Measurement of non-steady-state free fatty acid turnover

The accuracy of non-steady-state equations for measuring changes in free fatty acid rate of appearance (R(a)) is unknown. In the present study, endogenous lipolysis (traced with [¹⁴C]-linoleate) was pharmacologically suppressed in six conscious mongrel dogs. A computer-responsive infusion pump was then used to deliver an intravenous oleic acid emulsion in both constant and linear gradient infusion modes. Both non-steady-state equations with various effective volumes of distribution (V) and steady-state equations were used to measure oleate R(a) ([¹⁴C]oleate). Endogenous lipolysis did not change during the experiment. When oleate R(a) increased in a linear gradient fahsion, only non-steady-state equations with a large (150 ml/kg) V resulted in erroneus values (9% overestimate, P < 0.05). In contrast, when oleate R(a) decreased in a similar fashion, steady-state and standard non-steady-state equations (V = plasma volume = 50 ml/kg) overestimated total oleate R(a) (18 and 7%, P < 0.001 and P < 0.05, respectively). Overall, non-steady-state equations with an effective V of 90 ml/kg (1.8 x plasma volume) allowed the most accurate estimates of oleate R(a).