Absolute oral bioavailability and metabolic turnover of β-sitosterol in healthy subjects.

The metabolic turnover, absolute oral bioavailability, clearance, and volume of distribution for beta-sitosterol were measured in healthy subjects. [C-14] beta-Sitosterol was used as an isotopic tracer to distinguish pulse doses from dietary sources and was administered by both oral and intravenous routes. The administered doses of [C-14] beta-sitosterol were in the region of 3 to 4 mu g, sufficiently low as not to perturb the kinetics of beta-sitosterol derived from the diet. Because the plasma concentrations of [C-14] beta-sitosterol arising from such low doses were anticipated to be very low, the ultrasensitive isotope ratio analytical method of accelerator mass spectrometry was used. The limit of quantification for [C-14] beta-sitosterol was approximately 0.1 pg/ml, the oral absolute bioavailability was just 0.41%, clearance was 85 ml/h, volume of distribution was 46 L, and the turnover was 5.8 mg/day. Given the steady-state concentrations of beta-sitosterol (2.83 mu g/ml), then the dietary load was calculated to be approximately 1400 mg/day.