Changes in Walking Energy Expenditure and Substrate Oxidation during Pregnancy

Introduction: obesity and unhealthy weight gain during pregnancy are associated with risk of pregnancy complications. Management of energy balance during pregnancy needs an accurate assessment of intake and energy expenditure. As pregnancy promotes specific physiological changes, energy expenditure during rest and activity throughout this period may be altered. The aim of this study was to estimate with accurate methods changes in energy expenditure, substrate oxidation and efficiency during the most common activity in pregnancy that is walking. Methodology: it was a prospective observational study during pregnancy. A graded steady state walking submaximal exercise test was used to calculate indirect calorimetry variables with a portable metabolic cart at each trimester of pregnancy in 30 healthy pregnant women. Walking energy expenditure (WEE), carbohydrate (CarbOx) and fat oxidation (FatOx) were calculated with exercise intensity specific stoichiometric equations from measured oxygen consumption (VO2) and carbon dioxide production (VCO2). Resting component was removed from WEE to calculate net walking energy expenditure (Net WEE). Moreover, participants were classified as having healthy or unhealthy gestational weight gain (GWG) according to the Institute of Medicine (IOM) recommendations. Changes in body weight during the study were used to adjust net WEE and substrate oxidation. Walking exercise efficiency was calculated as work rate (WR) divided by WEE or net WEE. Differences in main variables during pregnancy were analyzed using a general lineal model. Least square means analyses were utilized to compare differences in WEE, Net WEE, and substrate oxidation between healthy and unhealthy weight gain groups. Results: Net WEE increased significantly during pregnancy. Higher rates were found between the second and the third trimester of pregnancy at any given speed. This elevation was reflected in increased FatOx during moderate intensity exercise and higher CarbOx at the fastest walking speed. Therefore, a significant interaction time*speed for NetWEE and substrate oxidation was found. Weight gain was an important variable in energy expenditure and substrate oxidation as quantitative differences in NetWEE and CarbOx were due to change in weight registered during the study. Changes in Net WEE throughout pregnancy were higher in the unhealthy GWG group which expended significantly more CarbOx than the healthy group at each speed. Overall, efficiency decreases during pregnancy (1.48% from the second to the third trimester and 1.88% from the first to the third one) and pregnant women were more efficient in moderate intensity exercise (walking at 4 km/h) independently of the trimester of pregnancy. Conclusion: The results suggest a paradoxical intensity-dependent substrate oxidation selection during walking exercise throughout pregnancy as in no-trained pregnant women the increase in WEE relies on fat oxidation in a short bout of activity. The results may be relevant for the management strategies in obesity and excessive GWG during pregnancy.