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1 From the US Department of Agriculture/Agricultural Research Service Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston.
2 The contents of this publication do not necessarily reflect the views or policies of the US Department of the Army or the USDA, nor does mention of trade names, commercial products, or organizations imply endorsement by the US Government. 3 Supported by the US Department of the Army grant DAMD 17-95-1-5070 and the USDA/ARS under Cooperative Agreement no. 58-6250-6001. 4 Address reprint requests to NF Butte, Children’s Nutrition Research Center, 1100 Bates Street, Houston, TX 77030. E-mail: nbutte{at}bcm.tmc.edu.
ABSTRACT
Background: The energy requirements of women have been based on total energy expenditure (TEE) derived from the factorial approach or as multiples of basal metabolic rate (BMR).
Objective: This study was designed to reevaluate the energy requirements of healthy, moderately active underweight, normal-weight, and overweight women of reproductive age.
Design: The energy requirements of 116 women [n = 13 with a low body mass index (BMI), n = 70 with a normal BMI, and n = 33 with a high BMI] were estimated from TEE measured by the doubly labeled water method. Twenty-four–hour EE and BMR were measured by room respiration calorimetry, activity EE was estimated from nonbasal EE as TEE - BMR, and physical activity level was calculated as TEE/BMR. Body composition was derived from a multicomponent model. Fitness, strength, and physical activity level were assessed, and fasting serum indexes were measured.
Results: Energy requirements differed among the low-BMI (8.9 ± 0.9 MJ/d), normal-BMI (10.1 ± 1.4 MJ/d), and high-BMI (11.5 ± 1.9 MJ/d) groups (P = 0.02–0.001, all pairwise comparisons). Major predictors of BMR, 24-h EE, and TEE were weight, height, and body composition; minor predictors were fasting metabolic profile and fitness. Fat-free mass and fat mass accounted for the differences in EE seen between the BMI groups. The mean physical activity level of 1.86 suggested that the multiples of BMR used to estimate energy requirements have been underestimated.
Conclusion: Recommended energy intakes for healthy, moderately active women of reproductive age living in industrialized societies should be revised on the basis of TEE.Am J Clin Nutr 2003;77:–8.
Key Words: Energy requirements • total energy expenditure • basal metabolic rate • activity • body composition • doubly labeled water method • women
INTRODUCTION
Traditionally, the energy requirements of adults have been based on total energy expenditure (TEE) derived from the factorial approach or as multiples of basal metabolic rate (BMR) (1, 2). The factorial approach requires information on the time spent in various activities and the energy cost of each activity. The factorial method is subject to errors of recorded or recalled time durations and of estimation of the energy cost and intensity of discrete and spontaneous activities. Energy requirements derived as multiples of BMR depend on the accuracy of the predicted or measured BMR and assignment into light, moderate, or heavy categories of physical activity. The doubly labeled water (DLW) method noninvasively measures TEE with the stable isotopes deuterium and oxygen-18 over several days (3). The DLW method has the advantage that it captures both BMR and the energy expended in physical activity and diet-induced thermogenesis. This approach for the assessment of TEE has been advocated and used by others (4–6). Black et al (4) compiled a large database of DLW measurements in persons aged 2–95 y from affluent societies. In their meta-analysis, TEE, BMR, and activity EE (AEE) were positively correlated with weight and height and negatively correlated with age. Females were found to have lower TEE than males. In a separate publication (5), TEE measurements by DLW were analyzed according to body mass index (BMI; in kg/m2), and physical activity level (PAL) was found to be similar across BMI groups.
The purpose of this study was to define the energy requirements of women of reproductive age based on TEE measured by the DLW method. Energy requirements are determined by biological as well as sociocultural factors and, therefore, are population-specific. Our subjects were representative of healthy, moderately active women of reproductive age living in an industrialized society. Our specific objectives were 1) to determine the energy requirements of underweight, normal-weight, and overweight women; 2) to determine the effects of age, body composition, fasting metabolic profile, fitness, and strength on energy requirements; and 3) to determine significant predictors of BMR, 24-h EE, TEE, and AEE.
