Natural abundance deuterium and 18-oxygen effects on the precision of the doubly labeled water method

Comparison of isotope ratio mass spectrometry and cavity ring-down spectroscopy procedures and precision of the doubly labeled water method in different physiological specimens

RATIONALE

This study determines if saliva collection procedures for the doubly labeled water (DLW) method, used for measuring total energy expenditure (TEE), are comparable to urine and plasma collection. Both the cavity ring-down spectroscopy (CRDS) and isotope ratio mass spectrometry (IRMS) analyses techniques are compared.

METHODS

Saliva specimens were collected from participants for the DLW method. The specimens were collected under different conditions: after consumption of tap water, after chewing gum, and during exposure to conditions of high and low relative humidity. The isotopes in saliva were compared with simultaneous plasma and urine collection. TEE calculated from saliva and analyzed using CRDS was compared to that of plasma analyzed using IRMS.

RESULTS

The within-individual variances were not significantly different between the saliva specimens (0.4‰) and plasma (0.3‰). After the oral dose of DLW, the saliva specimens displayed a shorter equilibration time to urine. When participants consumed 500 mL of tap water, the enrichment of saliva specimens reached a new plateau value faster than urine. Saliva collection exposed to high ambient humidity conditions was slightly less enriched as compared to low-humidity conditions while urine enrichment was unaffected. In contrast, whereas the within-individual effects of gum chewing during saliva collection on ¹⁸ O were unaffected, the abundance of ² H in saliva was slightly lower after chewing the gum. The within-individual difference between TEE calculated from saliva and that calculated from plasma analyzed using IRMS did not differ from zero, and the standard deviation was not different from that predicted by a propagation of error analysis based on analytical error alone.

CONCLUSIONS

Our findings support using saliva specimens for the DLW method. The analysis of plasma and urine, however, requires reducing the memory effect resulting from contaminants. Also, it should be performed in a manner that minimizes exposure to air where specimens may be exposed to evaporation or contamination from water vapor during sampling.

Maximizing precision and accuracy of the doubly labeled water method via optimal sampling protocol, calculation choices, and incorporation of 17O measurements

BACKGROUND/OBJECTIVES

The doubly labeled water (DLW) method is the gold standard methodology for determination of free-living, total daily energy expenditure (TEE). However, there is no single accepted approach for either the sampling protocols (daily vs. two-point, in which samples are collected after dosing and at the end of the measurement period) or the calculations used in the determination of the rate of carbon dioxide production (rCO₂) and TEE. Moreover, fluctuations in natural background abundances introduce error in the calculation of rCO₂ and TEE. The advent of new technologies makes feasible the possibility of including additional isotope measures (¹⁷O) to account for background variation, which may improve accuracy.

SUBJECTS/METHODS

Sixteen subjects were studied for 7 consecutive days in a whole-room indirect calorimeter (IC) with concurrent measurement of TEE by DLW. Daily urine samples were obtained and isotope ratios were determined using off-axis integrated cavity output spectroscopy (OA-ICOS).

RESULTS

We determined the best combination of approaches for estimating dilution spaces and elimination rates and calculated average daily volume of carbon dioxide production (VCO₂) using six different published equations. Using this best combination, multi-point fitting of isotope elimination rates using the daily urine samples substantially improved the average precision (4.5% vs. 6.0%) and accuracy (-0.5% vs. -3.0%) compared with the two-point method. This improvement may partly reflect the less variable day-to-day chamber measurements of energy expenditure. Utilizing ¹⁷O measurements to correct for errors due to background isotope fluctuations provided additional but minor improvements in precision (4.2% vs. 4.5%) and accuracy (0.2% vs. 0.5%).

CONCLUSIONS

This work shows that optimizing sampling and calculation protocols can improve the accuracy and precision of DLW measurements.

Implications of the variation in biological 18 O natural abundance in body water to inform use of Bayesian methods for modelling total energy expenditure when using doubly labelled water

RATIONALE

Variation in ¹⁸ O natural abundance can lead to errors in the calculation of total energy expenditure (TEE) when using the doubly labelled water (DLW) method. The use of Bayesian statistics allows a distribution to be assigned to ¹⁸ O natural abundance, thus allowing a best-fit value to be used in the calculation. The aim of this study was to calculate within-subject variation in ¹⁸ O natural abundance and apply this to our original working model for TEE calculation.

