Carbohydrate sparing of fatty acid oxidation. I. The relation of fatty acid chain length to the degree of sparing. II. The mechanism by which carbohydrate spares the oxidation of palmitic acid

Abstract

I. Carboxyl labeled fatty acids of the even series (C8-C16) and triglycerides of carboxyl labeled palmitic and capric acids were injected into fasted and carbohydrate-fed rats. Expired CO2 was collected at various intervals for 24 hours, and the cumulative C14O2 and the specific activity-time curves were derived for each rat. The conversion of the C14 of the injected fatty acids to CO2 was lower in the fed than in the fasted rat. This sparing action of carbohydrate was more pronounced with the longer-chain fatty acids. Under the conditions employed, administered carbohydrate failed to spare the oxidation of acetate to CO2. The sparing action of carbohydrate lasted several hours and was followed by a secondary rise in specific activity of expired CO2. This secondary rise, suggestive of an increased breakdown of labeled fatty acid, was more pronounced in the longer-chain fatty acids. II. To study the mechanism of carbohydrate action in sparing oxidation of palmitic acid to CO2, the triglyceride and nonlipid fractions were isolated from carbohydrate-fed and fasted rats 5 hours after injection of palmitic acid-1-C14. The C14 contents of carboxyl carbon and decarboxylated moieties of the triglyceride 16- and 18-carbon fatty acids and the Cl4 contents of the nonlipids were determined. Results indicated that the extent of utilization of acetyl-S-CoA, derived from palmitic acid breakdown, for fatty acid synthesis was far greater in the carbohydrate-fed than in the fasted rat. Diversion of acetyl-S-CoA from an oxidative to a synthetic fat can account for only a small part of the difference in the C14O2 recoveries observed in fed and fasted rats. Results obtained for C14 nonlipids in carbohydrate-fed and fasted rats indicated that carbohydrate feeding did not bring about a diversion of C14 into this fraction. The principal carbohydrate action in sparing palmitic acid oxidation was presumably in restricting its breakdown. In both carbohydrate-fed and fasted rats, injected palmitic acid was recovered after 5 hours chiefly as 16-carbon fatty acids but, a much greater proportion of unoxidized palmitic acid was found to be converted to 18-carbon fatty acids in carbohydrate-fed than in fasted rats. Degree of sparing, by carbohydrate, of oxidation of fatty acids of chain lengths less than 16 carbons is probably related to capacity of the organism to convert a particular fatty acid directly to 16- and 18-carbon fatty acids.