Encyclopedia of

Vitamin A 707
mobilized, and then returned to plasma several times
before it undergoes irreversible degradation. Using
model-based compartmental analysis of plasma retinol
in a healthy young man who had consumed 105 mmol
of retinyl palmitate, it was calculated that 50 mmol of
retinol passed through his plasma each day, although
only 4 mmol=day was degraded. [22] Overall, the body’s
capacity for vitamin A storage is high, whereas its
ability to degrade and eliminate the vitamin seems to
be quite limited. These features of metabolism help
to explain the propensity for retinyl esters to accumulate
in tissues when vitamin A intake exceeds needs.
The relationship between the concentrations of
plasma retinol and liver vitamin A is far from linear;
in fact, plasma retinol is maintained at a nearly
constant level over a wide range of liver vitamin A concentrations.
[20] Only when liver vitamin A stores are
nearly exhausted (