Energy expenditure of elite female runners measured by respiratory chamber and doubly labeled water.

To determine whether female athletes have unusually low energy requirements as suggested by many food intake studies, energy expenditure (EE) and intake were assessed in nine elite distance runners [26 +/- 3 (SD) yr, 53 +/- 4 kg, 12 +/- 3% body fat, and 66 +/- 4 ml.kg-1.min-1 maximal O2 uptake]. Subjects were admitted to a metabolic ward for 40 h during which 24-h sedentary EE was measured in a respiratory chamber. Free-living EE was then assessed by the doubly labeled water method for the next 6 days while the women recorded all food intake, daily body weight, and training mileage (10 +/- 3 miles/day). Energy intakes estimated from free-living EE (2,826 +/- 312 kcal/day) and body weight changes (-84 +/- 71 g/day) averaged 221 +/- 550 kcal/day in excess of those calculated from food records (2,193 +/- 466 kcal/day). The energy cost of training (1,087 +/- 244 kcal/day) was calculated as the difference between free-living EE and 24-h EE in the respiratory chamber (1,681 +/- 84 kcal/day) corrected for the thermic effect of food of the extra energy intake. These data do not support the hypothesis that training as a distance runner results in metabolic adaptations that lower energy requirements in women.     

Blood-sucking bugs as a gentle method for blood-collection in water budget studies using doubly labelled water

During doubly-labelled water (DLW) experiments, blood collection by venous puncture may traumatize animals and consequently affect the animals' behaviour and energy budget. Recent studies have shown that blood-sucking bugs (Triatominae; Heteroptera) can be used instead of conventional needles to obtain blood from animals. In this paper, we validate the bug method in captive nectar-feeding bats, Glossophaga soricina, for water budget analysis by comparing the daily water flux estimated with the DLW method with values measured by an energy balance method. As the mean daily water flux of the DLW method was not significantly deviating from the expected value, blood-sucking bugs may substitute more invasive methods of blood collection in DLW experiments. Based on the DLW estimates, daily energy and water intake rates were calculated and compared to values measured with the energy balance method. The DLW method and the energy balance method yielded on average similar results regarding the daily energy intake (DLW method: 48.8+/-14.2 kJ d(-1) versus energy balance method: 48.1+/-9.9 kJ d(-1)) and daily water intake (DLW method: 13.7+/-2.4 mL d(-1) versus energy balance method: 14.7+/-3.0 mL d(-1)). Based on the calculated water and sugar intake per day, we estimated the sugar concentration of ingested nectar to equal on average 16.2+/-2.4% (mass/mass), which fell close to the measured sugar concentration of 17% (mass/mass) bats fed on during the experiment. We conclude that it is possible to extrapolate mean daily energy and water intake for animal groups, populations and species based on DLW estimates, but due to the large variance of results (low accuracy), it seems inadequate to calculate values for single individuals.     

Effect of bolus fluid intake on energy expenditure values as determined by the doubly labeled water method.

The doubly labeled water (DLW, 2H(2)18O) method is a highly accurate method for measuring energy expenditure (EE). A possible source of error is bolus fluid intake before body water sampling. If there is bolus fluid intake immediately before body water sampling, the saliva may reflect the ingested water disproportionately, because the ingested water may not have had time to mix fully with the body water pool. To ascertain the magnitude of this problem, EE was measured over a 5-day period by the DLW method. Six subjects were dosed with 2H2(18)O. After the reference salivas for the two-point determination were obtained, subjects drank water (700-1,000 ml), and serial saliva samples were collected for the next 3 h. Expressing the postbolus saliva enrichments as a percentage of the prebolus value, we found 1) a minimum in the saliva isotopic enrichments were reached at approximately 30 min with the minimum for 2H (95.48 +/- 0.43%) being significantly lower than the minimum for 18O (97.55 +/- 0.44, P less than 0.05) and 2) EE values calculated using the postbolus isotopic enrichments are appreciably higher (19.9 +/- 7.5%) than the prebolus reference values. In conclusion, it is not advisable to collect saliva samples for DLW measurements within approximately 1 h of bolus fluid intake.     

