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Article

Energy Requirements of Lactating Women Derived from Doubly Labeled Water and Milk Energy Output¹

U.S. Department of Agriculture/ARS Children’s Nutrition Research Center, Houston, TX 77030 and Department of Pediatrics, Baylor College of Medicine, Houston, TX 77030

²To whom correspondence should be addressed at 1100 Bates. E-mail: nbutte{at}bcm.tmc.edu

	ABSTRACT

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
Instead of using an incremental approach to assess the energy requirements of lactation, a more comprehensive approach may be taken by measuring total energy expenditure (TEE), milk energy output and energy mobilization from tissue stores. The latter approach avoids assumptions regarding energetic efficiency and changes in physical activity and adiposity. The purpose of this study was threefold: to assess the energy requirements of lactation; to compare these estimates with energy requirements in the nonpregnant, nonlactating state and to test for energetic adaptations in basal metabolic rate (BMR) and physical activity during the energy-demanding process of lactation. Milk production and composition, body weight and composition, TEE, BMR and physical activity levels were measured in 24 well-nourished women during exclusive breastfeeding at 3 mo postpartum and after the cessation of breastfeeding at 18 or 24 mo postpartum. TEE was measured by the doubly labeled water method, milk production by 3-d test-weighing, milk energy by bomb calorimetry on a 24-h milk sample, body composition by dual-energy x-ray absorptiometry and BMR by room respiration calorimetry. TEE, BMR and physical activity level (physical activity level = TEE/BMR) did not differ between the lactating and nonlactating state (TEE 10.0 ± 1.5 versus 10.6 ± 2.1 MJ/d). Mean milk energy output was equivalent to 2.02 ± 0.33 MJ/d. Total energy requirements were greater during lactation than afterward (12.0 ± 1.4 versus 10.6 ± 2.1 MJ/d, P = 0.002). Energy mobilization from tissue stores (-0.65 ± 0.97 MJ/d) resulted in net energy requirements during lactation of 11.4 ± 1.8 MJ/d. Because adaptations in basal metabolism and physical activity were not evident in these well-nourished women, energy requirements during lactation were met primarily from the diet and only partially by mobilization of tissue stores.

KEY WORDS: • lactation • breastfeeding • energy requirements • doubly labeled water • humans

	INTRODUCTION

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
An incremental approach has been taken to the determination of the energy requirements of lactating women. The cost of milk production has been added to the energy requirements of nonpregnant women, with an allowance made for energy mobilization from tissue stores, if replete. The Food and Agriculture Organization/World Health Organization/United Nations University (FAO/WHO/UNU)³ recommendation for lactation up to 6 mo is an increment of 2.93 MJ/d, or 2.10 MJ/d if subsidized by body fat mobilization (500 g/mo); after 6 mo, an increment of 2.1 MJ/d is recommended (FAO/WHO/UNU Expert Consultation 1985 ). The Recommended Dietary Allowance (RDA) for the U.S. population for the first 6 mo of lactation is 2.7 MJ/d, or 2.1 MJ/d if body fat (300–500 g/mo) is mobilized [National Research Council (U.S.) and Subcommittee on the Tenth Edition of the RDAs 1989 ]. This approach assumes that the energetic efficiency of milk synthesis is known and that base energy requirements of women are also known and unchanging. Energy-sparing mechanisms affecting basal metabolism, thermogenesis or physical activity could spare energy for milk synthesis (Prentice et al. 1996 , Prentice and Prentice 1988 ). It is controversial whether the basal metabolic rate (BMR) increases (Sadurskis et al. 1988 , Spaaij et al. 1994 ), decreases (Blackburn and Calloway 1976 , van Raaij et al. 1991 ) or remains the same (Goldberg et al. 1991 , Illingworth et al. 1986 , Motil et al. 1990 , Schutz et al. 1980 ) during lactation. Changes in physical activity brought about by child-rearing may conserve or augment total energy expenditure (TEE), depending on the woman’s habitual level of activity. A more comprehensive approach to assessing the energy requirements of lactating women would be to measure TEE, milk energy output and energy mobilization from tissue stores, thereby avoiding any assumptions regarding energetic efficiency, or changes in physical activity and adiposity. TEE encompasses the synthetic cost of milk production; therefore, no assumptions are made regarding the energetic efficiency of milk synthesis. Although there is consensus on average milk production rates and milk composition to estimate milk energy output, few studies are available on TEE and energy mobilization from tissue stores in well-nourished lactating women relative to their nonpregnant, nonlactating state (Forsum et al. 1992 , Goldberg et al. 1991 ).

