ABSTRACT
Until two decades ago, it was m;sumed that once retinol left the blood was taken up vitamin tissues and irreversibly utilized, Hn',"P'.l" work by Vahlquist in 1972 indicated that retinol may be cycled from tissues to the blood. More recently, this idea has been verified by whole-body tracer kinetic studies in rats ( 4-8). Since these studies showed that the whole-body vitamin A utilization rate (i.e., disposal rate) is only about 10% ofthe retinol turnover rate in rats at various levels of vitamin A status indicate that the majority of retinol that leaves the plasma is recycled rather than irreversibly utilized. In fact, kinetic analysis of data from these studies indicates that an average retinol molecule recycles to the 7-,13 times before irreversible utilization. In analogy to engineering control theory, this phenomenon has been called (5) a since changes in need in one location can presumably be accommodated without acutely altering vitamin elsewhere.