METHODS

Urine samples from a cohort of 99 women, dosed with 50 g of 20% ² H₂ O, undertaking a 14-day breast milk intake protocol, were analysed for ¹⁸ O. The within-subject variance was calculated and applied to a Bayesian model for the calculation of TEE in a separate cohort of 36 women. This cohort of 36 women had taken part in a DLW study and had been dosed with 80 mg/kg body weight ² H₂ O and 150 mg/kg body weight H₂ ¹⁸ O.

RESULTS

The average change in the δ¹⁸ O value from the 99 women was 1.14‰ (0.77) [0.99, 1.29], with the average within-subject ¹⁸ O natural abundance variance being 0.13‰² (0.25) [0.08, 0.18]. There were no significant differences in TEE (9745 (1414), 9804 (1460) and 9789 (1455) kJ/day, non-Bayesian, Bluck Bayesian and modified Bayesian models, respectively) between methods.

CONCLUSIONS

Our findings demonstrate that using a reduced natural variation in ¹⁸ O as calculated from a population does not impact significantly on the calculation of TEE in our model. It may therefore be more conservative to allow a larger variance to account for individual extremes.

Total energy expenditure and body composition of children with developmental disabilities

BACKGROUND

Obesity prevalence is increased in children with developmental disabilities, specifically in children with spina bifida and Down syndrome. Energy expenditure, a critical aspect of weight management, has been extensively studied in the typically developing population, but not adequately studied in children with developmental disabilities.

OBJECTIVE

Determine energy expenditure, fat-free mass and body fat percentile and the impact of these findings on recommended caloric intake in children with spina bifida and Down syndrome.

METHODS/MEASURES

This pilot study included 36 children, 18 with spina bifida, 9 with Down syndrome and 9 typically developing children. Half of the children with spina bifida were non-ambulatory. Doubly labeled water was used to measure energy expenditure and body composition. Descriptive statistics described the sample and MANOVA and ANOVA methods were used to evaluate differences between groups.

RESULTS

Energy expenditure was significantly less for children with spina bifida who primarily used a wheelchair (p = .001) and children with Down syndrome (p = .041) when compared to children without a disability when adjusted for fat-free mass. However, no significant difference was detected in children with spina bifida who ambulated without assistance (p = .072).

CONCLUSIONS

Children with spina bifida and Down syndrome have a significantly decreased energy expenditure which directly impacts recommended caloric intake. No significant difference was detected for children with spina bifida who ambulated, although the small sample size of this pilot study may have limited these findings. Validating these results in a larger study is integral to supporting successful weight management of these children.

Seasonal changes in background levels of deuterium and oxygen-18 prove water drinking by harp seals, which affects the use of the doubly labelled water method

The aim of this study was to monitor seasonal changes in stable isotopes of pool freshwater and harp seal (Phoca groenlandica) body water, and to study whether these potential seasonal changes might bias results obtained using the doubly labelled water (DLW) method when measuring energy expenditure in animals with access to freshwater. Seasonal changes in the background levels of deuterium and oxygen-18 in the body water of four captive harp seals and in the freshwater pool in which they were kept were measured over a time period of 1 year. The seals were offered daily amounts of capelin and kept under a seasonal photoperiod of 69°N. Large seasonal variations of deuterium and oxygen-18 in the pool water were measured, and the isotope abundance in the body water showed similar seasonal changes to the pool water. This shows that the seals were continuously equilibrating with the surrounding water as a result of significant daily water drinking. Variations in background levels of deuterium and oxygen-18 in freshwater sources may be due to seasonal changes in physical processes such as precipitation and evaporation that cause fractionation of isotopes. Rapid and abrupt changes in the background levels of deuterium and oxygen-18 may complicate calculation of energy expenditure by use of the DLW method. It is therefore strongly recommended that analysis of seasonal changes in background levels of isotopes is performed before the DLW method is applied on (free-ranging) animals, and to use a control group in order to correct for changes in background levels.

Validity of the Remote Food Photography Method Against Doubly Labeled Water Among Minority Preschoolers

OBJECTIVE

The aim of this study was to determine the validity of energy intake (EI) estimations made using the remote food photography method (RFPM) compared to the doubly labeled water (DLW) method in minority preschool children in a free-living environment.

METHODS

Seven days of food intake and spot urine samples excluding first void collections for DLW analysis were obtained on thirty-nine 3- to 5-year-old Hispanic and African American children. Using an iPhone, caregivers captured before and after pictures of each child's intake, pictures were wirelessly transmitted to trained raters who estimated portion size using existing visual estimation procedures, and energy and macronutrients were calculated. Paired t tests, mean differences, and Bland-Altman limits of agreement were performed.