The purpose of this study was threefold: to estimate the energy requirements of lactation by measuring milk production and composition, body weight and composition and TEE; to compare these estimates to energy requirements in the nonpregnant, nonlactating state; and to test for energetic adaptations in BMR and physical activity during the energy-demanding process of lactation.

	SUBJECTS AND METHODS

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
Study design and subjects.

Measurements of TEE, BMR, anthropometry and body composition were performed on 24 healthy women at 3 mo and again at 18 or 24 mo postpartum at the Children’s Nutrition Research Center. Subjects were recruited in their third trimester of pregnancy and were required to be healthy, normotensive, nonsmoking and nondiabetic with the intention to breastfeed exclusively for a minimum of 4 mo. At 3 mo postpartum, the women were exclusively breastfeeding and milk production was assessed. By 6 mo, 4 women were exclusively breastfeeding, 15 were partially breastfeeding and 5 had weaned. The mean duration of breastfeeding was 320 ± 154 d. At 18 or 24 mo, the women were no longer breastfeeding. Body composition measurements taken at 0.5 mo postpartum were used to compute changes in weight, fat-free mass (FFM) and fat mass (FM) during the 0.5- to 3-mo period of exclusive breastfeeding. This study was approved by the Baylor Affiliates Review Board for Human Subject Research, and informed written consent was obtained from each woman.

The women had unremarkable health histories and pregnancies. Maternal age averaged 30.4 ± 3.2 y. Ethnic distribution consisted of 20 Caucasian, one African-American, one Hispanic and two Asian women. Median gravidity and parity were 2 (range 1–5) and 0.5 (range: 0–3), respectively. Attained level of education was 17 ± 2 y. Maternal prepregnancy weight (by subject recall) and body mass index averaged 59.1 ± 9.0 kg and 22.1 ± 3.1 kg/m², respectively. Mean gestational weight gain was 16.0 ± 3.8 kg; 50% of the women made gains within the Institute of Medicine–recommended ranges for gestational weight gain (Institute of Medicine and Food and Nutrition Board 1990 ). Family income was distributed as follows: 4% under $20,000, 25% between $20,000 and $34,999, 17% between $35,000 and $49,999 and 54% above $50,000. Infants were born healthy at full term with mean birth weights and lengths of 3.48 ± 0.52 kg and 50.7 ± 2.6 cm, respectively.

Anthropometry and body composition.

Body weight to the nearest 0.1 kg and height to the nearest 0.1 cm were measured using an electronic balance (Healthometer, Bridgeview, IL) and stadiometer (Holtain Limited, Crymych, U.K.), respectively. Dual-energy x-ray absorptiometry (QDR2000; Hologic, Madison, WI; software version 5.56) was used to measure FFM and FM. Energetic equivalents of changes in FFM and FM were taken as 5 and 39 kJ/g, respectively (Forbes 1987 ).

Milk energy output.

Milk energy output was assessed from milk production and the energy concentration of the milk. The amount of milk produced was assessed by a 3-d test-weighing session at home (Butte et al. 1984 ), with a correction for insensible water loss of 2 g · kg^-1 · d^-1 feeding time (Butte 1996 ). Infant weights were measured before and after each feeding on electronic, integrating scales with a precision of ±1.0 g (model 3862MP; Sartorious, Göttingen, Germany). For the determination of milk composition, mothers expressed all their milk during a 24-h period with an electric pump (Egnell, Cary, IL) while in the metabolic research unit for other studies. After each pumping session, the milk was weighed, and a 10% aliquot was refrigerated and later pooled for the 24-h analysis. The energy concentration of human milk was determined by adiabatic bomb calorimetry (Parr Instruments, Moline, IL).

BMR by respiration calorimetry.

Oxygen consumption (O₂), carbon dioxide production (CO₂) and energy expenditure under basal conditions were measured by room calorimetry. The performance of the respiration calorimeters has been described in detail previously (Moon et al. 1995 ). At 3 mo postpartum, subjects spent the night in the calorimeter; they were awakened at 0700 h after a 12-h fast, were asked to urinate and return to bed, and remained supine. After a 20-min equilibration period, BMR was measured for 40 min. At 18- to 24-mo postpartum, subjects arrived at the Children’s Nutrition Research Center at 0700 h in the fasted state and rested supine for 30 min before the 40-min measurement of BMR.

