Lifestyle intervention with dietary counseling and increased physical activity is the first-line strategy for prevention of gestational diabetes mellitus (GDM). This approach is based on epidemiological data suggesting that the etiology of GDM is similar to type 2 diabetes, with common risk factors including an unhealthy, hypercaloric diet and a sedentary lifestyle (Agha-Jaffar et al., 2016). Preliminary studies in obese pregnant mothers at risk for GDM indeed supported the notion that lifestyle modification may reduce GDM incidence through restriction of pregnancy weight gain (eg, Thornton et al., 2009). Based on such preliminary findings, well-powered randomized controlled lifestyle intervention trials such as LIMIT (Dodd et al., 2014b), UPBEAT (Poston et al., 2015), RADIEL (Koivusalo et al., 2016), DALI (Simmons et al., 2017), and LifeMoms (Peaceman et al., 2018), which collectively involved more than 5,000 patients, were more recently undertaken to assess the efficacy of lifestyle interventions to reduce GDM incidence. With the exception of RADIEL (Koivusalo et al., 2016), all other trials were ineffective at lowering GDM incidence. This is surprising, since these trials achieved acceptable levels of patient adherence (Koivusalo et al., 2016; Poston et al., 2015) and successful alterations of lifestyle factors targeted by the interventions—such as reduced dietary intake (Dodd et al., 2014a; Poston et al., 2015), improved diet quality (Dodd et al., 2014a; Koivusalo et al., 2016; Poston et al., 2015; Simmons et al., 2017), and increased physical activity (Dodd et al., 2014b; Koivusalo et al., 2016; Poston et al., 2015)—which indeed translated to lower rates of gestational weight gain (Koivusalo et al., 2016; Peaceman et al., 2018; Poston et al., 2015; Simmons et al., 2017) in all but one trial (Dodd et al., 2014b) without lowering GDM incidence.

The inability of lifestyle modification to prevent GDM despite successful restriction of weight gain raises the essential question of whether energy imbalance (energy intake> energy expenditure) is involved in the development or prevention of GDM. To date, information regarding energy balance is based on subjective data (eg, self-reported diet and physical activity), which is prone to recall bias, rather than objective data (eg, energy intake and expenditure by stable isotopes) derived from state-of-the art methods in human nutrition research. Consequently, empirical evidence to indicate that a more positive energy balance early in pregnancy favors the development of GDM is lacking, and yet this is the chief scientific premise supporting many lifestyle modification programs for GDM prevention (Dodd et al., 2014b; Koivusalo et al., 2016; Peaceman et al., 2018; Simmons et al., 2017). In a prospective, observational energy balance study in 62 pregnant women with obesity (BMI> 30 kg/m2) we used state-of-the-art methodology to simultaneously measure energy intake and energy expenditure over 12 weeks starting between 13 and 16 weeks of gestation and concluding between 24 and 27 weeks. Unlike previous studies based on dietary self-report, we assessed energy intake across the 12-week period