Effect of Fasting on the Oxidation of C14-Labeled Glucose, Palmitate and Valine in Growing Pigs

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Abstract
The rate of oxidation of C14-labeled glucose, palmitate and valine injected into pigs at 1, 6, 12 and 24 hr. after feeding was determined by placing the pigs in a respiration chamber and measuring the C14 activity of expired CO2. The peak rate of expired C14O2 occurred earliest with C14-palmitate (15 min.) and latest with C14-glucose (45 to 105 min.). The time at which the peak oxidation of C14-glucose occurred was delayed up to 1 hr. with longer periods of fasting. The length of fasting interval before injections had little effect on the total C14O2 expired over the following 24 hr. period when C14-glucose (29–43% of the dose recovered) or C14-valine (11–13%) were given, but six times as much C14-palmitate (36%) was oxidized when injected after a 24-hr, fast than after a 1-hr. fast. Once a peak in C14O2 activity had been attained, the rate of decline was found to be exponential for a period of 3 to 4 hr. with a half-life of 1 to 3 hr. When C14-glucose was given orally with the feed, the peak in C14O2 activity was lower and broader than when the tracer was injected 1–24 hr. after feeding. There was very little C14O2 expired by pigs given oral C14-palmitate and no peak in C14O2 was detected during the next 24 hours. Peak oxidation of oral C14 valine occurred 1 hr. later than with injections but 40% more of the dose was expired as C14O2 in the 24 hr. following administration. Tissue analysis 24 hr. after the pigs received the tracers orally indicated that most of the unoxidized C14 of C14-palmitate could be accounted for in carcass fat with very little found in muscle protein precipitated with trichloroacetic acid. The C14-activity of muscle protein was highest when C14-valine was injected but much of the unoxidized C14-valine could not be accounted for in the fat and protein of the carcass, and it appeared that most of it was still present in the trichloroacetic acid-soluble portion of the tissues.