TEE by doubly labeled water method.

TEE was measured by the doubly labeled water method (International Dietary Energy Consulting Group 1990 ). After the collection of a baseline saliva sample, the women received via mouth 100 mg of ²H₂O (Cambridge Isotope Laboratories, Andover, MA) and 125 mg of H₂¹⁸O (Cambridge Isotope Laboratories) per kg body weight. One daily saliva sample was collected at home for the next 13 d. Saliva samples were collected at least 30 min after eating or drinking. Using a small straw, the subject expectorated approximately 1 mL of saliva directly into an o-ring–sealed vial. Samples were stored frozen at -20°C in the o-ring–sealed vials. The time of collection was recorded. Saliva samples were analyzed for hydrogen and oxygen isotope ratio measurements by gas-isotope-ratio mass spectrometry (Wong et al. 1987 ). For hydrogen isotope ratio measurements, 10 µL of saliva without further treatment was reduced to hydrogen gas with 200 mg zinc reagent at 500°C for 30 min (Wong et al. 1992 ). The ²H/¹H isotope ratios of the hydrogen gas were measured with a Finnigan Delta-E gas-isotope-ratio mass spectrometer (Finnigan MAT, San Jose, CA). For oxygen isotope ratio measurements, 100 µL of saliva was allowed to equilibrate with 300 mbar of CO₂ of known ¹⁸O content at 25°C for 10 h using a VG ISOPREP-18 water-CO₂ equilibration system (VG Isogas, Limited, Cheshire, U.K.). At the end of the equilibration, the ¹⁸O/¹⁶O isotope ratios of the CO₂ were measured with a VG SIRA-12 gas-isotope-ratio mass spectrometer (VG Isogas, Limited).

The isotope dilution spaces for ²H (N_H) and ¹⁸O (N_O) were calculated as follows:

where d is the dose of ²H₂O or H₂¹⁸O in g, A is the amount of laboratory water in g used in the dose dilution, a is the amount of ²H₂O or H₂¹⁸O in g added to the laboratory water in the dose dilution, E_a is the rise in ²H or ¹⁸O abundance in the laboratory water after the addition of the isotopic water and E_d is obtained from the zero-time intercepts of the ²H and ¹⁸O decay curves in the saliva samples.

CO₂ was calculated from the fractional turnover rates of ²H (k_H) and ¹⁸O (k_O) as follows:

In this equation, the in vivo isotope fractionation factors of 0.945 [f₁, ²H₂O_(liquid) ²H₂O_(gas)], 0.990 [f₂, H₂¹⁸O_(liquid) H₂¹⁸O_(gas)] and 1.039 [f₃, H₂¹⁸O_(liquid) + C¹⁶O_2(gas) H₂¹⁶O_(liquid) + C¹⁸O¹⁶O _(gas)] measured at 37°C were used (Wong et al. 1988 ). CO₂ was converted to TEE using the Weir equation (de V. Weir 1949 ) as follows:

where oxygen consumption (O₂) was calculated from an assumed food quotient (FQ) of 0.86 using the relationship O₂ = CO₂/FQ according to Black et al. (1986) .

Activity energy expenditure (AEE) was estimated as TEE - (BMR + 0.1 x TEE), assuming that 10% of TEE was due to the thermic effect of feeding (TEF). There is no clear evidence that TEF is either increased or decreased in lactation (Illingworth et al. 1986 , Motil et al. 1990 , Spaaij et al. 1994 ); therefore, the assumption that TEF remains constant during lactation seems reasonable. Physical activity level (PAL) was defined as the ratio of TEE to BMR. Participation in physical activities of moderate, hard and very hard intensity was assessed by the 7-d recall method published by Blair (1984) .

Energy requirements.

At 3 mo postpartum, energy requirements during lactation were defined as the sum of TEE and milk energy output; net energy requirements during lactation were defined as the sum of TEE, milk energy output and the energy mobilization from tissues. At 18–24 mo postpartum, the energy requirements were taken as TEE.

Statistics.

