Storage of vitamin A

It has long been established that vitamin A (retinol) is an essential component of human diet and must be acquired from the diet as either preformed A or provitamin carotenoids, which subsequently converted in the body to the biologically active retinoids, retina and all-trans retinoic acid.

Vitamin A is normally transported in the blood linked to a specific protein, retinol binding protein (RBP). Specific proteins on cell surfaces and within cells are also involved with intracellular transport of the vitamin.

RBP circulates as a 1:1 molar complex; filtration and loss from the kidney are prevented by prealbumin, The normal concentrations in plasma are 40 to 50 ug per mL for RBP and 200 to 300 ug per mL for prealbumin.

Human liver

RBP binds retinol and this RBP-retinol complex is then secreted into circulation where it is transported to the target tissue to meet tissue vitamin A needs.

Vitamin A is fat soluble and is primarily stored in the liver, where RBP is synthesized. The liver holds over 90 percent of the body’s vitamin A reserves, with the rest deposited in fat tissue, lungs and kidneys.

The liver plays a pivotal role in the uptake, storage and maintenance of circuiting plasma vitamin A levels by mobilizing its vitamin A store.

In a well nourished person, vitamin A stores are generally sufficient to last many months on a vitamins A-deficient diet before signs of deficiency appear.
Storage of vitamin A