We previously reported that the stimulatory effect of exercise on the metabolic clearance of
ketone bodies in postabsorptive subjects is abolished when plasma
ketone body concentrations are elevated above 4 mmol/L by prior fasting. In this study we determined whether this process is related to fasting or to hyperketonemia itself. Eight normal postabsorptive subjects were rendered artificially hyperketonemic (approximately 6 mmol/L) by a constant infusion of
acetoacetate and exercised moderately for 2 h. The kinetics of
ketone bodies were determined with [14C]
acetoacetate or beta-[14C]hydroxybutyrate. The metabolic clearance was slightly increased (approximately 25%) at the beginning of exercise, but this phenomenon was subsequently amplified by the progressive fall in
ketonemia, which decreased to about 4 mmol/L at the end of exercise. Taking into account the fact that the metabolic clearance of
ketones is inversely related to their concentration, it could be estimated that the direct effect of exercise on the metabolic clearance is negligible. Thus, the inability of exercise to enhance the metabolic clearance of
ketones at high physiological plasma
ketone levels is a general phenomenon that applies to both endogenous and exogenous
ketosis.