Minitab (release 13; Minitab, College Station, PA, 1998) was used for data description and statistical analysis, including Pearson’s correlation, paired t test, ² and linear regression. ANOVA was used to test for differences in outcome variables between the lactating and nonlactating states; the model included subjects, time postpartum and covariates of weight, FFM and FM in some applications. Differences were considered statistically significant at P 0.05.

	RESULTS

TOP ABSTRACT INTRODUCTION SUBJECTS AND METHODS RESULTS DISCUSSION REFERENCES

 
The anthropometric and body composition measurements are presented in Table 1 . Changes in weight and FFM were not significant between 3 and 18–24 mo postpartum (P < 0.15). FM and %FM decreased significantly during this time interval (P < 0.02). Weight loss during the period of exclusive breastfeeding, 0.5 to 3 mo postpartum, was -29 ± 22 g/d, or 0.88 ± 0.67 kg/mo. None of the women reported dieting at this time. Changes in FFM and FM during this time were -14 ± 16 and -15 ± 26 g/d, respectively. The rate of weight change correlated significantly with the change in FM (r = 0.82, P = 0.001). By linear regression, each kilogram change in weight was associated with a 0.66-kg change in FM. Based on the changes in body composition, energy mobilization was equal to -0.65 ± 0.97 MJ/d. After the cessation of breastfeeding, the change in weight was relatively minor, averaging 1.0 ± 10 g/d between 18 and 24 mo postpartum.

The isotope dilution spaces, N_H and N_O, did not change significantly between 3 and 18–24 mo postpartum (Table 2 ). Fractional turnover rates of ¹⁸O and ²H were significantly higher during lactation than afterward (P = 0.001). CO₂ and O₂ were not significantly different between lactating and nonlactating states.

View larger version (19K): [in this window] [in a new window]   Figure 1. The relationship between milk energy output and total energy expenditure (TEE; top) and activity energy expenditure (AEE; bottom) in lactating women at 3 mo postpartum.

	DISCUSSION

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The changes in weight and, therefore, energy mobilization from tissues (-0.65 MJ/d) of these lactating women were typical of other well-nourished women (Butte and Hopkinson 1998 , Institute of Medicine and Subcommittee on Nutrition During Lactation 1991 ). On average, these women achieved their nadir weight at 12 mo postpartum (60.5 kg), which was close to their prepregnancy weight, and then regained between 12 and 24 mo (61.6 kg). Well-nourished lactating women experience a mild, gradual weight loss in the first 6 mo postpartum, averaging -0.8 kg/mo, whereas undernourished women experience negligible weight loss. Sadurskis et al. (1988) monitored FM in 23 Swedish women for 6 mo postpartum; FM decreased from 30.4 to 29.6% by ¹⁸O dilution and from 32.9 to 31.9% by total body potassium counting. Consistent with a minor weight loss and sedentary lifestyle, British women (n = 10) displayed a nonsignificant increase (30.3–31.4% between 1 and 3 mo postpartum) in FM estimated by ²H and ¹⁸O dilution (Goldberg et al. 1991 ). In U.S. women (n = 45), FM decreased from 28.0% at 1 mo to 26.3% at 4 mo postpartum by underwater weighing (Butte et al. 1984 ). Changes in adipose tissue volume in 15 Swedish women were measured by magnetic resonance imaging (Sohlström and Forsum 1995 ). In the first 6 mo postpartum, the subcutaneous region accounted for the entire reduction in adipose tissue volume, which decreased from 30.6 to 27.4 L; nonsubcutaneous adipose tissue volume actually increased. Mobilization of tissue reserves is a general, but not obligatory, feature of lactation.

Milk production and milk energy content were within normal limits for exclusively breastfeeding women at 3 mo postpartum (Institute of Medicine and Subcommittee on Nutrition During Lactation 1991 , Jensen 1995 , Prentice and Prentice 1988 ). We avoided the common difficulties of estimating the energy content of human milk due to diurnal, within-feed and between-breast variation in milk fat by obtaining a complete 24-h milk expression. Lactation performance may also be judged by infant growth, which was adequate in these infants (Butte et al. 2000 ). Interestingly, milk energy output tended to be lower in women with higher TEE and AEE.

