Doubly Labeled Water Estimates Research Articles

High levels of isotope elimination improve precision and allow individual-based measurements of metabolic rates in animals using the doubly labeled water method.

Doubly labeled water (DLW) can be used to measure energy expenditure in free-ranging animals, but questions have been raised about its accuracy in different species or contexts. We investigated whether differences in the extent of isotope elimination affects the precision and accuracy of the DLW method, which can vary according to the experimental design or metabolic rate of the species. Estimated total energy expenditure by the DLW method (TEEdlw) was compared with actual total energy expenditure simultaneously measured via respirometry (TEEresp) in streaked shearwaters Calonectris leucomelas, a pelagic seabird. Subjects were divided into three groups with different experimental conditions: at rest on the ground for 24 h (Group A) or for 48 h (Group B), and at rest on the water for 24 h (Group C). TEEdlw in Group A matched TEEresp, whereas there was an overestimation of TEEdlw in both Groups B and C compared with TEEresp. However, compared with Group A, TEEdlw in Groups B and C had reduced the isotopic analytical variability and thus higher precision. The best regression model (TEEdlw = 1.37 TEEresp − 14.12) showed a high correlation (R2 = 0.82) between TEEdlw and TEEresp and allows a correction factor for field metabolic rates in streaked shearwaters. Our results demonstrate that the commonly made assumption that the DLW method is not appropriate for individual-based estimates may be incorrect in certain circumstances. Although a correction factor may be necessary when using the DLW method to estimate metabolic rate, greater levels of isotope eliminations provides DLW estimates with high precision, which can adequately represent relative individual estimates. Nevertheless, the DLW method, should be used with caution when characterizing interspecies difference of energy expenditures.

The Sedentary Time and Activity Reporting Questionnaire (STAR-Q): reliability and validity against doubly labeled water and 7-day activity diaries.

We determined measurement properties of the Sedentary Time and Activity Reporting Questionnaire (STAR-Q), which was designed to estimate past-month activity energy expenditure (AEE). STAR-Q validity and reliability were assessed in 102 adults in Alberta, Canada (2009-2011), who completed 14-day doubly labeled water (DLW) protocols, 7-day activity diaries on day 15, and the STAR-Q on day 14 and again at 3 and 6 months. Three-month reliability was substantial for total energy expenditure (TEE) and AEE (intraclass correlation coefficients of 0.84 and 0.73, respectively), while 6-month reliability was moderate. STAR-Q-derived TEE and AEE were moderately correlated with DLW estimates (Spearman's ρs of 0.53 and 0.40, respectively; P < 0.001), and on average, the STAR-Q overestimated TEE and AEE (median differences were 367 kcal/day and 293 kcal/day, respectively). Body mass index-, age-, sex-, and season-adjusted concordance correlation coefficients (CCCs) were 0.24 (95% confidence interval (CI): 0.07, 0.36) and 0.21 (95% CI: 0.11, 0.32) for STAR-Q-derived versus DLW-derived TEE and AEE, respectively. Agreement between the diaries and STAR-Q (metabolic equivalent-hours/day) was strongest for occupational sedentary time (adjusted CCC = 0.76, 95% CI: 0.64, 0.85) and overall strenuous activity (adjusted CCC = 0.64, 95% CI: 0.49, 0.76). The STAR-Q demonstrated substantial validity for estimating occupational sedentary time and strenuous activity and fair validity for ranking individuals by AEE.

Accuracy of Armband Monitors for Measuring Daily Energy Expenditure in Healthy Adults

There is a need to develop accurate devices for measuring daily energy expenditure under free-living conditions, particularly given our current obesity epidemic. The purpose of the present study was to evaluate the validity of energy expenditure estimates from two portable armband devices, the SenseWear Pro3 Armband (SWA) monitor and the SenseWear Mini Armband (Mini) monitor, under free-living conditions. Participants in the study (30 healthy adults aged 24-60 yr) wore both monitors for 14 consecutive days, including while sleeping. Criterion values for total energy expenditure (TEE) were determined using doubly labeled water (DLW), the established criterion standard method for free-living energy expenditure assessment. The average TEE estimates were within 112 kcal·d−¹ for the SWA and within 22 kcal·d−¹ for the Mini, but the absolute error rates (computed as the average absolute value of the individual errors) were similar for the two monitors (SWA = 8.1% ± 6.8%, Mini = 8.3% ± 6.5%). Using intraclass correlation (ICC) analysis, significant agreements were found between the SWA and DLW estimates of energy expenditure (ICC = 0.80, 95% CI = 0.89-0.70) and between the Mini and DLW (ICC = 0.85, 95% CI = 0.92-0.76). Graphical plots of the DLW TEE values against the difference between DLW and monitor estimates of TEE showed that the agreement was consistent across a range of TEE values. The SenseWear Pro3 and the SenseWear Mini armbands show promise for accurately measuring daily energy expenditure under free-living conditions. However, more work is needed to improve the ability of these monitors to accurately measure energy expenditure at higher levels of expenditure.

