Abstract

Seven normal subjects fasted for 3 days were exercised for 30 min on a bicycleergometer at 60 +/- 3 (mean +/- SE) of their maximal aerobic capacity. Rates of transport and oxidation of ketone bodies were determined at rest and duringexercise using a primed constant infusion of (14C) acetoacetate. During the initial 7.5 min of exercise, ketone concentration abruptly decreased (-19.4 +/- 3.1) as the result of a 22.2 +/- 7.0 reduction in ketone production rate(RD) of ketones. As work progressed, blood ketones continued to fall slowly until the 15th min of exercise and thereafter plateaued at a level which was 21.9 +/- 4.1 lower than resting values. During this apparent new steady state, both Ra and Rd were below control values but tended to be equal and to return simultaneously to their preexercise level. The metabolic clearance ratewas increased throughout the entire period of exercise, the rise being more marked during the initial 7.5 min (+40.7 +/- 7.5) than at later periods (+ 19.0 +/- 7.4). Exercise greatly enhanced ketone oxidation which became virtually complete; despite this, the C02 derived from ketones, which averaged 17.6 +/- 1.6 at rest, decreased to 10.1 +/- 0.6 after 30 min of work. This indicates that even during the hyperketonemia of fasting, the increased energy needs related to exercise are met primarily by fuels other thanketones. Verf.-Referat