Clinical Trial Details
— Status: Completed
Administrative data
| NCT number |
NCT01938794 |
| Other study ID # |
13-1497 |
| Secondary ID |
R44DK093362 |
| Status |
Completed |
| Phase |
|
| First received |
|
| Last updated |
|
| Start date |
September 2013 |
| Est. completion date |
December 31, 2017 |
Study information
| Verified date |
October 2019 |
| Source |
University of Colorado, Denver |
| Contact |
n/a |
| Is FDA regulated |
No |
| Health authority |
|
| Study type |
Observational
|
Clinical Trial Summary
Accurately measuring how many calories a person burns each day is difficult to do.
Researchers can do this with a technique called doubly labeled water (DLW). This involves
drinking water that is "labeled" with a non-radioactive tracer. After a few hours, the
labeled water can be detected in the urine. To measure how many calories are burned (Total
daily energy expenditure, TDEE), urine samples are collected several days apart. Although
this technique is accurate, it is also challenging for two reasons. First, the labeled water
is expensive. Second, the urine samples are analyzed using equipment (Isotope Ratio Mass
Spectrometer, or IRMS) that is expensive and difficult to operate. The goal of this project
is to develop a new instrument to perform DLW measurements of TDEE. This instrument, called a
triple isotope water analyzer (TIWA) is less expensive and easier to operate than IRMS.
Additionally, since the TIWA is more accurate than IRMS, it may potentially reduce the amount
of labeled water required to measure TDEE, and thus reduce costs. The purpose of this study
is to compare the accuracy of measuring TDEE from labeled water using the new instrument
(TIWA) and from the traditional approach (IRMS). We will also compare the accuracy to the
measurement of TDEE from whole-room indirect calorimetry (metabolic room), which is
considered the most accurate way to measure TDEE.
Description:
The high prevalence of obesity in the US (17) is a major public health concern, as overweight
and obese individuals are at increased risk for many chronic diseases (5, 7, 15, 18). Obesity
stems from an imbalance between total caloric consumption and total energy expenditure (TEE),
although the causes of this imbalance remain debated (29). Accurate and precise measurements
of TEE therefore play a pivotal role in understanding and ultimately reversing this epidemic.
TEE can be measured using direct (measurement of heat production) or indirect (measurement of
respiratory gas exchange) calorimetry (4), but neither of these approaches are practical for
measuring TEE in free living subjects. The gold standard for measuring TEE in free-living
individuals is the doubly labeled water (DLW) method, which is based on the principle that
the oxygen in body water is in complete isotopic equilibrium with the oxygen in dissolved
respiratory carbon dioxide due to the action of carbonic anhydrase. The consequence of this
exchange is that an isotopic label of oxygen introduced into body water is eliminated by the
combined flux of body water and the exhaled carbon dioxide. Lifson and colleagues reasoned
that, since hydrogen is found only in water and not in carbon dioxide, the elimination of a
hydrogen isotope would be affected solely by the flux of body water (11). Thus the difference
in the rates of isotope elimination of simultaneously administered oxygen and hydrogen labels
is a measure of CO2 production.
However, despite its widespread use (6, 9, 10, 20, 25, 29), the DLW method has some major
limitations. Individual measurements are only precise to ± 7 % at best (23), so the method is
currently most suitable for studies of groups rather than individual variation. A second
problem is that the test is expensive to perform due to the need for relatively large sample
sizes to achieve sufficient statistical power, the large quantities of H218O needed for
dosing (23), and IRMS analysis. High levels of 18O are required to distinguish the dose from
background isotope levels after 10 - 21 days of elimination. It currently costs $500 - $750
for the 18O required to perform a DLW measurement on an adult subject (50 - 75 kg fat free
mass) and the cost is unpredictable due to fluctuations in demand from the medical diagnostic
PET scan. The need for high 18O enrichments is caused by fluctuations in the background
isotope levels over time (8). This uncertainty in the background levels increases the isotope
dose that must be administered and contributes to the uncertainty in the DLW measurements as
compared to the reference calorimetry measurements of TEE in validation studies. Finally,
IRMS analysis presents its own set of challenges, including the need for sophisticated,
expensive instrumentation with dedicated, highly trained operators, and, in general,
measurement of only one isotope ratio at a time, reducing analytical throughput. Because of
these challenges, most researchers conducting DLW tests do not maintain in-house IRMS
facilities, relying instead on expensive and slow analyses by outside measurement
laboratories. The proposed work will address these problems by developing a new
triple-isotope method for DLW analysis, significantly improving the individual accuracy of
the measurements and reducing the cost of the DLW method, leading to more widespread use of
the DLW method in both clinical and research applications.
The overall goal of this Small Business Innovation Research (SBIR) Phase II grant is to
develop and validate a new instrument to measure and correct for the background isotope
levels of 18O and 2H during DLW analysis by measuring the 17O stable isotope of oxygen in
body water. This approach will address the two major limitations addressed above. First, by
using 17O measurements to correct for background fluctuations in 18O and 2H, this approach
will reduce the amount of 18O, and thus cost, of performing DLW studies. Results from our
Phase I studies (see Preliminary Data below) show that background fluctuations in 18O and 17O
in body water are correlated with an R2 of 0.96, background fluctuations in 2H and 17O are
correlated with an R2 of 0.89, and background fluctuations in 2H and 18O are correlated with
an R2 of 0.92. Based on these correlations, using 17O measurements to estimate the background
fluctuations of the 2H and 18O will provide an estimated forty percent decrease in the
uncertainty of the DLW method due to background fluctuation. Second, The proposed instrument
will be utilized in the new, triple-isotope method for DLW which will reduce existing
barriers to widespread use of the DLW method by improving precision, reducing costs, reducing
the technical expertise required to perform the analysis, and increasing throughput.
Development of the new instrument will be performed by our business partners, Los Gatos
Research, and validation studies will be performed at the University of Colorado Anschutz
Medical campus.
In this work, we will apply Los Gatos Research's ultrasensitive absorption spectroscopy
technology, Off-Axis Integrated Cavity Output Spectroscopy (Off-Axis ICOS), to simultaneously
and inexpensively (< $50 per sample) measure 2H, 18O, and 17O in liquid water samples.
Briefly, in Off-Axis ICOS, laser light is coupled to an optical cavity in an off-axis fashion
and is continuously measured similar to a standard absorption experiment (Figure 1) (1). The
cavity provides an extraordinarily long effective optical pathlength (e.g. typically 5 - 10
km) allowing for the accurate quantification of weakly absorbing molecules. Moreover, since
the off-axis beam path is not unique, the system is extremely insensitive to changes in
alignment, making it robust. This robustness combined with the long effective optical
pathlength makes it possible to measure water isotopomers with very high precision. Since its
development, Los Gatos Research (LGR) and its commercial customers have performed many
experiments to validate the sensitivity and robustness of Off-Axis ICOS to measure a variety
of trace gases including water isotopomers H2O, 1H2HO, and H218O (2, 12, 14, 19, 26, 27) and
most recently water isotopomers in undistilled human urine (3).
