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* Jean Mayer U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, MA 02111 and
Tufts-New England Medical Center Hospital, Boston, MA 02111
2To whom correspondence should be addressed. E-mail: sai.das{at}tufts.edu.
| ABSTRACT |
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KEY WORDS: extreme obesity energy expenditure fat-free mass prediction equations
The prevalence of extreme obesity, as defined by BMI
40 kg/m2, is now 5% in U.S. adults (1), representing a 3-fold increase over the last 40 years. This previously rare condition is associated with profound adverse health consequences; however, very little is known about factors that promote and maintain extreme obesity. Studies using doubly labeled water indicated that individuals with higher than recommended body weight (BMI
25 kg/m2) have high absolute total energy expenditure (TEE)3 compared with individuals of lower BMI (24). This implies that a high energy intake is required to maintain the excess weight. However, most of those studies were conducted in overweight or marginally obese individuals, and there is very little information available on the extremely obese individuals who may be more likely to have metabolic abnormalities that could predispose to low energy requirements.
The aim of the present study was to test the hypothesis that the weight maintenance energy expenditure of extremely obese individuals is low relative to both published data for nonobese individuals (3) and TEE predicted by the equations of the U.S. Dietary Reference Intakes (DRIs) (5). In addition, the accuracy of equations for predicting resting energy expenditure (REE) was assessed using prediction equations developed for use with nonobese (57) individuals and equations developed for use in obese individuals (8,9).
| SUBJECTS AND METHODS |
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Measurements. A 15-d doubly labeled water study was conducted to measure TEE. REE and TEF were measured using indirect calorimetry (Deltatrac portable metabolic cart, Sensor Medics). The Siri 3-compartment model was used to estimate the percentage of body fat and body composition, and standard anthropometric measures were also obtained. Biochemical measures such as thyroid function tests and leptin were determined in blood from fasting subjects. Insulin and glucose were also measured in fasting subjects, and the homeostasis model assessment for insulin sensitivity (HOMA) was calculated using these values (10,11). Self-reported leisure time activity during the previous 12-mo period was determined by a structured interview using the Minnesota Leisure Time Physical Activity (LTPA) questionnaire; occupational activity over the previous 12 mo was also assessed using the self-administered Tecumseh Occupational Activity Questionnaire. All of the above methods were detailed and referenced previously (12,13).
Statistics. Statistical analyses were performed using SPSS 10.0.7 and SYSTAT 9.0.1 (SPPS) and SAS (version 8, SAS Institute). Values are expressed as means ± SEM unless otherwise specified. Given the wide BMI range (37.577 kg/m2) in our study subjects, BMI was divided into tertiles to examine trends across different BMI levels. Baseline characteristics were compared among tertiles using 1-way ANOVA, and energy expenditure variables were compared using both ANOVA and analysis of covariance (ANCOVA), adjusting for fat-free mass (FFM). Tukeys Honestly Significant Difference (HSD) test for multiple comparisons was used to determine significance among groups. Linear multiple regression analysis was performed to determine the best predictors of energy expenditure and suitable models to predict TEE, REE, and activity energy expenditure (AEE).
To test the hypothesis that TEE was low, measured TEE was compared with published TEE data for nonobese individuals (3) and data for nonobese from our own laboratory (14) using a 1-sample t test. Using ANOVA and Tukeys HSD test, measured TEE was also compared with TEE predicted using the equations of the U.S. DRIs (5). ANOVA with Dunnetts post-hoc test was used to determine whether predicted REE using the WHO equations (6), the Harris-Benedict equation (7), and with 2 equations developed or recommended for use in the obese (8,9) were significantly different from measured REE. In addition, the sum of squared differences between measured and predicted REE was used to assess the accuracy of the predictions. For all tests, differences were considered significant at P < 0.05.
| RESULTS |
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body weight (mean ± SD), 1.46 ± 0.35 kg of starting weight] and were weight stable during the study period (
body weight, 4.2 ± 26 g/d). Physical characteristics of the subjects except for body weight after fasting and BMI were similar among the tertiles of BMI in these extremely obese women (Table 1), and values of all biochemical variables were within normal range for all subjects. Glucose was significantly higher in fasting subjects in the highest BMI tertile compared with the middle tertile (P < 0.05), and leptin was elevated in fasting subjects in the middle BMI tertile (P < 0.05) compared with the lowest tertile. Energy expenditure variables by tertile of BMI are shown unadjusted for body composition (Table 2) and therefore illustrate the effect of increasing BMI on energy expenditure. Both TEE and REE were significantly higher in the highest BMI tertile compared with the lowest BMI tertile. However, when energy expenditure variables such as TEE and REE were adjusted for FFM, there were no significant differences among tertiles. This indicated that the differences in energy expenditure between the highest and lowest tertiles were due to differences in FFM.