Adjusted for weight or FFM and FM, BMR did not differ in these women during and after lactation. This stands in contrast to the higher 24-h energy expenditure, sleeping metabolic rate and BMR, adjusted for FFM and FM, that we observed in lactating than nonlactating women at 3 mo postpartum estimated by room calorimetry (Butte et al. 1999 ). The discrepancy in our own results might be attributed to the different conditions under which BMR was measured at 18–24 mo. Although the women had fasted and rested before the measurement, they did not sleep overnight in the calorimeter. If lactation is a continuous, inefficient process, BMR would be expected to be elevated due to the energy cost of milk synthesis. However, there is no consensus as to whether BMR is elevated (Forsum et al. 1992 , Sadurskis et al. 1988 , Spaaij et al. 1994 ), depressed (Guillermo-Tuazon et al. 1992 , Lawrence et al. 1986 ) or unchanged (Goldberg et al. 1991 , Illingworth et al. 1986 , Madhavapeddi and Rao1992 , Motil et al. 1990 , Piers et al. 1995 , Schutz et al. 1980 , Singh et al. 1989 , van Raaij et al. 1991 ) during lactation. Interpretation of these studies is difficult because BMR was not always adjusted for differences in body weight or body composition between comparison groups. In general, it would appear that BMR is unchanged or slightly elevated during lactation; there is little evidence of energy conservation.

TEE of lactating women has been measured by the doubly labeled water (DLW) method in five other studies (Butte et al. 1997 , Forsum et al. 1992 , Goldberg et al. 1991 , Lovelady et al. 1993 , Singh et al. 1989 ). Potential sources of error in the DLW method unique to lactation are isotope exchange and sequestration during de novo synthesis of milk fat and lactose and increased water flux into milk. Theoretically, the export of exchangeable hydrogen bound to solids in milk may result in a 1.0–1.3% underestimation of CO₂ (International Dietary Energy Consulting Group 1990 ). ²H sequestration may increase this underestimation to 1.5–3.4%. Mean TEE values of 10.0 MJ/d (PAL = 1.79) in our subjects are higher than values reported for British (8.9 MJ/d, PAL = 1.54) (Goldberg et al. 1991 ) and lactating Otomi women (9.2 MJ/d, PAL = 1.6) (Butte et al. 1997 ), similar to those reported for U.S. women (10.1 MJ/d, PAL = 1.76) (Lovelady et al. 1993 ) and slightly lower than those reported for Gambian (10.4 MJ/d, PAL = 1.95) (Singh et al. 1989 ) and Swedish (10.7 MJ/d, PAL = 1.80) (Forsum et al. 1992 ) women during lactation. The physical activity of our women would be assessed as "moderate" or "heavy" according to the FAO/WHO/UNU classification (FAO/WHO/UNU Expert Consultation 1985 ), consistent with the active lives of these working mothers. Theoretically, the energy cost of lactation could be met by a reduction in the time spent in physical activity or an increase in the efficiency of performing routine tasks. Adaptations in the level of physical activity were not seen in these lactating women; AEE and PAL did not differ significantly during and after lactation. Reductions in physical activity have been reported in early lactation (4–5 wk postpartum) in Swedish (van Raaij et al. 1991 ) and British (Goldberg et al. 1991 ) women. By 27 wk, the activity of the Swedish women, however, was similar to prepregnancy levels. By 3 mo postpartum, the majority of the women in our study had resumed their characteristic occupational and recreational lifestyles, with additional child-rearing responsibilities.

The energy requirements of lactating women were estimated from measurements of TEE, milk energy output and energy mobilization from tissue stores in the following five studies in which doubly labeled water was used (Butte et al. 1997 , Forsum et al. 1992 , Goldberg et al. 1991 , Lovelady et al. 1993 , Singh et al. 1989 ). In 10 lactating British women, the total energy requirements (and net energy requirements, because there was no fat mobilization) were 11.1, 11.3 and 11.2 MJ/d at 1, 2 and 3 mo postpartum, respectively. Milk energy output averaged 2.2 MJ/d (Goldberg et al. 1991 ). In 23 lactating Swedish women, the total energy requirement at 2 mo postpartum was 12.7 MJ/d, offset by 0.3 MJ/d from tissue stores to yield a net requirement of 12.4 MJ/d (Forsum et al. 1992 ). In nine lactating U.S. women, the total energy requirement was 12.4 MJ/d, with 2.2 MJ/d exported into milk; 1.2 MJ/d was mobilized from tissues, yielding a net requirement of 11.2 MJ/d (Lovelady et al. 1993 ). In 14 fully lactating Gambian women, the total energy requirement was 12.5 MJ/d, subsidized by 0.4 MJ/d to yield a net energy requirement of 12.1 MJ/d at 5–6 mo postpartum (Singh et al. 1989 ). The energy requirements of Otomi women from Mexico were determined by the same approach. Total energy requirements were 11.2 MJ/d, of which 2.2 MJ/d was exported into milk (Butte et al. 1997 ). In the Otomi women, the energy cost of lactation was subsidized minimally by mobilization of tissue stores (0.1 MJ/d) to yield a net energy requirement of 11.1 MJ/d.

