The total energy cost of pregnancy is positively associated with prepregnancy fat mass, % body fat, and pregnancy weight gain (6), but maintenance costs are only associated with prepregnancy fatness. This might be explained by the fact that prepregnancy fatness is a marker of overall nutritional status or that prepregnancy fatness may indicate a positive energy balance before conception, and this energy balance might be maintained throughout pregnancy. Either mechanism would explain the wide variability in metabolic response to pregnancy and serve to match energy requirements to energy availability, hence optimizing fetal growth. Leptin has been suggested to be the signal that may link prepregnancy fatness with the maternal metabolic response to pregnancy (5).
Butte et al. (13) compared energy metabolism in women with a low, normal, and high body mass index (BMI). The increase in BMR during pregnancy was highest in a high BMI (>26 kg/m2) group at 16.3 (5.4) kcal/week compared with a normal BMI (19.8-26.0 kg/m2) group at 9.5 (4.6) kcal/week and a low BMI (£19.8 kg/m2) group at 8.8 (4.5) kcal/week (13). Increments in BMR and 24-h energy expenditure were correlated with change in weight and fat-free mass but also with prepregnancy BMI or percentage fat (13). The change in BMR at 24 weeks gestation was significantly correlated with maternal obesity prior to pregnancy. Women who were lean prepregnancy were more likely to attenuate the increase in BMR in order to spare energy, whereas obese women had larger increases in BMR in response to energy excess (8).
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