Barry is a vegan. His diet does not include foods that contain preformed vitamin A and he does not take any supplements. Nevertheless, his body has adequate vitamin A stores. Which of the following statements provides the best explanation for Barry's adequate vitamin A status?
a. He consumes foods that supply tryptophan, which his liver converts to preformed vitamin A
b. His fat cells synthesize preformed vitamin A from essential fatty acids
C. He eats plenty of foods that contain beta-carotene
d. His body does not require sources of vitamin A
e. His kidneys are able to store vitamin A