Table 5.

Comparison of vitamin A transport via holo-RBP in the blood vs. retinyl esters in the blood.

	RBP-Bound Retinol in Blood	Retinyl Ester in Blood
Tissue Origin	Primarily the liver	Primarily the small intestine
Source of Vitamin A	Vitamin A stored in the liver, the primary organ for vitamin A storage	Dietary vitamin A immediately after absorption by the small intestine
Ability to Mobilize Liver-Stored Vitamin A	Yes	No
Dependence on Immediate Diatary Intake	No	Yes
Regulation of its Concentration in the Blood	Yes	No
As a Source of Vitamin A During the Absence of Food	Yes	No
As a Source of Vitamin A in the Absence of Vitamin A in Food	Yes	No
Nature of the Carrier Protein(s) in the Blood	The only known natural ligand of RBP is retinol	Retinyl esters are carried by lipoproteins such as chylomicron remnants, which contain many kinds of lipids
Cellular Uptake Specificity	Cellular retinol uptake by the RBP receptor is not associated with cellular uptake of many other kinds of lipids	Cellular retinyl ester uptake is associated with cellular uptake of many other kinds of lipids
Regulatory Mechanism of Vitamin A Uptake	Unknown	Unknown
As a Cause of Vitamin A Toxicity in Human	No (Healthy people maintain micromolar concentrations in the blood)	Yes (An increase above 10% in retinyl esters in the blood is a sign of vitamin A overload in human)