Validity of the Remote Food Photography Method (RFPM) for estimating energy and nutrient intake in near real-time

Two studies are reported; a pilot study to demonstrate feasibility followed by a larger validity study. Study 1's objective was to test the effect of two ecological momentary assessment (EMA) approaches that varied in intensity on the validity/accuracy of estimating energy intake (EI) with the Remote Food Photography Method (RFPM) over 6 days in free-living conditions. When using the RFPM, Smartphones are used to capture images of food selection and plate waste and to send the images to a server for food intake estimation. Consistent with EMA, prompts are sent to the Smartphones reminding participants to capture food images. During Study 1, EI estimated with the RFPM and the gold standard, doubly labeled water (DLW), were compared. Participants were assigned to receive Standard EMA Prompts (n = 24) or Customized Prompts (n = 16) (the latter received more reminders delivered at personalized meal times). The RFPM differed significantly from DLW at estimating EI when Standard (mean ± s.d. = -895 ± 770 kcal/day, P < 0.0001), but not Customized Prompts (-270 ± 748 kcal/day, P = 0.22) were used. Error (EI from the RFPM minus that from DLW) was significantly smaller with Customized vs. Standard Prompts. The objectives of Study 2 included testing the RFPM's ability to accurately estimate EI in free-living adults (N = 50) over 6 days, and energy and nutrient intake in laboratory-based meals. The RFPM did not differ significantly from DLW at estimating free-living EI (-152 ± 694 kcal/day, P = 0.16). During laboratory-based meals, estimating energy and macronutrient intake with the RFPM did not differ significantly compared to directly weighed intake.     

Use of doubly labeled water technique in soldiers training for jungle warfare

The doubly labeled water method was used to estimate the energy expended by four members of an Australian Army platoon (34 soldiers) engaged in training for jungle warfare. Each subject received an oral isotope dose sufficient to raise isotope levels by 200-250 (18O) and 100-120 ppm (2H). The experimental period was 7 days. Concurrently, a factorial estimate of the energy expenditure of the platoon was conducted. Also, a food intake-energy balance study was conducted for the platoon. Mean daily energy expenditure by the doubly labeled water method was 4,750 kcal (range 4,152-5,394 kcal). The factorial estimate of mean daily energy expenditure was 4,535 kcal. Because of inherent inaccuracies in the food intake-energy balance technique, we were able to conclude only that energy expenditure, as measured by this method, was greater than the estimated mean daily intake of 4,040 kcal. The doubly labeled water technique was well tolerated, is noninvasive, and appears to be suitable in a wide range of field applications.     

Assessment of physical activity in older individuals: a doubly labeled water study.

We compared the accuracy of two physical activity recall questionnaires and a motion detector in 45- to 84-yr-old women (n = 35) and men (n = 32), using doubly labeled water (DLW) in conjunction with indirect calorimetry as the criterion measure. Subjects were administered the Yale Physical Activity Survey (YPAS) and Minnesota Leisure Time Physical Activity Questionnaire (LTA). Physical activity energy expenditure was determined over a 10-day period by using a Caltrac uniaxial accelerometer and DLW in conjunction with indirect calorimetry. In older women, Minnesota LTA (386 +/- 228 kcal/day) and Caltrac (379 +/- 162 kcal/day) underestimated physical activity by approximately 55% compared with DLW (873 +/- 244 kcal/day). No difference was observed between daily physical activity measured by the YPAS (863 +/- 447 kcal/day) and DLW in older women. In older men, Minnesota LTA (459 +/- 288 kcal/day) and Caltrac (554 +/- 242 kcal/day) underestimated daily physical activity by approximately 50-60% compared with DLW (1,211 +/- 429 kcal/day). No difference was found between physical activity measured by the YPAS (1,107 +/- 612 kcal/day) and DLW in older men. Despite no difference in mean physical activity levels between YPAS and DLW in women and men, Bland and Altman (Lancet 1: 307-310, 1986) analyses demonstrated poor concordance between DLW and YPAS (i.e., limits of agreement = -1,310-1,518 kcal/day). Our data suggest that the Minnesota LTA recall and Caltrac uniaxial accelerometer may significantly underestimate free-living daily physical activity energy expenditure in older women and men. Although the YPAS compares favorably with DLW on a group basis, its use as a proxy measure of individual daily physical activity energy expenditure may be limited in older women and men.     

