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provide valuable data that describe the increase in resting metabolic rate (RMR) during pregnancy and its relationship with changes in body composition. The authors showed RMR increased 19 ± 6% from early to late pregnancy.
Expressing RMR per kilogram of fat-free mass (FFM), the most important determinant of RMR, the authors account for pregnancy-related changes in FFM and showed that RMR was 7 ± 11% higher in late pregnancy and therefore concluded that there was an adaptive thermogenesis (wasting of energy). A negative correlation between the change in RMR/FFM and the change in maternal fat mass then led to the conclusion that low adaptive thermogenesis (or the inability to waste energy) may contribute to fat accumulation for some women.
The energy metabolism field has spent extensive time considering different approaches for estimating adaptive thermogenesis.
Age and race are also significant determinants of RMR in especially large heterogeneous cohorts.
The accepted practice is to use linear regression to model independent variables of interest on the initial measured energy expenditure (ie, early pregnancy). This approach provides an equation (ie, RMR = a∗[fat-free mass] + b∗[fat mass] + c∗[age] + intercept) that, when individual data for each independent variable is imputed, yields an RMR for each person that is now proportional to these variables (RMRadjusted). RMRadjusted can then be used in statistical analyses to understand differences between individuals or changes throughout pregnancy that are dependent on body composition and to quantify adaptive thermogenesis that is independent of body composition (ie, RMR minus RMRadjusted).
(A, 18 ± 8% as absolute values, and B, 5 ± 7% as RMR/FFM). When using linear regression considering FFM only, the adaptive thermogenesis in RMR is 15 ± 6% (C). Yet when FM and age are also included, the adaptive thermogenesis is 8 ± 6% (D).
Hence, there are important methodological considerations needed for appropriate analysis and interpretation of RMR data (Figure). Importantly, the accepted analytical approach supports the conclusion of Berggren et al.
There are significant variations in gestational weight gain, with many women gaining in excess of the Institute of Medicine guidelines. Unfortunately, efforts to improve appropriate gestational weight gain have had only limited success. To date, interventions have focused primarily on decreasing energy intake and/or increasing physical activity. Maternal resting energy expenditure, which comprises ∼60% of total energy expenditure compared with the ∼20% that comes from physical activity, may be an important consideration in understanding variations in gestational weight gain.
We thank Drs Most and Redman for their interest and comments on our publication evaluating the relationship between changes in resting metabolic rate (RMR) and fat accretion during pregnancy. As noted in the original manuscript, our primary objective was to quantify the changes in resting energy expenditure during pregnancy and their relationship with gestational weight gain and fat accretion among healthy women.