SUBJECTS AND METHODS
Subjects and study design
Subjects were classified as underweight, normal weight, or overweight: the low-BMI ( 18.5), normal-BMI (> 18.5 but < 25), and high-BMI ( 25) groups, respectively. Enrollment criteria included nonsmoking, ages 18–40 y, parity 4, physically active (ie, 20–30 min moderate exercise 3 times/wk), and no chronic use of medications or alcohol or drug abuse. Fasting serum indexes, anthropometry, body composition, fitness, strength, respiration calorimetry, TEE, and physical activity were measured at the Children’s Nutrition Research Center, Houston. This study was approved by the Baylor Affiliates Review Board for Human Subject Research, and written informed consent was obtained from each woman.
Serum indexes
A blood sample was obtained after the subjects had fasted for 12 h. Serum iron, iron saturation, and hemoglobin were determined by spectrophotometric methods. Hematocrit was measured by flow cytometry; ferritin by an automated chemiluminescence system (Bayer Corporation, Norwood, MA); transferrin receptor (TfR) by enzyme immunoassay (Ramco Laboratories, Inc, Houston); insulin and leptin by radioimmunoassay (Linco Research, Inc, St Charles, MO); glucose by an enzymatic method using glucose oxidase (EC 1.1.3.4); triacylglycerol by an enzymatic method using lipoprotein lipase (EC 3.1.1.34), glycerol kinase (EC 2.7.1.30), glycerol-1-phosphate dehydrogenase (EC 1.1.1.261), and diaphorase (EC 1.8.1.4); free fatty acids by an enzymatic method using acyl–CoA oxidase (EC 1.3.3.6); and thyrotropin, total and free thyroxine (T3), and total and free triiodothyronine (T4) by radioimmunoassay (Diagnostic Products Corp, Los Angeles).
Anthropometry and body composition
Body weight and height were measured with an electronic balance (Healthometer, Bridgeview, IL) and stadiometer (Holtain Limited, Crymych, United Kingdom), respectively. Anthropometric measurements were made by a single investigator.
Total body water (TBW) was determined by dilution of an orally administered dose of deuterium oxide (100 mg 2H2O/kg). Deuterium dilution space (NH) was converted to TBW by dividing by 1.04. Body density was measured with an underwater weighing system that uses "force cube" transducers (Precision Biomedical Systems, Inc, State College, PA) (7). Body volume was corrected for residual lung volume measured by the simplified nitrogen washout method (8).
Dual-energy X-ray absorptiometry (DXA; QDR2000, software version 5.56; Hologic, Inc, Madison, WI) was used to measure total-body bone mineral content (BMC).
A 4-component body-composition model that uses body weight, TBW from 2H dilution, body volume from densitometry, and BMC from DXA was used to compute fat mass (FM) and fat-free mass (FFM) (9):
RESULTS
Subject description
Of the total of 116 women studied, 13 were underweight, 70 were of normal weight, and 33 were overweight. No significant differences in age, ethnicity, or family income were observed among the low-, normal-, and high-BMI groups. The mean age of the subjects was 31 ± 4 y (range: 21–40 y). The ethnic distribution was 78% white, 11% African American, 9% Hispanic, and 2% Asian. Family income was > $50 000 in 80% of the sample. Mean education attainment was 17 ± 2 y, and was slightly lower in the high-BMI group than in the other groups (P 0.04). Sixty-seven percent of the women were nulliparous, 28% had one child, 4% had 2 children, and 1% had 3 children. Most (91%) of the women worked outside of the home: 36% were in business or administrative positions in an office setting; 6% worked in laboratories; 22% were teachers, professors, or students; 20% were health care providers; 7% were physical trainers; and 9% were homemakers.
