Whole body oxidation of dietary fatty acids: implications for energy utilization

Whole body oxidation of dietary stearic, oleic, and linoleic acid was measured in males consuming a test diet of normal foods at a level commensurate with energy requirements for 16 days. Labeled stearic, oleic or linoleic acid was consumed with the breakfast meal on either day 8, 11, or 14. Breath samples were analyzed for total CO2 content and 13CO2 abundance. Breath enrichment of 13CO2 after ingestion of labeled substrate was calculated over background 13C abundance with diet only and expressed as fraction of substrate dose absorbed. Fecal excretion of labeled and diet fatty acids was determined for pooled stool collections. Stearic, oleic, and linoleic acids were separated from fecal fat extracts and combusted to determine 13CO2 enrichment over background. Both dietary and labeled stearic acid were less well absorbed than either oleic or linoleic acids. At hours 7-9 after ingestion of the labeled breakfast, significant differences in percent of absorbed dose excreted in breath were observed between all three fatty acids. Significant differences were observed in apparent amounts of labeled oleate, linoleate, and stearate oxidized after 3 to 9 h. This difference in fatty acid oxidation challenges the assumption that dietary fat is oxidized at a rate independent of its long chain fatty acid composition.