RESULTS

The mean EI was 1,191 ± 256 kcal/d using the RFPM and 1,412 ± 220 kcal/d using the DLW method, resulting in a mean underestimate of 222 kcal/d (-15.6%; P < 0.0001) that was consistent regardless of intake. The RFPM underestimated EI by -28.5% in 34 children and overestimated EI by 15.6% in 5 children.

CONCLUSIONS

The RFPM underestimated total EI when compared to the DLW method among preschoolers. Further refinement of the RFPM is needed for assessing the EI of young children.

Using doubly-labelled water to measure free-living energy expenditure: Some old things to remember and some new things to consider

The doubly-labelled water (DLW) method provides the ability to measure the energy expenditure of free-living animals based only on the injection of two isotopes in water (one of oxygen and one of hydrogen) and traditionally the collection of 2 blood samples. We review here the fundamental basis of how the method works, and highlight how the choice of the appropriate calculation equation can have a large impact on the resultant estimates, particularly in species where the difference between the isotope elimination constants is small. This knowledge is not new, but is worth reiterating given the potential for error by making the wrong choice. In particular, it is important to remember that for mammals weighing less than 5kg, and birds weighing less than 2kg, the single pool models perform best in validation studies, while in mammals above 15kg the two-pool models perform best. Above 2kg in birds and between 5 and 15kg in mammals, however, the model superiority is uncertain. Even where the choice based on body mass would appear clear, the decision may need to be tempered by species specific information regarding potential additional sources for hydrogen turnover, such as de novo lipogenesis or methanogenesis. Recent advances in the technique have included attempts to make the method less invasive by using innovative methods for dosing and sample collection. In addition, the advent of laser spectroscopy, as a replacement technology for mass spectrometry, may open up many new opportunities in the field. These potentially include direct sampling of breath in the field and tracking background isotope drift using ¹⁷oxygen levels.

Special Considerations for Measuring Energy Expenditure with Doubly Labeled Water under Atypical Conditions

The global increase in the prevalence of obesity has dramatically increased interest in understanding the factors that influence human total energy expenditure (TEE). This in turn has increased interest in the doubly labeled water (DLW) method, a technique for measurement of total energy expenditure in free-living humans. The increasing use of this method is attributed to its portability, objectivity, minimal invasiveness, high accuracy and good precision. Although a relatively standard protocol for the method has emerged, the new generation of users often is unfamiliar with rationale behind aspects of the protocol as well as the approaches to avoid or correct for in situations that are not covered by the standard protocol procedure. The primary uncommon situations like introduction of isotopically different diet and fluids with or without geographical relocation, seasonal and temperature variations, activity level of participants etc. during or prior to the DLW measurements can lead to shift in baseline abundance of ²H and ¹⁸O or tracer elimination, resulting in moderate to large errors in the measured TEE. These unique situations call for special modifications to the conventional protocol to minimize errors. The objective of the present review was to assemble a list of frequently asked questions and the issues they represent, and then examine the available literature to describe and explain the modifications to the standard DLW protocol to maintain the method's accuracy. This discussion of DLW protocol modification can be an excellent resource for investigators who intend to use this measurement technique for interesting and uncommon study designs.

Seasonal variation in natural abundance of 2H and 18O in urine samples from rural Nigeria

The doubly labeled water (DLW) method is used to measure free-living energy expenditure in humans. Inherent to this technique is the assumption that natural abundances of stable isotopes (2)H and (18)O in body water remain constant over the course of the measurement period and after elimination of the loading dose of DLW will return to the same predose level. To determine variability in the natural abundances of (2)H and (18)O in humans living in a region with seasonal shifts in rain patterns and sources of drinking water, over the course of 12 mo we collected weekly urine samples from four individuals living in southwest Nigeria as well as samples of their drinking water. From ongoing regional studies of hypertension, obesity, and energy expenditure, we estimated average water turnover rate, urine volumes, and sodium and potassium excretion. Results suggest that (2)H and (18)O in urine, mean concentrations of urinary sodium and potassium, urine volume, and total body turnover differed significantly from dry to rainy season. Additionally, seasonal weather variables (mean monthly maximum temperatures, total monthly rainfall, and minimum relative humidity) were all significantly associated with natural abundances in urine. No seasonal difference was observed in drinking water samples. Findings suggest that natural abundances in urine may not remain constant as assumed, and studies incorporating DLW measurements across the transition of seasons should interpret results with caution unless appropriate doses of the tracers are used.

