Retinol is an active form of vitamin A circulating in the blood
Retinol is an active form of vitamin A circulating in the blood. It is a fat soluble vitamin that participates in controlling of embryonic development, photoreception, maintenance of immune status, and many other processes. Retinol is the precursor of the retinoid group (or retinoids) – biologically active derivatives of vitamin A, particularly, retinal and retinoic acid.
Retinoic acid serves as a ligand for the superfamily of nuclear receptors, including receptors for steroid hormones (cortisol, testosterone), vitamin D, triiodothyronine, prostaglandins, and transcription factors. Thus, it is an essential component for the expression of genes that are involved in the processes of cell development and providing the sensitivity to hormones and growth stimuli. Due to these functions retinoic acid can: 1) regulate the normal growth and differentiation of cells of the embryo; 2) stimulate the division and differentiation of cells of rapidly dividing tissues – cartilage, bone tissue, spermatogenic epithelium, placenta, skin epithelium, mucous membranes, cells of the immune system.
Serum retinol levels reflect vitamin A reserve in the liver only when it’s extremely low (below 0.07 ?mol/g liver) or extremely high (above 1.05 ?mol/g liver). Otherwise, serum retinol is homeostatically controlled and for that reason don’t always correlate with vitamin A intake or clinical signs of deficiency. Consequently, serum retinol isn’t a good tool for assessing the vitamin A status and may not respond to interventions. Instead, the distribution of serum retinol values in a population and the prevalence of individuals with serum retinol values below a given cut-off can provide important information on the vitamin A status of a population and may reflect the severity of vitamin A deficiency as a public health problem, especially when the degree of underlying infection or inflammation is taken into account. Serum retinol values are usually checked in young children, since they belong to a group of individuals highly vulnerable to vitamin deficiency.