Twenty-four-hour metabolic responses to resistance exercise in women

Seven nonobese adult females (40 +/- 8 years) were studied in a room calorimeter on a day that resistance exercise (REX) was performed (4 sets of 10 exercises) and on a nonexercise control day (CON). Twenty-four-hour energy expenditure (EE) on the REX day (mean +/- SD, 2,328 +/- 327 kcal.d(-1)) was greater than CON (2,001 +/- 369 kcal.d(-1), p < 0.001). The net increase in EE during and immediately after (30 minutes) exercise represented 76 +/- 12% of the total increase in 24-hour EE. Twenty four-hour RQ on the REX day (0.86 +/- 0.06) did not differ from CON (0.87 +/- 0.02). Twenty four-hour carbohydrate oxidation was elevated on the REX day, but 24-hour fat and protein oxidation were not different. Thus, in women, the increase in EE due to resistance exercise is largely seen during and immediately after the exercise. The increased energy demand is met by increased carbohydrate oxidation, with no increase in 24-hour fat oxidation.     

Improved insulin action following short-term exercise training: role of energy and carbohydrate balance.

Short-term exercise training improves insulin action, but the impact of replacing the energy expended during exercise to prevent energy deficit is unclear. The purpose of this study was to establish the role of an energy deficit in mediating improved insulin action after short-term exercise training. Two groups of previously sedentary, overweight/obese subjects performed 6 consecutive days of moderate-intensity walking to expend approximately 500 kcal/day. In one group, energy and carbohydrate expended during exercise was replaced [balance group (BAL), n = 8] and in the other group, energy was not replaced [deficit group (DEF), n = 8]. Insulin action (blood glucose uptake during glucose infusion) and selected lipids and adipokines were measured pre- and posttraining. Training increased estimated daily energy expenditure by approximately 500 kcal/day (DEF = 469 +/- 45, BAL = 521 +/- 48), generating an energy deficit in DEF (-481 +/- 24 kcal/day) but not BAL (+8 +/- 20 kcal/day). Insulin action increased 40% in DEF (P = 0.032) but not BAL (-8.4%, P = 0.107). Hepatic glucose production was suppressed during glucose infusion in DEF (30.2 +/- 9.5%, P = 0.037) but not BAL (-10.0 +/- 7.4%, P = 0.417). Fasting leptin concentrations declined in DEF but not BAL. Six days of exercise training without energy replacement significantly increased insulin action. Restoring energy balance by refeeding the energy and carbohydrate expended during exercise resulted in no change in insulin action. These findings suggest that changes in short-term energy and/or carbohydrate balance play a key role in mediating the beneficial effects of exercise on whole body and hepatic insulin action.     

Field use of D2 18O to measure energy expenditure of soldiers at different energy intakes

To test the application of doubly labeled water under adverse field conditions, energy expenditures of 16 special operations soldiers were measured during a 28-day field training exercise. Subjects were matched by fat-free mass and divided equally between an ad libitum ready-to-eat meal diet and a 2,000 kcal/day lightweight ration. Subjects recorded intakes daily, and body composition was measured before and after the exercise. At the beginning of the study, subjects moved to a new northerly location and, therefore, a new water supply. To compensate for this, a group of soldiers who did not receive heavy water was followed to measure isotopic base-line changes. Energy expenditure by doubly labeled water was in agreement with intake/balance (3,400 +/- 260 vs. 3,230 +/- 520 kcal/day). The overall coefficient of variation of energy expenditure by doubly labeled water was half that of intake/balance (7.6 vs. 16.1%). The coefficient of variation of repeat measures with doubly labeled water was 7.3%. Energy expenditure of the ready-to-eat meal group, 3,540 +/- 180 kcal/day, was not significantly different from the lightweight ration group, 3,330 +/- 301 kcal/day. Doubly labeled water was valid under field conditions.     

