Fuel utilization in exercise and pregnancy

Measurements by indirect calorimetry reveal preferential use of carbohydrates during exercise in pregnancy [53]. The respiratory exchange ratio (RER) reflects the ratio between CO2 output and oxygen uptake (VO2). The RER provides information on the proportion of substrate derived from various macronutrients. For carbohydrate to be completely oxidized to CO2 and H2O, one volume of CO2 is produced for each volume of O2 consumed. An RER of 1 indicates carbohydrates are being utilized, while an RER of 0.85 indicates mixed substrate. Assessment of fuel utilization during pregnancy is important because of the possible effect of exercise-induced maternal hypoglycemia [53]. Such events are unlikely to occur during 45 min of moderate exercise, but could occur after 60 min of continuous moderate to strenuous exercise (Fig. 3.2). The tendency for higher respiratory exchange ratios during pregnancy and during exercise in pregnancy suggests a preferential utilization of carbohydrates. Soultanakis et al. [53] found that with exercise greater than 20 min in length, both glucose and glycogen stores were depleted, resulting in higher levels of ketones and free fatty acids being used as fuel sources. Increased carbohydrate metabolism along with lower glycogen stores may further predispose women to hypoglycemia during pregnancy. Protein utilization in pregnancy during exercise does not increase above nonpregnancy levels, and since most people in the United States get more than the required amount of protein in their diets, additional/supplemental dietary protein is unnecessary fuel during moderate bouts of exercise [54].

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