Estimating activity energy expenditure: how valid are physical activity questionnaires?

Activity energy expenditure (AEE) is the modifiable component of total energy expenditure (TEE) derived from all activities, both volitional and nonvolitional. Because AEE may affect health, there is interest in its estimation in free-living people. Physical activity questionnaires (PAQs) could be a feasible approach to AEE estimation in large populations, but it is unclear whether or not any PAQ is valid for this purpose. Our aim was to explore the validity of existing PAQs for estimating usual AEE in adults, using doubly labeled water (DLW) as a criterion measure. We reviewed 20 publications that described PAQ-to-DLW comparisons, summarized study design factors, and appraised criterion validity using mean differences (AEEPAQ − AEEDLW, or TEEPAQ − TEEDLW), 95% limits of agreement, and correlation coefficients (AEEPAQ versus AEEDLW or TEEPAQ versus TEEDLW). Only 2 of 23 PAQs assessed most types of activity over the past year and indicated acceptable criterion validity, with mean differences (TEEPAQ − TEEDLW) of 10% and 2% and correlation coefficients of 0.62 and 0.63, respectively. At the group level, neither overreporting nor underreporting was more prevalent across studies. We speculate that, aside from reporting error, discrepancies between PAQ and DLW estimates may be partly attributable to 1) PAQs not including key activities related to AEE, 2) PAQs and DLW ascertaining different time periods, or 3) inaccurate assignment of metabolic equivalents to self-reported activities. Small sample sizes, use of correlation coefficients, and limited information on individual validity were problematic. Future research should address these issues to clarify the true validity of PAQs for estimating AEE.

Using doubly-labeled water measurements of human energy expenditure to estimate inhalation rates

Doubly-labeled water (DLW) data is recognized as an improvement over alternative methods to quantify human energy expenditure. Previously, energy expenditure has been estimated indirectly using heart-rate monitoring, calorimetry, or accelerometer measurements. Inhalation rate estimates can benefit from improved energy expenditure estimates using equations developed by Layton. DLW methods are advantageous for several reasons: the database is robust, they are direct measures, subjects are free-living, and the observation period is longer than what is possible from staged activity measures. DLW energy data is an improvement over previous inhalation estimates based on dietary recall survey data. Mean long-term inhalation rates of 16 m3/day and 13 m3/day, for physically active adult men and women, respectively, were derived based on DLW estimates of energy expended. The range of human energy expenditure is narrow with the maximum energy expenditure not likely greater than twice the minimum.

Validation of the doubly‐labeled water (DLW) method for estimating CO2 production and water flux in growing poultry chicks

