Assessment of percent cholesterol absorption in humans with stable isotopes

Dietary cholesterol restriction is a general recommendation for the medical community and emphasizes the importance of intestinal cholesterol absorption and metabolism in humans. However, several methods that may accurately quantify cholesterol absorption utilize radioactive isotopes that are undesirable for younger individuals, women, children, and normal subjects. To eliminate this hazard, we have developed a procedure for measurement of percent cholesterol absorption, based on that of Zilversmit (1972. Proc. Soc. Exp. Med. Biol. 140: 862-865), using stable nonradioactive isotopic tracers of cholesterol. [26,26,26,27,27,27-2H]cholesterol (30 mg) was administered orally and [23,24,25,26,27-13C]cholesterol (15 mg) was administered intravenously on day 0 and percent cholesterol absorption was calculated as the plasma ratio of oral/intravenous isotopic tracer on day 3 as determined by gas chromatography-mass spectrometry with selected ion monitoring. Tracer cholesterol given orally peaked in plasma on day 2 and then slowly declined in parallel with the intravenous tracer. Cholesterol absorption in 16 healthy subjects (on no medication and not ingesting alcohol) consuming a Step One Diet was 53.5% +/- 8.5 SD%. Five subjects underwent repeat testing after 4-6 weeks with excellent replication (SD of difference between tests = 2.8%). No differences in the metabolism of [13C5]cholesterol, [2H6]cholesterol, and [14C]cholesterol were observed. The use of stable isotopes for the study of percent cholesterol absorption is precise and safe, allowing repeated measurements in normal individuals and thus facilitating clinical investigation of this key component of human cholesterol metabolism.