Metabolism of parenterally administered fat emulsions in the rat: studies of fatty acid oxidation with 1-13C- and 8-13C-labelled triolein.

To reassess the hypothesis that fatty acid catabolism occurs to completion via β-oxidation, male Sprague-Dawley rats receiving continuous total parenteral nutrition (TPN) including 43% energy as fat were infused with [1- 13 C]- or [8- 13 C]triolein. Expired CO 2 was collected continuously for 4h and its 13 C: 12 C ratio determined by isotope-ratio mass spectrometry. Bicarbonate retention was also assessed over 4 h by infusion of NaH 14 CO 3 and measurement of the expired 14 CO 2 . A possible loss of label from [8- 13 C]oleic acid from the citric acid cycle via labelled acetyl-CoA without oxidation to CO 2 was assessed by infusing further animals with acetate labelled with 14 C either at C atoms I or 2 and determination of its conversion to expired 14 CO 2 . At isotopic steady state, 63.2 (SE 1.6)% (n 8) of the infused [1- 14 C]acetate and 46.0 (SE 1.2)% (n 8) of [2- 14 C]acetate was recovered as expired 14 CO 2 . After correction for bicarbonate retention and non-oxidative isotope loss, 37.3 (SE 1.2)% (n 20) of the [1- 13 C]triolein was found to have been oxidized, whereas 32.6 (SE 1.0)% (n 20) of the [8- 13 C]triolein was oxidized (P ≤ 0.01). The lower oxidation of the C atom at position 8 of oleic acid than that at position 1 indicates incomplete oxidative breakdown of the fatty acid after entering β-oxidation.