This study is the first validation of the doubly‐labeled water (DLW) method on birds (1) to evaluate the accuracy of 2 points versus multiple points for computing fractional isotopic washout rates (k) and CO2 production (rCO2), (2) to measure CO2 production and water flux each day over a 4‐day period, (3) to compare measured fractional evaporative water loss (rG) with assumed values that provide DLW estimates of rCO2 with zero error, and (4) to measure the effect of assumed rG on the error of estimating water influx and efflux. Percent error of CO2 production of six growing poultry chicks estimated by the DLW method was not correlated with mean daily relative growth rates of up to 5% nor with daily rates of energy retained in growth of up to 320 kJ/day/kg, nor was it significantly reduced by using multiple points (5 points) rather than 2 points to compute fractional isotopic washout rates (k) and isotope pool sizes. Its seems clear from our study and the previous 5 validations on growing birds that average relative daily growth rates of up to about 20% do not increase the error of estimating rCO2 by the DLW method. Arithmetic error was significantly less when using one isotopic pool, rather than two pools, to compute rCO2 and was less when using an assumed fractional evaporative water loss (rG) of 0.45 rather than an assumed rG of 0.25 or 0.5 (the two values used predominantly in previous DLW studies). Our study supports Speakman's (1997) suggestion that the one‐pool model is more appropriate than the two‐pool model for birds weighing&lt;1 kg. We recommend using an assumed rG of 0.45 to compute rCO2 of poultry, which is a compromise between the two schools of rG useage, i.e., rG=0.25 or 0.5, however we hesitate to recommend 0.45 for all birds in all settings. Close agreement between measured rG and an assumed rG that produced zero rCO2 error supports the validity of using the pooled fractionation correction factors (fpool) of 0.0339 for tritiated water and 0.0249 for deuterated water. Absolute error decreased with the percent washout of during measurement periods of 1 to 4 days. Accuracy of estimating rCO2 was not significantly different for durations of 2, 3, and 4 days using either tritiated or deuterated water. The arithmetic error of estimating rCO2 using a one isotopic pool model, 2 points, an rG of 0.5, and tritiated water was −1.9% (SD=13.5) for the first day of a 4‐day period and −4.0% (SD=8.9) for the entire period. Percent arithmetic error of water influx (rH2Oinf) and efflux (rH2Oeff) estimated for day 1 from tritiated water washout and an assumed rG of 0.5 was −0.5 (SD=6.4) and 0.1% (SD=11.1), respectively. An rG of 0.5 produced significantly less arithmetic error than an rG of 0.25 or an rG of zero (i.e., no fractionation correction), and less absolute error in rH2Oinf. Errors were slightly more negative (underestimates) with an rG of 0.25, i.e., −2.2 and −2.0%, respectively and even more negative with no correction for isotopic fractionation (i.e., an assumed rG of zero). Tritiated water estimates of water influx and efflux during the first day had no error when using an rG of 0.57 and 0.48, respectively. With assumed rGs of 0.25 and 0.5, the errors of water influx were −7.8 and −5.9%, and the errors of water efflux were 3.4 and 5.6%, respectively, over 4 days. We recommend using an assumed rG of 0.45 to compute rH2Oeff for poultry. The error of rCO2 was about 3 to 4 times more sensitive to values of assumed rG than the error of water flux.

Validation of the doubly labeled water method under low and high humidity to estimate metabolic rate and water flux in a tropical snake (Boiga irregularis).

This study uses indirect calorimetry to assess the effects of humidity on the accuracy of the doubly labeled water (DLW) technique to predict metabolic rate and water flux in brown treesnakes (Boiga irregularis). The DLW technique accurately predicted total water efflux in brown treesnakes under low-humidity conditions and found that the total number of water molecules exchanged with the environment under humid conditions was not significantly different than maximum net total evaporative water loss under low humidity conditions plus fecal water loss. Because of changes of total body water of >12%, the DLW technique overestimated metabolic rate by a factor of 2.2 under low-humidity conditions. Under high-humidity conditions, the DLW technique overestimated metabolic rate in brown treesnakes by a factor of 4.6. Researchers using the DLW technique in humid or moist environments should be cautious because this study indicates that DLW estimates of metabolic rate may be inflated when large amounts of water vapor are exchanged through the skin or respiratory passages.

Precision of DLW energy expenditure measurements: contribution of natural abundance variations.

Accuracy and precision of the the doubly labeled water (DLW) measurements of energy expenditure are commonly estimated with models that do not account for 2H and 18O natural abundance variation. A new and simple treatment that completes the ratio-product model is derived in the present study. It uses both linear regression of multipoint data with time and information on within-subject 2H and 18O natural abundance variation. Use of this treatment is demonstrated in a group of seven subjects in whom 2H and 18O natural abundance variation was assessed by collecting predose samples for 6-8 days before dosing. In this set of 16 measurements, the precision (coefficient of variation) of individual DLW estimates of CO2 production was 4.90 +/- 2.14%, of which 3.23 +/- 1.20% arose from natural abundance variation.

Estimation of precision in DLW studies using the two-point methodology.