Comparisons of energy intake and energy expenditure in overweight and obese women with and without binge eating disorder

The purpose of this study was to determine whether there are differences in energy intake or energy expenditure that distinguish overweight/obese women with and without binge eating disorder (BED). Seventeen overweight/obese women with BED and 17 overweight/obese controls completed random 24-h dietary recall interviews, and had total daily energy expenditure (TDEE) assessed by the doubly labeled water (DLW) technique with concurrent food log data collection. Participants received two baseline dual-energy X-ray absorptiometry (DXA) scans and had basal metabolic rate (BMR) and thermic effect of food (TEF) measured using indirect calorimetry. Results indicated no between group differences in TDEE, BMR, and TEF. As in our previous work, according to dietary recall data, the BED group had significantly higher caloric intake on days when they had binge eating episodes than on days when they did not (3,255 vs. 2,343 kcal). There was no difference between BED nonbinge day intake and control group intake (2,233 vs. 2,140 kcal). Similar results were found for food log data. Dietary recall data indicated a trend toward higher average daily intake in the BED group (2,587 vs. 2,140 kcal). Furthermore, when comparing TDEE to dietary recall and food log data, both groups displayed significant under-reporting of caloric intake of similar magnitudes ranging from 20 to 33%. Predicted energy requirements estimated via the Harris-Benedict equation (HBE) underestimated measured TDEE by 23-24%. Our data suggest that increased energy intake reported by BED individuals is due to increased food consumption and not metabolic or under-reporting differences.     

Age-related decline in RMR in physically active men: relation to exercise volume and energy intake

We tested the hypothesis that resting metabolic rate (RMR) declines with age in physically active men (endurance exercise > or =3 times/wk) and that this decline is related to weekly exercise volume (h/wk) and/or daily energy intake. Accordingly, we studied 137 healthy adult men who had been weight stable for > or =6 mo: 32 young [26 +/- 1 (SE) yr] and 34 older (62 +/- 1 yr) sedentary males (internal controls); and 39 young (27 +/- 1 yr) and 32 older (63 +/- 2 yr) physically active males (regular endurance exercise). RMR was measured by indirect calorimetry (ventilated hood system) after an overnight fast and approximately 24 h after exercise. Because RMR is related to fat-free mass (FFM; r = 0.76, P < 0.001, current study), FFM was covaried to adjust RMR (RMR(adj)). RMR(adj) was lower with age in both the sedentary (72.0 +/- 2.0 vs. 64.0 +/- 1.3 kcal/h, P < 0.01) and the physically active (76.6 +/- 1.1 vs. 67.9 +/- 1.2 kcal/h, P < 0.01) males. In the physically active men, RMR(adj) was related to both exercise volume (no. of h/wk, regardless of intensity; r = 0.56, P < 0.001) and estimated energy intake (r = 0.58, P < 0.001). Consistent with these relations, RMR(adj) was not significantly different in subgroups of young and older physically active men matched either for exercise volume (h/wk; n = 11 each) or estimated energy intake (kcal/day; n = 6 each). These results indicate that 1) RMR, per unit FFM, declines with age in highly physically active men; and 2) this decline is related to age-associated reductions in exercise volume and energy intake and does not occur in men who maintain exercise volume and/or energy intake at a level similar to that of young physically active men.     

Effect of leg exercise training on vascular volumes during 30 days of 6 degrees head-down bed rest.

Plasma and red cell volumes, body density, and water balance were measured in 19 men (32-42 yr) confined to bed rest (BR). One group (n = 5) had no exercise training (NOE), another near-maximal variable-intensity isotonic exercise for 60 min/day (ITE; n = 7), and the third near-maximal intermittent isokinetic exercise for 60 min/day (IKE; n = 7). Caloric intake was 2,678-2,840 kcal/day; mean body weight (n = 19) decreased by 0.58 +/- 0.35 (SE) kg during BR due to a negative fluid balance (diuresis) on day 1. Mean energy costs for the NOE, and IKE, and ITE regimens were 83 (3.6 +/- 0.2 ml O2.min-1.kg-1), 214 (8.9 +/- 0.5 ml.min-1.kg-1), and 446 kcal/h (18.8 +/- 1.6 ml.min-1.kg-1), respectively. Body densities within groups and mean urine volumes (1,752-1,846 ml/day) between groups were unchanged during BR. Resting changes in plasma volume (ml/kg) after BR were -1.5 +/- 2.3% (NS) in ITE, -14.7 +/- 2.8% (P less than 0.05) in NOE, and -16.8 +/- 2.9% (P less than 0.05) in IKE, and mean water balances during BR were +295, -106, and +169 ml/24 h, respectively. Changes in red cell volume followed changes in plasma volume. The significant chronic decreases in plasma volume in the IKE and NOE groups and its maintenance in the ITE group could not be accounted for by water balance or by responses of the plasma osmotic, protein, vasopressin, or aldosterone concentrations or plasma renin activity. There was close coupling between resting plasma volume and plasma protein and osmotic content.(ABSTRACT TRUNCATED AT 250 WORDS)     

