Using a continuous intravenous infusion of D-(-)-3-hydroxyl4,4,4_2H31butyrate tracer, we measured total ketone body transport in 12 infants: six newborns, four 1-6-mo-olds, one diabetic, and one hyperinsulinemic infant. Ketone body inflow-outflow transport (flux) averaged 17.3±1.A Mmol kg-' min' in the neonates, a value not different from that of 20.6±0.9 !mol kg-' min' measured in the older infants. This rate was accelerated to 32.2 jAmol kg-' min' in the diabetic and slowed to 5.0 gsmol kg-' min-' in the hyperinsulinemic child. As in the adult, ketone turnover was directly proportional to free fatty acid and ketone body concentrations, while ketone clearance declined as the circulatory content of ketone bodies increased.Compared with the adult, however, ketone body turnover rates of 12.8-21.9 jsmol kg-' min' in newborns fasted for <8 h, and rates of 17.9-26.0 gsmol kg-' min-' in older infants fasted for <10 h, were in a range found in adults only after several days of total fasting. If the bulk of transported ketone body fuels are oxidized in the infant as they are in the adult, ketone bodies could account for as much as 25% of the neonate's basal energy requirements in the first several days of life. These studies demonstrate active ketogenesis and quantitatively important ketone body fuel transport in the human infant. Furthermore, the qualitatively similar relationships between the newborn and the adult relative to free fatty acid concentration and ketone inflow, and with regard to ketone concentration and clearance rate, suggest that intrahepatic and extrahepatic regulatory systems controlling ketone body metabolism are well established by early postnatal life in humans.
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