Maternal Metabolic Limits In Endurance Athletes During Pregnancy

In pregnancy, mothers face increasing energy demands to support fetal growth. Previous studies have proposed a limit to sustained energy metabolism in humans (~2.5× basal metabolic rate, BMR; kilocalories/day). It is unclear if pregnancy, particularly coupled with high levels of physical activity, pushes maternal energy expenditure beyond the metabolic limit. PURPOSE: We hypothesize that there is a metabolic limit to maternal energy expenditure, ~2.5× preconception BMR, which is not surpassed during pregnancy. METHODS: We recruited 12 female endurance athletes (mean age: 32.6 ± 2.0 years, mean preconception BMI: 21.5 ± 2.8) planning to conceive or in early pregnancy (<17 weeks) who self-reported >300 minutes/week of moderate-to-vigorous physical activity (MVPA) before conception, including >32 kilometers/week of running. We measured total energy expenditure (TEE; kilocalories/day) using the doubly labeled water method in early pregnancy (0-17 weeks) and in the third trimester (32-34 weeks). MVPA was measured using ActiGraph accelerometers and Withings fitness trackers. Body composition was measured using the doubly labeled water method and Withings scales. Statistical analyses included paired t-tests and linear regression. RESULTS: Mean gestational weight gain was 12.7 ± 4.2 kilograms. Weekly running distance decreased from early pregnancy (51.4 ± 20.9 kilometers/week) to the third trimester (25.3 ± 33.8 kilometers/week) (p = 0.002), while weekly MVPA was largely maintained (682.3 ± 169.1 minutes/week vs. 565.6 ± 178.3 minutes/week; p = 0.05). Mean metabolic scope (TEE/estimated preconception BMR) increased from 2.01 ± 0.31 in early pregnancy to 2.23 ± 0.30 in the third trimester (p = 0.001). In linear regression models accounting for age and gestational age, third trimester metabolic scope showed non-significant, negative trends with both weekly rate of weight gain in the third trimester (β = -0.25, p > 0.05) and total gestational weight gain (β = -1.27, p > 0.05). CONCLUSION: These results support the existence of a maternal metabolic limit, even in conditions of high energy demands due to physical activity, which may shape gestational weight gain and pregnancy outcomes in endurance athletes. This study was funded by The National Science Foundation (BA-DDRIG #2316583), The Leakey Foundation, and the Triangle Center for Evolutionary Medicine.