A new approach to calculate energy expenditure as assessed with doubly labelled water during high energy turnover: a proof of concept study.

Doubly labelled water is considered the gold standard technique to assess daily
life energy expenditure over a period of several days up to 3 weeks. The
technique is based on the principle that when a dose of water containing two
stable isotopes (2H and 18O) is given to a subject, the 2H isotope will be
eliminated from the body as water (H2O) and the 18O isotope will be eliminated
as both water and carbon dioxide (CO2). The difference in disappearance rates
of both isotopes is hence a measure of CO2-production, from which energy
expenditure can be calculated. In principle, there are two different
calculation techniques used to determine the dilution space (distribution
volume) of these isotopes in the body. The way the dilution space is calculated
also affects the calculated CO2-production. These two techniques are generally
referred to as the *plateau* technique and the *slope-intercept* technique. In
the current study, we investigate the possibility to calculate CO2-production
more accurately with a newly developed approach which will be referred to as
*overnight slope* technique. This technique will also allow to calculate
day-to-day variability in energy expenditure, rather then just energy
expenditure over the entire observation period (i.e. usually 2 weeks).

The aim of this study is to test whether our new *overnight slope* technique to
calculate the isotope dilution space results in a more accurate measurement of
doubly labelled water assessed CO2-production, which will be compared to
indirect calorimetry (respiration chambers) as the gold standard.

Methodological proof of concept study

The study poses a very low risk but a high burden for the subjects. As doubly
labelled water assesses CO2-production over several days (at least 5) and the
respiration chambers are being used as the reference technique, subjects will
have to reside inside the respiration chambers for 5 consecutive days. This
does not pose any risks but is a considerable burden for the subject. Also,
subjects will be cycling for several hours per day inside the chamber in order
to obtain a high physical activity level (PAL = 2.5 - 3.0). Because of this
burden, subjects will be allowed to exit the chamber every evening for
approximately 30 min in order to take a shower. CO2-production from this 30 min
outside the chamber will be estimated based on measured CO2-production over 30
min during the day when not exercising.
Doubly labelled water is safe for use in humans and the required dose for
energy expenditure assessment does not pose any risks to the subjects. Only
urine and saliva samples will be taken.
To monitor all movement inside the chamber, wearable sensors (accelerometers)
will be used. Physical activity as assessed with these accelerometers will be
used to assess biological variation in spontaneous physical activity over the
different days. Accelerometers pose a minimal burden to the subject