Resting Metabolic Rate Rises in Pregnancy, but Energy Intake and Active Expenditure Remain Stable: A Pilot Study

BACKGROUND: Adequate nutrition during pregnancy is well established; however, how pregnancy affects energy metabolism, particularly the relationship between energy intake, expenditure and maternal mass, remains poorly understood. Current dietary recommendations advise increasing energy intake across pregnancy, but these are based on theoretical estimates rather than contemporary empirical data. AIM: To describe changes in energy intake, total daily energy expenditure (TDEE) and body composition across pregnancy in a pilot study of pregnant people living in Australia and to assess the associations with maternal fat mass. MATERIALS AND METHODS: Pregnant people aged ≥ 18 years with singleton pregnancies were included. Assessments were conducted in each trimester (first: 8-13 weeks'; second: 23-25 weeks'; third: 34-37 weeks'). Dietary intake was measured using a 3-day weighed food diary. Total and active energy expenditure was assessed by a thigh-worn accelerometer and resting metabolic rate assessed using indirect calorimetry. RESULTS: Twenty-seven people were included. The mean (SD) total daily energy intake (8895 (2279) kJ to 8973 (1745) kJ), TDEE (10343 (2056) kJ to 11307 (1922) kJ) and active energy expenditure (3212 (1225) kJ to 3161 (1199) kJ) remained consistent over pregnancy (all p > 0.05), whereas resting metabolic rate increased (6081 (972) kJ to 7041 (1052) kJ, p = 0.002). Higher TDEE and active energy expenditure were positively associated with fat mass. CONCLUSION: Total energy intake, TDEE and activity-related energy expenditure was stable during pregnancy while resting metabolic rate increased. Larger, adequately powered studies are needed to better characterise energy balance in pregnancy and its clinical implications.