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No significant differences were observed among BMI tertiles for AEE, TEF, physical activity level (PAL), or food quotient (FQ). In addition, the respiratory quotient (RQ) in fasting subjects was significantly higher in the lowest BMI tertile compared with the highest BMI tertile, but postprandial RQ (averaged over the 4 h TEF) did not differ significantly among BMI tertiles. There were no significant differences among BMI tertiles for self-reported physical activity when expressed in min/d or MJ/d (data for the latter not shown) or for any of the activity categories (data also not shown). In addition, there was no significant relation between measured AEE and energy expenditure calculated from LTPA and occupational activities (r = 0.04; P = 0.8). PAL was also not predicted by LTPA, occupational, or total activity in the 3 activity categories i.e., light, moderate, or heavy activity (P = 0.140.76).
Regression analyses (Table 3) showed that TEE was best predicted by FFM (adjusted R2 = 0.56; P < 0.001). Although FFM alone significantly predicted REE (adjusted R2 = 0.63; P < 0.001), the addition of fat mass to the regression model substantially improved the prediction for REE (adjusted R2 = 0.71; P < 0.001). AEE was also significantly predicted by FFM (adjusted R2 = 0.12; P < 0.05).
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| DISCUSSION |
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30.0 kg/m2). These results indicate that extremely obese individuals have high rather than low rates of TEE, and therefore must consume very high levels of dietary energy to maintain their excess weight. This conclusion is inconsistent with numerous reports of low energy intake in obese individuals (1518), but is entirely consistent with previous TEE data from smaller studies of extremely obese persons (34 subjects total) (1926). There are several aspects of our study design that deserve mention. The use of doubly labeled water to measure TEE and a multicompartment model for the measurement of body composition was important. Doubly labeled water is the only recognized accurate method for estimation of TEE in free-living subjects (27,28) and is the only method that is relatively independent of inaccurate dietary reporting (1518). Although 1 previous study that compared TEE measured in a calorimetric chamber with that of TEE from doubly labeled water measurements indicated that doubly labeled water may underestimate TEE in obese subjects (20), the degree of underestimation in that study was small (mean ± SD, 4.4 ± 5.2
). Furthermore, an underestimation of TEE in obese individuals would only strengthen our findings of high TEE in our population. In addition to finding high TEE in this extremely obese population, we also observed that PAL values (an index of the proportion of TEE attributable to physical activity) were somewhat low compared with published summaries of PAL determined by doubly labeled water in nonobese populations (3). Although differences in measurement errors between laboratories necessarily limit between-laboratory comparisons, all methods were validated against reference techniques in our laboratory and in the published data. It is also interesting to note that the observed PAL values in our study, although apparently substantially lower than those of nonobese individuals (3), were actually higher than anticipated for sedentary adults on the basis of current (5) U.S. DRIs.
In addition to the finding of high levels of TEE, varying degrees of accuracy were found in the prediction of REE in our population with the equations recommended for use in the nonobese and obese (8,9). Contrary to expectation, the widely used WHO equation and Harris-Benedict equations (6,7,29) developed in normal weight individuals provided the closest values to measured REE. The Harris-Benedict equation was the most accurate of all. The 2 obese-specific equations substantially under- or overpredicted REE, indicating that the standard equations provide better predictions of REE in extremely obese individuals.
In conclusion, absolute values for TEE and REE in extremely obese individuals are high and increase with increasing body mass. In addition, TEE and REE in our extremely obese subjects were higher than published values for nonobese individuals (3), and also higher than the U.S. DRI estimates of dietary energy requirements in nonobese individuals (5,6,29). These observations strongly suggest that high energy intakes contribute to the maintenance of excess weight in extremely obese individuals. Finally, the most widely used Harris-Benedict equations and WHO equations provided the closest estimate for predicted REE, and the obese-specific equations were inaccurate for use in our extremely obese individuals
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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3 Abbreviations used: AEE, activity energy expenditure; CSU, clinical studies unit; DRI, dietary reference intakes; FFM, fat-free mass; FQ, food quotient; GBP, gastric bypass; LTPA, leisure time physical activity; PAL, physical activity level; REE, resting energy expenditure; RQ, respiratory quotient; TEE, total energy expenditure; TEF, thermic effect of feeding. ![]()
Manuscript received 8 December 2003. Initial review completed 4 January 2004. Revision accepted 29 March 2004.
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