The women were nonanemic, normoglycemic, and euthyroidic. Hemoglobin, hematocrit, serum iron, iron saturation, ferritin, TfR, and TfR/ferritin were within normal limits for women of reproductive age (Table 1). Serum iron (P = 0.02) and iron saturation (P = 0.05) were significantly lower in the low-BMI group than in the other 2 groups. TfR was higher in the high-BMI group than in the normal-BMI group (P = 0.04). Although the women were normoglycemic, serum insulin (P = 0.001) and glucose (P = 0.008) were significantly higher in the high-BMI group than in the other 2 groups. Serum leptin was higher in the high-BMI group than in the other 2 groups (P = 0.001). Serum triacylglycerol was higher in the high-BMI group than in the normal-BMI group (P = 0.001). BMI, weight, FM, and %FM were significantly correlated with insulin (r = 0.68–0.74, P = 0.001), leptin (r = 0.80–0.87, P = 0.001), triacylglycerol (r = 0.32–0.43, P = 0.001), glucose (r = 0.31–0.36, P = 0.001), and thyrotropin (r = 0.19–0.22, P = 0.05).
View this table:
TABLE 1 . Fasting serum indexes by BMI grouping1
Anthropometry and body composition
Body size, dimension, and composition are presented by BMI group in Table 2. Except for height, highly significant differences for all variables were seen among the low-, normal- and high-BMI groups.
View this table:
TABLE 2 . Anthropometric and body-composition measures by BMI grouping1
Fitness and strength
Approximately 41% of the variance in ·VO2max could be explained by FFM and FM, with FM having a significant negative effect (Table 3). Absolute ·VO2max tended to differ by BMI group (P = 0.08), whereas the maximal workload achieved was significantly different (P = 0.02); the low-BMI group tended to perform more poorly in terms of ·VO2max than did the other groups (P = 0.07–0.100). The differences among BMI groups were accounted for by weight or FFM and FM. ·VO2max was predicted by using Equation 7 [SEE = 0.30, r2(adjusted) = 40.8%]:
DISCUSSION
The purpose of this study was to determine the energy requirements of healthy underweight, normal-weight, and overweight women of reproductive age on the basis of TEE. The effects of age, body composition, fasting metabolic profile, fitness, strength, and occupational and recreational activities on TEE and its components were examined. Major predictors of BMR, 24-h EE, and TEE were body size and body composition; minor predictors were fasting metabolic profile and fitness. FFM and FM accounted for the differences in BMR, 24-h EE, and TEE seen between the underweight, normal-weight, and overweight women. The predictability of AEE and PAL of these women was low; no significant differences in age, body size, body composition, or serum indexes were detected by PAL quartile.
The approach taken to define the energy requirements of women was based on TEE measured by the DLW method. The definition of energy requirements is population-specific. For this reason, we carefully described the health status, anthropometric indexes, body composition, fitness, and lifestyles of our subjects. Our study population consisted of healthy, moderately active women living in an urban, industrialized setting; most of the women worked outside of the home, 33% had young children, and most participated in moderate exercise 3 times/wk. By design, the women represented a wide spectrum of body sizes and compositions. In terms of strength and ·VO2max, the women in the low-BMI group did not perform as well as did the other groups.
Factors influencing the energy expenditure of women
To understand the variability in energy requirements of the women, we and others explored the effects of age, body size, and body composition on EE (4–6, 25–30). EE declines with age throughout life (4, 30), but we did not see a significant decline within our subjects’ limited age span of 21–40 y. The effects of body size and composition on BMR, 24-h EE, TEE, and AEE were examined in our study. Body size or composition accounted for 64% of the variance in BMR, 64–66% of the variance in 24-h EE, 29–34% of the variance in TEE, and 7–10% of the variance in AEE. The lower predictability of TEE and AEE was due to the fact that activity patterns are influenced by behavioral choices and, therefore, are less definable with the use of biological measures. To better predict TEE and AEE, we tested physical activity–related variables, such as leisure time activities, ·VO2max and strength, which slightly improved the prediction of TEE and AEE.