Previous attempts to estimate precision of doubly labeled water (DLW) estimates of CO2 production, using propagation of error analyses, have necessarily made simplifying assumptions which may compromise the resultant error estimate. Using an empirical iteration approach, error distributions for the DLW calculation were generated which overcome these problems. The error distribution for CO2 estimates generated by DLW is symmetrical but not normal. The distribution is significantly truncated such that the 99% inclusion limits are 2.034 SD and not 2.58 SD. The precision error (99% CI for mean as percent of the mean) in DLW experiments, when using duplicate analyses, varies between approximately 3% and 47% depending on the ratio of the elimination constants of the two labels (ko/kd), experimental duration and initial isotope dose. The error could be improved by approximately 10 fold by increasing the number of replicates at all six isotope determinations from 2 to 5. Estimating precision in actual experiments can be made using the same empirical approach. The resultant estimates can be of extreme value in evaluating the role of precision as a factor influencing deviations during validation studies, and also for weighting mean estimates in applications of the technique. The deviations of DLW estimates from those made simultaneously by indirect calorimetry in a small mammal validation study were generally greater than the precision of the DLW estimates of CO2 production. This may indicate there are more problems with the technique than precision alone.

Validation of the Doubly Labeled Water Method (3HH18O) for Measuring Water Flux and CO2 Production in the Tropical Hummingbird Amazilia saucerottei

We validated the doubly labeled water (DLW) method for a tropical hummingbird (Amazilia saucerottei) in a humid environment. We compared daily water influx and energy intake, expenditure, and storage using two independent techniques: the DL W method (using 3HH18O [HTO-18] with and without fractionation corrections) and a gravimetric method. Uncorrected DLW values differed significantly from gravimetric values, with water influx 4.5 % lower, energy intake 4.6 % lower, and energy expenditure 15.1 % higher than for the gravimetric method. In contrast, the corrected DLW values were statistically equivalent to the gravimetric values, with differences ranging from 1.5 % lower to 0.1 % higher. This suggests that physical fractionation of isotopes during evaporative water loss was appreciable but that corrections based on published fractionation constants yielded highly accurate DLW estimates. We conclude that two potential problems in using the DLW method on tropical hummingbirds, water vapor input from humid air and dynamic biological effects from high rates of water turnover and energy metabolism, did not contribute significant error to DLW estimates corrected for fractionation.

Validation of the doubly labeled water technique for measuring energy metabolism in starlings and sparrows

1. 1. I have tested the idea that doubly labeled water (DLW) can accurately predict CO 2; production in savannah sparrows, song sparrows, white-throated sparrows, starlings, and a single house sparrow by comparing DLW estimates with those obtained simultaneously by capturing expired CO; in Ascarite. In addition I used the energy balance method to see if metabolic rates generated from DLW measurements accurately reflected the actual metabolic rates of these birds. 2. 2. I found close agreement in DLW and the gravimetric and energy balance methods, with DLW underestimating CO 2; production on average by −3.5% in sparrows, and −7.1% in starlings. Similarly, the energy balance method indicated a −3.1% underestimate by DLW for sparrows and a −5.1% for starlings. 3. 3. The DLW method can yield reasonable estimates of CO 2; production in a variety of passerine birds.

Daily Energy Expenditure of Savannah Sparrows: Comparison of Time-Energy Budget and Doubly-Labeled Water Estimates

Abstract Using the doubly-labeled water (DLW) technique, we measured the CO2 production of six free-living male Savannah Sparrows (Passerculus sandwichensis, mean mass 19.1 g) and found a mean (±1 SD) of 7.28 ± 1.36 ml ${\rm CO}_{2}/{\rm h}$. Assuming an R.Q. of 0.85 and 24 J/ml CO2, we calculated a mean $\dot{H}_{TD}$ (daily energy expenditure) of 80.3 ± 16.6 kJ/day. Simultaneously to our DLW measurements, we constructed time budgets for the sparrows during their normal activities with the intent of exploring the agreement between predictions of a number of time-energy budget (TEB) models from the literature and DLW measurements. Our results reveal that several models yield reasonable results, the mean value for the models of Kendeigh et al. (1977), Walsberg (1977), and Mugaas and King (1981) differing from DLW measurements by only +4.7, -5.6%, and +5.7%, respectively. Comparisons of $\dot{H}_{TD}$ as predicted by these TEB models between species and within species between nesting phases might be made with some confidence. However, the variation in TEB estimates is not related to variation in DLW measurements within our study. This precludes statements about differences between individuals.