Rate and extent of compensatory changes in energy intake and expenditure in response to altered exercise and diet composition in humans

We assessed the effect of no exercise (Nex; control) and high exercise level (Hex; approximately 4 MJ/day) and two dietary manipulations [a high-fat diet (HF; 50% of energy, 700 kJ/100 g) and low-fat diet (LF; 20% of energy, 300 kJ/100 g)] on compensatory changes in energy intake (EI) and energy expenditure (EE) over 7-day periods. Eight lean men were each studied four times in a 2 x 2 randomized design. EI was directly quantified by weight of food consumed. EE was assessed by heart rate (HR) monitoring. Body weight was measured daily. Mean daily EE was 17.6 and 11.5 MJ/day (P < 0.001) on the pooled Hex and Nex treatments, respectively. EI was higher on HF diets (13.4 MJ/day pooled) compared with the LF diets (9.0 MJ/day). Regression analysis showed that these energy imbalances induced significant compensatory changes in EB over time of approximately 0.3-0.4 MJ/day (P < 0.05). These were due to changes in both EI and EE in the opposite direction to the perturbation in energy balance. These changes were significant, small but persistent, amounting to approximately 0.2 and approximately 0.35 MJ/day for EI and EE, respectively.     

Increased energy intake minimizes weight loss in men at high altitude.

The hypothesis that high-altitude weight loss can be prevented by increasing energy intake to meet energy requirement was tested in seven men, 23.7 +/- 4.3 (SD) yr, taken to 4,300 m for 21 days. Energy intake required to maintain body weight at sea level was found to be 3,118 +/- 300 kcal/day, as confirmed by nitrogen balance. Basal metabolic rate (BMR), determined by indirect calorimetry, increased 27% on day 2 at altitude and then decreased and reached a plateau at 17% above the sea level BMR by day 10. Energy expended during strenuous activities was 37% lower at altitude than at sea level. Fecal excretion of energy, nitrogen, total fiber, and total volatile fatty acids was not significantly affected by altitude. Energy intake at altitude was adjusted after 1 wk, on the basis of the increased BMR, to 3,452 +/- 452 kcal/day. Mean nitrogen balance at altitude was negative (-0.25 +/- 0.71 g/day) before energy intake was adjusted but rose significantly thereafter (0.20 +/- 0.71 and 0.44 +/- 0.66 g/day during weeks 2 and 3). Mean body weight decreased 2.1 +/- 1.0 kg over the 3 wk of the study, but the rate of weight loss was significantly diminished after the increase in energy intake (201 +/- 75 vs. 72 +/- 48 g/day). Individual regression lines drawn through 7-day segments of body weight showed that in four of seven subjects the slopes of body weight were not significantly different from zero after the 2nd wk. Thus weight loss ceased in four of seven men in whom increased BMR at altitude was compensated with increased energy intake.(ABSTRACT TRUNCATED AT 250 WORDS)     

Validation of a non-invasive blood-sampling technique for doubly-labelled water experiments

