FIGURE 2.

Working compartmental model for retinol kinetics in children. Circles represent compartments; the rectangle is a delay component and DT(3) is the delay time retinol spent in component 3; and interconnectivities between compartments (arrows) are fractional transfer coefficients [L(I,J)s, or the fraction of retinol in compartment J transferred to compartment I each day]. For example, L(6,5) is the fraction of retinol in compartment 5 transferred to compartment 6 each day and L(5,6) is the fraction in compartment 6 transferred to compartment 5 each day. Compartment 1 is the site of input of orally ingested tracer (*) and dietary vitamin A [U(1)]; fractional absorption efficiency was fixed at 0.80. Components 1–4 represent the processing of preformed dietary vitamin A, chylomicron production and metabolism, hepatic uptake of chylomicron remnant retinyl esters, and processing of retinol (compartment 4); delay component 3 is the site of loss of unabsorbed tracer. Retinol is secreted from the liver bound to retinol-binding protein into plasma compartment 5, the site of sampling (triangles). Retinol in the plasma pool can exchange with vitamin A in 2 extravascular compartments: a faster turning-over pool (compartment 7) and a slower turning-over storage pool (compartment 6), which is also the site of irreversible loss from the system. The inset shows the simplified model used to analyze the reduced data sets (protocols 2 and 3). In the simplified model, delay component 3 is the site of input for tracer (*) and dietary vitamin A [U(3)]. TBS, total body stores.