The gastrointestinal handling and metabolism of [1-13C]palmitic acid in healthy women

The gastrointestinal handling and metabolism of 1-¹³C]palmitic acid given as the free fatty acid was examined in six healthy women by measuring the excretion of¹³C-label in stool and in breath as¹³CO₂. The gastrointestinal handling of 1-¹³C]palmitic acid was compared with the apparent absorption of dietary lipid by measuring lipid losses in stool. The variation both within and between subjects was determined by repeating the study in the same individuals on separate occasions. The time course for excretion of label in stool over the five-day study period followed a common pattern, with most of the label excreted over the first two days of the stool collection.¹³C-Label excreted in stool over the five-day study period was 14.3±9.8% of that administered and on repeating the trial was 31.6±24.7% (not significantly different due to variability); there was poor agreement within subjects. Lipid excreted in stool expressed as a percentage of ingested lipid was 5.2±4.4% in Trial 1 and 5.9±4.0% in Trial 2, and was the same in each individual on repeating the trial. There was no clear relationship between the excretion of¹³C-label and lipid in stool (Trial 1:R=−0.43,P>0.40; Trial 2:R=−0.02,P>0.97). On the first occasion, 22.0±4.5% of the administered label was excreted on breath over the 15-h study period and on repeating the trial was 15.8±9.5% (not significantly different) with poor repeatability in a given individual. There was an inverse relationship between the proportion of¹³C-label excreted in stool and that excreted on breath in Trial 1 (R=−0.80,P>0.06) with a weaker association observed in Trial 2 (R=−0.49,P>0.32). Correcting for differences in the apparent absorption of label reduced the variability in its excretion in breath observed between subjects, particularly in Trial 2. It is concluded that although there are differences in the gastrointestinal handling of 1-¹³C]palmitic acid both within and between healthy adults, the postprandial oxidation of absorbed substrate was similar. The assumptions underlying these observations need to be examined by characterizing the nature of¹³C-label in stool.