Retention of intravenously infused [13C] bicarbonate is transiently increased during recovery from hard exercise

The effects of exercise on energy substrate metabolism persist into the postexercise recovery period. We sought to derive bicarbonate retention factors ( k) to correct for carbon tracer oxidized, but retained from pulmonary excretion before, during, and after exercise. Ten men and nine women received a primed- continuous infusion of [ C-13] bicarbonate ( sodium salt) under three different conditions: 1) before, during, and 3 h after 90 min of exercise at 45% peak oxygen consumption ( V-O2peak); 2) before, during, and 3 h after 60 min of exercise at 65% V-O2peak; and 3) during a time- matched resting control trial, with breath samples collected for determination of (CO2)-C-13 excretion rates. Throughout the resting control trial, k was stable and averaged 0.83 in men and women. During exercise, average k in men was 0.93 at 45% V-O2peak and 0.94 at 65% VO2peak, and in women k was 0.91 at 45% V-O2peak and 0.92 at 65% V-O2peak, with no significant differences between intensities or sexes. After exercise at 45% V-O2peak, k returned rapidly to control values in men and women, but following exercise at 65% V-O2peak, k was significantly less than control at 30 and 60 min postexercise in men ( 0.74 and 0.72, respectively, P < 0.05) and women ( 0.75 and 0.76, respectively, P < 0.05) with no significant postexercise differences between men and women. We conclude that bicarbonate/ CO2 retention is transiently increased in men and women for the first hour of postexercise recovery following endurance exercise bouts of hard but not moderate intensity.