Inter- and intraindividual correlations of background abundances of (2)H, (18)O and (17)O in human urine and implications for DLW measurements

BACKGROUND/OBJECTIVES

The method of choice for measuring total energy expenditure in free-living individuals is the doubly labeled water (DLW) method. This experiment examined the behavior of natural background isotope abundance fluctuations within and between individuals over time to assess possible methods of accounting for variations in the background isotope abundances to potentially improve the precision of the DLW measurement.

SUBJECTS/METHODS

In this work, we measured natural background variations in (2)H, (18)O and (17)O in water from urine samples collected from 40 human subjects who resided in the same geographical area. Each subject provided a urine sample for 30 consecutive days. Isotopic abundances in the samples were measured using Off-Axis Integrated Cavity Output Spectroscopy.

RESULTS

Autocorrelation analyses demonstrated that the background isotopes in a given individual were not temporally correlated over the time scales of typical DLW studies. Using samples obtained from different individuals on the same calendar day, cross-correlation analyses demonstrated that the background variations of different individuals were not correlated in time. However, the measured ratios of the three isotopes (2)H, (18)O and (17)O were highly correlated (R(2)=0.89-0.96).

CONCLUSIONS

Although neither specific timing of DLW water studies nor intraindividual comparisons were found to be avenues for reducing the impact of background isotope abundance fluctuations on DLW studies, strong inter-isotope correlations within an individual confirm that use of a dosing ratio of 8‰:1‰ (0.6 p.p.m.: p.p.m.) optimizes DLW precision. Theoretical implications for the possible use of (17)O measurements within a DLW study require further study.

The doubly labeled water method produces highly reproducible longitudinal results in nutrition studies

The doubly labeled water (DLW) method is considered the reference method for the measurement of energy expenditure under free-living conditions. However, the reproducibility of the DLW method in longitudinal studies is not well documented. This study was designed to evaluate the longitudinal reproducibility of the DLW method using 2 protocols developed and implemented in a multicenter clinical trial-the Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE). To document the longitudinal reproducibility of the DLW method, 2 protocols, 1 based on repeated analysis of dose dilutions over the course of the clinical trial (dose-dilution protocol) and 1 based on repeated but blinded analysis of randomly selected DLW studies (test-retest protocol), were carried out. The dose-dilution protocol showed that the theoretical fractional turnover rates for (2)H and (18)O and the difference between the 2 fractional turnover rates were reproducible to within 1% and 5%, respectively, over 4.5 y. The Bland-Altman pair-wise comparisons of the results generated from 50 test-retest DLW studies showed that the fractional turnover rates and isotope dilution spaces for (2)H and (18)O, and total energy expenditure, were highly reproducible over 2.4 y. Our results show that the DLW method is reproducible in longitudinal studies and confirm the validity of this method to measure energy expenditure, define energy intake prescriptions, and monitor adherence and body composition changes over the period of 2.5-4.4 y. The 2 protocols can be adopted by other laboratories to document the longitudinal reproducibility of their measurements to ensure the long-term outcomes of interest are meaningful biologically. This trial was registered at clinicaltrials.gov as NCT00427193.

δ2H and δ18O of human body water: a GIS model to distinguish residents from non-residents in the contiguous USA

An understanding of the factors influencing the isotopic composition of body water is important to determine the isotopic composition of tissues that are used to reconstruct movement patterns of humans. The δ(2)H and δ(18)O values of body water (δ(2)H(bw) and δ(18)O(bw)) are related to the δ(2)H and δ(18)O values of drinking water (δ(2)H(dw) and δ(18)O(dw)), but clearly distinct because of other factors including the composition of food. Here, we develop a mechanistic geographical information system (GIS) model to produce spatial projections of δ(2)H(bw) and δ(18)O(bw) values for the USA. We investigate the influence of gender, food, and drinking water on the predicted values by comparing them with the published values. The strongest influence on the predicted values was related to the source of δ(2)H(dw) and δ(18)O(dw) values. We combine the model with equations that describe the rate of turnover to produce estimates for the time required for a non-resident to reach an isotopic equilibrium with a resident population.