Two techniques for bleeding small mammals have been used in doubly-labeled water (DLW) studies, including vena puncture and the use of starved nymphal stages of hematophagous reduviid bugs (Reduviidae, Hemiptera). In this study, we tested the validity of using reduviid bugs in doubly-labeled water experiments. We found that the isotope enrichment in initial blood samples collected with bugs was significantly lower compared to isotope enrichment in blood samples obtained using vena puncture. We therefore used the desiccation method for estimating total body water (TBW) in DLW experiments because TBW calculated using the isotope dilution method was overestimated when blood samples were collected using reduviid bugs. In our validation experiment with nectar-feeding bats (Glossophaga soricina), we compared estimates of daily energy expenditure (DEE) using DLW with those derived from the energy balance method. We considered Speakman's equation (controlling for 25% fractionated water loss) as the most appropriate for our study animal and calculated DEE accordingly. On average, DEE estimated with DLW was not significantly different from the mean value obtained with the energy balance method (mean deviation 1.2%). We conclude that although bug hemolymph or intestinal liquids most likely contaminate the samples, estimates of DEE are still valid because the DLW method does not depend on absolute isotope enrichments but on the rate of isotope decrease over time. However, dilution of blood with intestinal liquids or hemolymph from a bug may lead to larger variation in DEE estimates. We also tested how the relative error of DLW estimates changed with varying assumptions about fractionation. We used three additional equations for calculating DEE in DLW experiments. The basic equation for DLW experiments published by Lifson and McClintock (LM-6) assumes no fractionation, resulted in an overestimate of DEE by 10%. Nagy's equation (N-2) controls for changes in body mass but not for fractionation. Using Nagy's equation, DEE was overestimated by 8%. Under the assumption that 50% of total water flux fractionates, the alternative equation by Lifson and McClintock (LM-35) DEE was underestimated by 5%. The best fit between estimates of DEE based on DLW and energy balance measurements was derived by assuming that 32% of total water flux (TWF) is fractionated. We conclude that the outcome of DLW experiments is sensitive to assumptions regarding evaporative water loss, and thus recommend Speakman's equation 7.17 for use with bats.     

Milk intake and carbon dioxide production of piglets determined with the doubly labelled water technique

The present study was undertaken to study different methodological aspects of quantifying CO2 production and milk intake of suckling piglets using the doubly labelled water (DLW) technique. In total, 37 piglets were enriched intraperitoneally with DLW to study equilibration time of 18O (n = 3), to validate the estimation of milk intake and CO2 production (n = 10) of piglets fed milk replacer and to quantify milk intake and CO2 production of piglets nursed ordinarily by sows (n = 24). Enrichment of 18O in expired air was analysed without any sample preparation, whereas enrichment of 18O in serum was analysed after a minimum step of sample preparation, which included pipetting of the sample, blowing gaseous CO2 into the vial for 3 s and equilibrating for 24 h. The 18O enrichment of CO2 in expired air was constant within 30-40 min of intraperitoneal injection, suggesting that DLW was equilibrated within the body water by that time. For piglets fed milk replacer, the estimation of the daily CO2 production by the DLW method (64.0 ± 2.7 l CO2/day) was in agreement with that obtained by respiration trials (64.7 ± 1.8 l CO2/day). Furthermore, the intake of milk replacer (891 ± 63 g/day) determined by deuterium oxide (D2O) dilution was similar in magnitude to that found by weighing the milk disappearance (910 ± 58 g/day). The milk intake of piglets fed milk replacer was comparable with that of sucking piglets, but sucking piglets had a remarkably higher CO2 production than artificially reared piglets, which likely was caused by a higher intake of milk solids and a higher activity level. For sucking piglets, the daily CO2 production increased curvilinearly with increasing live weight (LW) in kg: piglet CO2 production (l/day) = 25.75 × LW - 1.01 × LW2. In conclusion, 18O equilibrates fast within the body water pool when administered intraperitoneally, and the accuracy of assessing milk intake and rate of CO2 production using the DLW technique is promising. Assessment of excess enrichment of 18O in serum proved to be robust. Finally, the CO2 production of piglets fed milk replacer differs considerably from that of sucking piglets.     

Influences of calorie restriction and age on energy expenditure in the rhesus monkey