Across these diverse populations, mean milk energy outputs during full lactation were similar (2.1–2.2 MJ/d). These values are less than estimates used in the FAO/WHO/UNU and U.S. RDA recommendations, which assume a milk energy concentration of 2.93 MJ/L and milk production rates of 819–848 and 750 mL/d, respectively [FAO/WHO/UNU Expert Consultation 1985 , National Research Council (U.S.) and Subcommittee on the Tenth Edition of the RDAs 1989 ]. The FAO/WHO/UNU and U.S. RDA recommendations also entail an 80% energetic efficiency of milk synthesis, which is encompassed in our TEE. TEE, body size and energy mobilization from tissues displayed the expected variability among these populations, resulting in net energy requirements of fully lactating women ranging from 11.1 to 12.4 MJ/d. Our estimates of the total and net energy requirements of lactating women at 3 mo postpartum are in line with estimates of other U.S. women (Lovelady et al. 1993 ) and British women (Goldberg et al. 1991 ). The women in these studies were fully breastfeeding their infants, who were <6 mo of age. Naturally, the stage and extent of breastfeeding affect the incremental energy requirements for lactation. Milk production rates increase during the first 6 mo of full lactation. Beyond 6 mo postpartum, typical milk production rates are variable and depend on weaning practices.

In conclusion, total energy requirements were augmented during lactation compared with the nonpregnant, nonlactating state (12.0 ± 1.4 versus 10.6 ± 2.1 MJ/d). Energy mobilization from tissue stores (-0.65 ± 0.97 MJ/d) subsidized the energy cost of lactation, resulting in net energy requirements of 11.4 ± 1.8 MJ/d. Because adaptations in basal metabolism and physical activity were not evident in these well-nourished women, energy requirements were met in part by mobilization of tissue stores but primarily from the diet.

	ACKNOWLEDGMENTS

 
The authors thank the women who participated in this study and acknowledge the contributions of Marilyn Navarrete for subject recruitment and Carolyn Heinz for study coordination; Sopar Seributra and Sandra Kattner for nursing and dietary support; Nitesh Mehta, Maurice Puyau, Firoz Vohra, Judy Joo Posada, JoAnn Pratt, Roman Shypailo, Zahira Colon, Kiyoko Usuki, Shide Zhang and Deborah Roose for technical assistance; Anne Adolph for data management; Leslie Loddeke for editorial review and Idelle Tapper for secretarial assistance.

	FOOTNOTES

 
¹ This work is a publication of the U.S. Department of Agriculture (U.S. Department of Agriculture)/Agricultural Research Service (ARS) Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine and Texas Children’s Hospital, Houston, Texas. This project was funded in part with federal funds from the U.S. Department of Agriculture/ARS under Cooperative Agreement 58-6250-6001. The contents of this publication do not necessarily reflect the views or policies of the U.S. Department of Agriculture, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. Government.

³ Abbreviations used: AEE, activity energy expenditure; BMR, basal metabolic rate; DLW, doubly labeled water; FAO/WHO/UNU, Food and Agriculture Organization/World Health Organization/United Nations University; FFM, fat-free mass; FM, fat mass; FQ, food quotient; k_H, deuterium fractional turnover rate; k_O, oxygen-18 fractional turnover rate; N_H, deuterium dilution space; N_O, oxygen-18 dilution space; PAL, physical activity level; RDA, Recommended Dietary Allowance; TEE, total energy expenditure; CO₂, carbon dioxide production; O₂, oxygen consumption.

Manuscript received June 22, 2000. Initial review completed July 24, 2000. Revision accepted September 22, 2000.

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