Evaluation of specific metabolic rates of major organs and tissues: comparison between nonobese and obese women

Abstract

Elia (1992) identified the specific resting metabolic rates (K(i)) of major organs and tissues in young adults with normal weight: 200 for liver, 240 for brain, 440 for heart and kidneys, 13 for skeletal muscle, 4.5 for adipose tissue and 12 for residual mass (all units in kcal/kg per day). The aim of the present study was to assess the applicability of Elia's K(i) values for obese adults. A sample of young women (n = 80) was divided into two groups, nonobese (BMI <29.9 kg/m(2)) and obese (BMI 30.0-43.2 kg/m(2)). This study was based on the mechanistic model: REE = σ (K(i) × T(i)), where REE is whole-body resting energy expenditure measured by indirect calorimetry and T(i) is the mass of individual organs and tissues measured by magnetic resonance imaging. For each organ/tissue, the corresponding Elia's K(i) value was analyzed respectively for nonobese and obese groups by using stepwise univariate regression analysis. Elia's K(i) values were within the range of 95% confidence intervals (CIs) in the nonobese group. However, Elia's K(i) values were outside the right boundaries of 95% CIs in the obese group and a corresponding obesity-adjusted coefficient was calculated as 0.98, indicating that Elia's values overestimate K(i) by 2.0% in obese adults. Obesity-adjusted K(i) values were 196 for liver, 235 for brain, 431 for heart and kidneys, 12.7 for skeletal muscle, 4.4 for adipose tissue, and 11.8 for residual mass. In conclusion, although Elia's K(i) values were validated in nonobese women, obesity-adjustments are appropriate for application in obese women.

Figure 1

Measured resting energy expenditure (REEm, in kcal/day) vs. predicted REE (REEp) for the (a) nonobese women and (b) obese women. REEp were calculated by the K_i values suggested by Elia (1), according to equation 3. The lines of identity are shown. (a) REEm = 0.925 × REEp + 112, r = 0.879, P < 0.001, n = 51 nonobese women. (b) REEm = 0.906 × REEp + 138, r = 0.946, P < 0.001, n = 29 obese women.

Figure 2

The difference between measured and predicted resting energy expenditure (REEm – REEp, in kcal/day) vs. (a) BMI (in kg/m²) and (b) %fat for all subjects (n = 80). REEp was calculated using the K_i values suggested by Elia (1), according to equation 3. The zero difference line and the lines representing 2 s.d. for the REE differences (−155 and 139 kcal/day) are shown. (a) (REEm – REEp) = 85.7 − 3.43 × BMI; r = −0.314, P < 0.01. (b) (REEm – REEp) = 34.7 − 1.52 × %fat; r = −0.276, P < 0.05.

Figure 3

95% Confidence intervals (CIs) for the K_i values of seven organs and tissues, fitted by stepwise univariate analysis are shown on a logarithmic scale, for the nonobese women (upper line) and the obese women (lower line). The Xs represent the K_i values suggested by Elia (1). AT, adipose tissue; Res, residual mass; SM, skeletal muscle.