Validation of deuterium and oxygen18 in urine and saliva samples from children using on-line continuous-flow isotope ratio mass spectrometry

The doubly labelled water method is valuable for measuring energy expenditure in humans. It usually involves blood or urine sampling, which might be difficult in neonates and children with cerebral palsy or other disabilities. We therefore aimed to validate a method making use of saliva samples analyzed by automated thermal conversion elemental analyzer in combination with isotope ratio mass spectrometry (TC-EA/IRMS). The subjects received labelled water orally and urine and saliva samples were collected and analyzed. Deuterium as well as oxygen18 was measured in one single run using a peak jump method. Excellent linearity was found for measurement of enrichments of deuterium (R2 = 0.9999) and oxygen18 (R2 = 0.9999). The intra-assay precision and the inter-assay precision of the measurement of two standards were good for both deuterium and oxygen18. The variation between urine and saliva samples was small (4.83% for deuterium and 2.33% for oxygen18 n = 40). Saliva sampling is to be preferred, therefore, as it can be easily collected and is non-invasive. Moreover, its time of production is almost exactly known. The TC-EA/IRMS method is a good alternative to the more laborious off-line IRMS measurements.

D and 18O enrichment measurements in biological fluids in a continuous-flow elemental analyser with an isotope-ratio mass spectrometer using two configurations

In doubly labelled water studies, biological sample enrichments are mainly measured using off-line techniques (equilibration followed by dual-inlet introduction) or high-temperature elemental analysis (HT-EA), coupled with an isotope-ratio mass spectrometer (IRMS). Here another continuous-flow method, (CF-EA/IRMS), initially dedicated to water, is tested for plasma and urine analyses. The elemental analyser configuration is adapted for each stable isotope: chromium tube for deuterium reduction and glassy carbon reactor for 18O pyrolysis. Before on-line conversion of water into gas, each matrix is submitted to a short and easy treatment, which is the same for the analysis of the two isotopes. Plasma is passed through centrifugal filters. Urine is cleaned with black carbon and filtered (0.45 microm diameter). Tested between 150 and 300 ppm in these fluids, the D/H ratio response is linear with good repeatability (SD<0.2 ppm) and reproducibility (SD<0.5 ppm). For 18O/16O ratios (from 2000 to 2200 ppm), the same repeatability is obtained with a between-day precision lower than 1.4 ppm. The accuracy on biological samples is validated by comparison to classical dual-inlet methods: 18O analyses give more accurate results. The data show that enriched physiological fluids can be successfully analysed in CF-EA/IRMS.

The role of technology in the past and future development of the doubly labelled water method

The doubly labelled water method is an isotope-based technique that is used to measure the energy demands of free-living animals and humans. It is based on the observation that, in the body, the oxygen in carbon dioxide is in complete isotope exchange equilibrium with the oxygen in body water. Hence, a label of isotopic oxygen in body water is eliminated by both respiratory CO(2) and water turnover, whereas a similarly introduced label of deuterium is eliminated only by water flux. The difference in isotope fluxes therefore permits estimation of CO(2) production, which is correlated to energy demands. The doubly labelled water method has been advanced predominantly by technological advances in mass spectrometry. Although it was first described in the 1950s, it was only used on small animals and in low numbers because the costs of the isotopes were a primary constraint. However, advances in mass spectrometry precision and accuracy in the 1980s made it possible to reduce the quantities of isotope used, and hence apply the method on humans, although still in small numbers. The advent of continuous flow inlets in the 1990s made possible the processing of samples in much larger numbers and the sample sizes of studies have expanded. Ironically, however, the technique is now under treat because of technological advances in another area (positron emission tomography), which has generated an enormous demand for (18)O and pushed up the price of isotopes. A continuation of this trend might drive prices to levels where sustained application of the method in human studies is questionable. Replacing determination of isotope enrichments currently performed by isotope ratio mass spectrometry with determinations made by stable isotope infrared laser spectrometry may be a technological advance that will get us out of this problem.

Validation of habitual energy intake

OBJECTIVE

To provide a framework for use of the doubly labelled water method to measure energy expenditure in order to validate dietary instruments for the assessment of energy.

DESIGN

Review and description of the use of doubly labelled water method for use as a biomarker for habitual energy intake.

RESULTS

The doubly labelled water method has a relative accuracy of 1% and within-subject precision of 5 to 8%. Comparison of self-reported energy intake with energy expenditure demonstrated that over one-third of individuals may underreport energy intake by more than 25%.

CONCLUSIONS

The doubly labelled water method, although expensive and dependent on non-routine laboratory instrumentation, is an excellent biomarker of energy intake.