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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 isotopes2H and18O 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 of2H and18O 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 that2H and18O 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.

International audience; Background: Combining accelerometry (ACC) with heart rate (HR) monitoring is thought to improve activity energy expenditure (AEE) estimations compared to ACC alone. Objective: To evaluate the validity of ACC and HR used alone or combined to estimate AEE in free living conditions, compared to doubly-labeled water (DLW). Methods: Ten-day free-living AEE was measured by a DLW protocol in thirty-five 18-55 y men (11 lean active, 12 lean sedentary, 12 overweight sedentary) wearing an Actiheart (combining ACC and HR) and a RT3 accelerometer. AEE was estimated using group- or individual-calibration of the HR/AEE relationship, based on an exercise tolerance test. In a subset (n=21), AEE changes were measured after one month of detraining (active subjects) or an 8-week training (sedentary subjects). Results: Actiheart combined ACC/HR estimates were more accurate than estimates from HR or ACC alone. Accuracy of the Actiheart group-calibrated ACC/HR estimates was modest (intra-class correlation coefficient [ICC]=0.62), with no bias but high root mean square error (RMSE) and limits of agreement (LOA). The mean bias of the estimates was reduced by one third, like RMSE and LOA, by individual calibration (ICC=0.80). Contrasting with group-calibrated estimates, the Actiheart individual-calibrated ACC/HR estimates explained 38% of the variance of the DLW-AEE change (ICC=0.63). Conclusion: This study supports a good level of agreement between the Actiheart ACC/HR estimates and DLW-measured AEE in lean and overweight men with varying fitness levels. Individual calibration of the HR/AEE relationship is necessary for AEE-estimations at individual level rather than at group scale and for AEE change evaluation.

The potential of bioelectrical impedance spectroscopy (BIS) for assessing nutritional status in spaceflight was tested in two head-down-tilt bed-rest studies. BIS-predicted extracellular water (ECW), intracellular water (ICW), and total body water (TBW) measured using knee-elbow electrode placement were compared with deuterium and bromide dilution (DIL) volumes in healthy, 19- to 45-yr-old subjects. BIS was accurate during 44 h of head-down tilt with mean differences (BIS - DIL) of 0–0.1 kg for ECW, 0.3–0.5 for ICW, and 0.4–0.6 kg for TBW ( n = 28). At 44 h, BIS followed the within-individual change in body water compartments with a relative prediction error (standard error of the estimate/baseline volume) of 2.0–3.6% of water space. In the second study, BIS did not detect an acute decrease (-1.41 ± 0.91 kg) in ICW secondary to 48 h of a protein-free, 800 kcal/day diet ( n = 18). BIS's insensitivity to ICW losses may be because they were predominantly (65%) localized to the trunk and/or because there was a general failure of BIS to measure ICW independently of ECW and TBW. BIS may have potential for measuring nutritional status during spaceflight, but its limitations in precision and insensitivity to acute ICW changes warrant further validation studies.

This study evaluated the accuracy with which the dual-energy X-ray absorptiometer (Hologic QDR 4500A) measured fat-free mass (FFM), fat mass (FM), and hydration of FFM. In a study of 58 men and women (ages 70–79 yr), the QDR 4500A was found to provide a systematically higher estimate of FFM and lower estimate of FM than a four-component model of body composition. A correction factor from this study was developed and applied to two other samples ( n = 13 and 37). We found mean corrected levels of FFM and FM to be equivalent to that obtained by the four-component model or total body water. In addition, the hydration of the corrected FFM was closer to the established hydration level in adult samples and that obtained from the four-component model. These findings suggest that the current calibration of the fan-beam system of the Hologic QDR 4500A provides an overestimate of FFM and underestimate of FM compared with reference methods.

Gretebeck, Randall J., Dale A. Schoeller, Rick A. Socki, Janis Davis-Street, Everett K. Gibson, Leslie O. Schulz, and Helen W. Lane. Adaptation of the doubly labeled water method for subjects consuming isotopically enriched water. J. Appl. Physiol. 82(2): 563–570, 1997.—The use of doubly labeled water (DLW) to measure energy expenditure is subject to error if the background abundance of the oxygen and hydrogen isotope tracers changes during the test period. This study evaluated the accuracy and precision of different methods by which such background isotope changes can be corrected, including a modified method that allows prediction of the baseline that would be achieved if subjects were to consume water from a given source indefinitely. Subjects in this study were eight women (4 test subjects and 4 control subjects) who consumed for 28 days water enriched to resemble drinking water aboard the United States space shuttle. Test subjects and control subjects were given a DLW dose on days 1 and 15, respectively. The change to an enriched water source produced a bias in expenditure calculations that exceeded 2.9 MJ/day (35%), relative to calculations from intake-balance. The proposed correction based on the predicted final abundance of18O and deuterium after equilibration to the new water source eliminated this bias, as did the traditional use of a control group. This new modified correction method is advantageous under field conditions when subject numbers are limited.

This study assessed the effects of changes in skin temperature on multifrequency bioimpedance analysis (MF-BIA) and on the prediction of body water compartments. Skin temperature (baseline 29.3 +/- 2.1 degrees C) of six healthy adults was raised over 50 min to 35.8 +/- 0.6 degrees C, followed by cooling for 20 min to 26.9 +/- 1.3 degrees C, by using an external heating and cooling blanket. MF-BIA was measured at both distal (conventional) and proximal electrode placements. Both distal and proximal impedance varied inversely with a change in skin temperature across all frequencies (5–500 kHz). The change in proximal impedance per degree centigrade change in skin surface temperature was approximately 60% of distal impedance. The change in measured impedance at 50 kHz erroneously increased predicted total body water (TBW) by 2.6 +/- 0.9 liters (P < 0.001) and underpredicted fat mass by 3.3 +/- 1.3 kg (P < 0.0001). Computer modeling of the MF-BIA data indicated changes in predicted water compartments with temperature modifications; however, the ratio of extracellular water (ECW) to TBW did not significantly change (P < 0.4). This change in impedance was not due to a change in the movement of water of the ECW compartment and thus probably represents a change in cutaneous impedance of the skin. Controlled ambient and skin temperatures should be included in the standardization of BIA measurements. The error in predicted TBW is < 1% within an ambient temperature range of 22.3 to 27.7 degrees C (72.1–81.9 degrees F).

to investigate the role of physical activity in maintaining health, the traditional argument has been that the number of outcome events is limiting, and therefore large numbers of individuals must be enrolled and costs per assessment must be small. Based on both this rationale and the fact that

to the editor: We thank those who took the time to write commentaries on our Viewpoint (see Ref. [2][1]). While we were surprised that many were in basic agreement, we were pleased by the many insightful disagreements. Clearly there will always be specific situations that will necessitate self