A New Approach to Measuring Energy Expenditure in Humans
Condition: Human Energy Expenditure
Study type: Observational

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.
    

Detailed 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).
    

Observational model: Other
Time perspective: Cross-Sectional
Sampling method: Probability Sample
Study population: 
        Healthy adults (>18 yrs)