FIGURE 1.

Proposed compartmental model for vitamin A kinetics in children. Circles represent compartments; the rectangles are delay elements; interconnectivities between components (arrows) are fractional transfer coefficients [L(I, J)s, or the fraction of retinol in compartment J transferred to compartment I each day]; and DT(I)s are delay times (or the time spent in delay element I). Delay element 3 is the site of introduction of ingested tracer (*) and dietary vitamin A [U(3)]. Components 3 and 4 represent vitamin A digestion and absorption plus chylomicron processing until vitamin A uptake by hepatocytes (compartment 4). Retinol bound to retinol-binding protein is secreted from compartment 4 into plasma compartment 5, the site of sampling (triangle). Retinol in plasma can exchange with vitamin A in 2 extravascular pools (a larger compartment 6 and a smaller compartment 7) and it can also irreversibly enter component 8, hypothesized to be tissues from which vitamin A does not recycle. Compartment 6 and component 8 are the sites of irreversible loss from the system.