand the factors that drive the tremendous individual variability that is evident in these plots.

Conclusion
The DLW method enables unrestricted measures of free-living TEE over periods of 1-3 weeks and is the gold standard method for such measurements. However, the expense of using it means individual studies generally have small sample sizes. The IAEA-sponsored DLW database aims to combine data from different studies to enable analyses that would not be feasible using individual smaller studies alone. As of May 2019, the database contains 6,621 measurements of TEE.
The database is generously supported by the IAEA and by the companies Taiyo Nippon Sanso, SERCON and ISOTEC. We are grateful to these companies for their support and especially to Takashi Oono for his tremendous efforts at fund raising on our be-