FIGURE 1.

Proposed compartmental model for retinol kinetics in humans. Circles represent compartments; components 3 and 13, shown as rectangles, are delay elements, and interconnectivities between compartments [L(I,J)s] are fractional transfer coefficients or the fraction of retinol in compartment J that is transferred to compartment I each day. Panel A shows the model for [¹³C₁₀]retinol and panel B shows the parallel model for [¹³C₅]retinol (which was derived from the [¹³C₁₀]β-carotene dose). In panel A, compartments 1–4 (including component 3) correspond to vitamin A digestion and absorption, loss of unabsorbed vitamin A, chylomicron production and metabolism, hepatocyte uptake of chylomicron remnant retinyl esters, and hepatic processing of retinol. Compartment 5 represents plasma retinol bound to retinol-binding protein and transthyretin; this retinol exchanges with vitamin A in 1 extravascular pool (compartment 6), which includes vitamin A stores in liver and other organs; thus, the fractional transfer of retinol to compartment 6 (stores) from compartment 5 (plasma) is L(6,5) (d⁻¹) and the fractional transfer of retinol back to plasma from stores is L(5,6) (d⁻¹). The asterisks (*) represent the site of input of the orally administered [¹³C] stable isotopes, and U(1) represents dietary vitamin A input. Absorption efficiency was fixed at 75%; thus, the fraction of the dose not absorbed is L(0,1) = 0.333 × L(2,1) because 25% is one-third of 75%. L(5,2) [and L(15,12) in the parallel model] represents the possibility that some retinol may be absorbed via the portal vein or picked up from the surface of chylomicrons; this parameter was required to fit the first data point. In panel B, compartments 11 and 12 represent β-carotene digestion and absorption; compartment 12 corresponds to the bioconversion of absorbed β-carotene to retinol in enterocytes. The remaining parts of the model are the same as in panel A, with L(16,15) = L(6,5), L(15,16) = L(5,6), and L(20,16) = L(10,6). L(I,J), fractional transfer coefficient. Adapted from reference 4 with permission.