Background: Energy-conserving processes reported in undernourished women during pregnancy are a recognized strategy for providing the energy required to support fetal development. Women who are obese before conceiving arguably have sufficient fat stores to support the energy demands of pregnancy without the need to provoke energy-conserving mechanisms. Objective: We tested the hypothesis that obese women would show behavioral adaptation [ie, a decrease in self-selected walking (SSW) speed] but not metabolic compensation [ie, a decrease in resting metabolic rate (RMR) or the metabolic cost of walking] during gestation. Design: RMR, SSW speed, metabolic cost of walking, and anthropometric variables were measured in 23 women aged 31 6 4 y with a BMI (in kg/m 2 ) of 33.6 6 2.5 (mean 6 SD) at ;15 and 30 wk of gestation. RMR was also measured in 2 cohorts of nonpregnant control subjects matched for the age, weight, and height of the pregnant cohort at 15 (n = 23) and 30 (n = 23) wk. Results: Gestational weight gain varied widely (11.3 6 5.4 kg), and 52% of the women gained more weight than is recommended. RMR increased significantly by an average of 177 6 176 kcal/d (11 6 12%; P , 0.0001); however, the within-group variability was large. Both the metabolic cost of walking and SSW speed decreased significantly (P , 0.01). Whereas RMR increased in .80% of the cohort, the net oxygen cost of walking decreased in the same proportion of women. Conclusion: Although the increase in RMR was greater than that explained by weight gain, evidence of both behavioral and biological compensation in the metabolic cost of walking was observed in obese women during gestation. The trial is registered with the Australian Clinical Trials Registry as ACTRN012606000271505.Am J Clin Nutr 2011;94:819-30.

Gestational weight gain: arguments for a transgenerational weight-control process Dear Sir:We are writing to comment on a recent publication in the Journal entitled ''Changes in resting and walking energy expenditure and walking speed during pregnancy in obese women'' by Byrne et al (1).The observed results did not fully support the hypothesis raised by authors that excess fat mass in obese women would blunt energyconserving processes during pregnancy. The authors reported that 50% of obese pregnant women gain within or less than the Institute of Medicine's 2009 recommendations (2). We, like others, observed a negative correlation between gestational weight gain (GWG) and pregestational BMI (1452 single pregnancies; Pearson's r, 20.22; P , 0.0001) and a lower GWG in obese women without any structured weight-control program during pregnancy. Although GWG was highly variable between pregnant women, 40% of them were below the Institute of Medicine's 2009 recommendations (2). We also observed a lower weight gain at 1 mo in children from obese mothers who were exclusively breastfed (3), and we are currently conducting research to better understand the mechanisms involved. We believe that GWG is determined by a transgenerational regulation process aimed at optimizing fetal growth. Although the population in Byrne et al's study is small and lacks a pregnant control group and body composition assessment, the unexplained increase in resting metabolic rate fits with a process that limits weight gain. This makes sense if GWG determines birth weight and the health status of the offspring as suggested by Fraser et al (4). The same mechanism might occur after birth in infants who are exclusively breastfed. Because there might be naturally occurring weight-control regulation during pregnancy, weight-control programs should be used very cautiously and the impact on fetal development carefully monitored.None of the authors declared a conflict of interest.

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