Caloric restriction (CR) is known to retard the aging process, and a marker of aging is decreased energy expenditure (EE). To assess longitudinal effects of CR on EE in rhesus monkeys (Macaca mulatta), data from 41 males (M) and 26 females (F) subjected to 9 or 15 yr of CR were studied. EE and body composition of monkeys 11-28 yr of age were measured using indirect calorimetry and dual X-ray absorptiometry. Total EE (24-h EE) was divided into daytime (day EE), nighttime (night EE), and daytime minus nighttime (D - N EE). M calorie-restricted monkeys showed a lower 24-h EE (means +/- SD = 568 +/- 96 kcal/day, P < 0.0001) than controls (C; 630 +/- 129 kcal/day). Calorie-restricted M had a lower night EE (difference = 36 kcal P < 0.0001) compared with C M, but after adjusting for FFM and FM, night EE was not different between calorie-restricted and C males (P = 0.72). The 24-h EE decreased with age (13 kcal decrease/yr, P < 0.0001), but there was no difference between CR and C. Adjusted for FFM and FM, D - N EE decreased with age (9 kcal/yr, P < 0.0001), with no interaction with age (P = 0.72). The F were compared with age-matched M selected from the male cohort. F had a lower 24-h EE (496 +/- 84 kcal/day) than M (636 +/- 139 kcal/day) (P < 0.0001). Adjusting for FFM and FM, night EE was lower in F compared with M (difference = 18 kcal, P = 0.077). Night EE did not differ between calorie-restricted and C younger monkeys after adjusting for FFM and FM. In conclusion, CR did not alter the age-related decrease in EE with CR.     

Comparison of different approaches for the calculation of energy expenditure using doubly labeled water in a small mammal

The doubly labeled water (DLW) method is an isotope-based technique for the estimation of the CO(2) production, and hence energy expenditure, of free-living animals and humans. Several methods are available for the calculation of CO(2) production from the isotope fluxes, depending on different assumptions about the behavior of isotopes during the elimination process. We used the DLW method to estimate the daily energy expenditures (DEE) of 55 field voles (Microtus agrestis) held in a captive facility at 8 degrees C. We calculated DEE using both plateau and intercept approaches for estimating the sizes of the isotope dilution spaces, three different assumptions about fractionation processes, and two ways of treating the different dilution spaces of the oxygen and hydrogen isotopes. We compared the resultant DEE estimates with metabolizable energy intake (MEI) measured during a 3-d feeding trial immediately before the DLW measurements, during which the animals were in energy balance. By making different assumptions about the apparent energy absorption efficiency, we generated a range of direct estimates of MEI. When we compared DEE and MEI, we found that the two-pool model formulations consistently underestimated energy demands by up to 29.8%, depending on the assumptions made in the reference calculation. However, while our data suggest that some correction for fractionation is necessary, with the present data we were unable to separate the two most common treatments of fractionation. These data strongly support the previous suggestion that for small mammals single-pool models provide more accurate estimates of energy demands than two-pool formulation of the DLW method.     

Increased energy expenditure and leptin sensitivity account for low fat mass in myostatin-deficient mice

Myostatin deficiency causes dramatically increased skeletal muscle mass and reduced fat mass. Previously, myostatin-deficient mice were reported to have unexpectedly low total energy expenditure (EE) after normalizing to body mass, and thus, a metabolic cause for low fat mass was discounted. To clarify how myostatin deficiency affects the control of body fat mass and energy balance, we compared rates of oxygen consumption, body composition, and food intake in young myostatin-deficient mice relative to wild-type (WT) and heterozygous (HET) controls. We report that after adjusting for total body mass using regression analysis, young myostatin-deficient mice display significantly increased EE relative to both WT (+0.81 ± 0.28 kcal/day, P = 0.004) and HET controls (+0.92 ± 0.31 kcal/day, P = 0.005). Since food intake was not different between groups, increased EE likely accounts for the reduced body fat mass (KO: 8.8 ± 1.1% vs. WT: 14.5 ± 1.3%, P = 0.003) and circulating leptin levels (KO: 0.7 ± 0.2 ng/ml vs. WT: 1.9 ± 0.3 ng/ml, P = 0.008). Interestingly, the observed increase in adjusted EE in myostatin-deficient mice occurred despite dramatically reduced ambulatory activity levels (-50% vs. WT, P < 0.05). The absence of hyperphagia together with increased EE in myostatin-deficient mice suggests that increased leptin sensitivity may contribute to their lean phenotype. Indeed, leptin-induced anorexia (KO: -17 ± 1.2% vs. WT: -5 ± 0.3%) and weight loss (KO: -2.2 ± 0.2 g vs. WT: -1.6 ± 0.1, P < 0.05) were increased in myostatin-deficient mice compared with WT controls. We conclude that increased EE, together with increased leptin sensitivity, contributes to low fat mass in mice lacking myostatin.