We found minor contributions of fasting serum hormones and metabolites to the variance observed in EE. Independent of FFM and FM, free fatty acids and thyrotropin were related to BMR, 24-h EE, and TEE. The positive association between fasting serum free fatty acids and rates of EE may reflect higher free fatty acid flux, oxidation, or both. Thyrotropin is a stimulator of T3 and T4 release, which in turn increase ·VO2 and heat production.
Energy expenditure by body mass index
Absolute rates of BMR, 24-h EE, TEE, and AEE were substantially higher in the women who were overweight than in those who were not, as was found by other investigators (5, 26, 27, 31). After adjustment for body size or composition, no significant differences in EE were found between BMI groups. PAL has been shown to be similar between BMI categories in women and men (5, 27). The higher 24-h EE and TEE values observed in the overweight women were attributable to the higher BMR and energy cost of physical activities, as exemplified by the cost of cycling at 50 W and walking at 4 km/h in the calorimeter, which were 14% and 21% higher, respectively, in the high-BMI group than in the normal-BMI group. Although PALs were similar among BMI groups, the amount of time spent in comparable physical activities would be less in the overweight women. For instance, if the observed AEE entailed only walking at 4 km/h, the duration of walking would be equivalent to 324, 332, and 285 min in the low-, normal- and high-BMI groups, respectively.
Energy requirements based on total energy expenditure and physical activity level
PAL provides a convenient way of controlling for age, sex, weight, and body composition. To validate the PAL index, regression of the logarithms of 24-h EE and TEE on the logarithms of BMR yields coefficients of 0.91 and 0.73, indicating that the ratio approach in this case does completely adjust for BMR. In an analysis by Black et al (4), the logarithm of TEE was regressed on the logarithm of BMR in 574 adults from affluent societies. The resultant regression coefficients were 1.00 for all subjects, 0.98 for males, and 0.99 for females. These findings indicated that the PAL index was not correlated with BMR and was thus a valid index of TEE adjusted for BMR. The larger sample size in the study by Black et al favors its findings. The PAL provides a useful index of physical activity and a practical approach for estimating energy requirements.
Our results suggest that the multiples of BMR used to estimate the energy requirements of moderately active women have been underestimated in the 1985 FAO/WHO/UNU energy and protein requirements (1) and in the 1989 US recommended dietary allowances (2). In the FAO/WHO/UNU publication, multiples of 1.56, 1.64, and 1.82 were used to represent light, moderate, and heavy PALs in women. In the US recommended dietary allowances, activity factors of 1.60 and 1.55 were assigned to women aged 19–24 and 25–50 y, respectively, engaged in light-to-moderate activity. In our study, the mean PAL within the calorimeter was 1.35, representing sedentary conditions with 30 min of moderate walking. Exclusion of walking would decrease the PAL to 1.29, which represents a minimal survival level of physical activity. The mean free-living PAL of our women, as determined by DLW measurements, was 1.86. Assigning a multiple of 1.60 to these women would underestimate their energy requirements by an average of 1109 kJ/d.
Although significant interindividual variation in TEE was observed, TEE may be used to estimate the energy intakes required to sustain the lifestyles of moderately active women of reproductive age in industrialized societies. On the basis of TEE, current recommended energy intakes for healthy, moderately active women of reproductive age living in industrialized societies should be revised.
ACKNOWLEDGMENTS
We thank the women who participated in this study and we acknowledge the contributions of Carolyn Heinz and Marilyn Navarrete for study coordination, Sopar Seributra for nursing, Sandra Kattner for dietary support, and Maurice Puyau, Firoz Vohra, Anne Adolph, Roman Shypailo, JoAnn Pratt, and Shide Zhang for technical assistance.
NFB was responsible for the study design and analysis and for writing the manuscript; MST and JMH were responsible for data collection; WWW, NRM, and EOS were responsible for sample analysis; and EOS was responsible for the statistical analysis. None of the authors had any financial or personal affiliation with any company or organization that sponsored this research.
REFERENCES