**2. Biological roles of vitamin A**

 Vitamin A is an essential micronutrient required in small amounts by human throughout the life cycle to perform multiple metabolic functions. It is important for growth and development, the maintenance of immune function and maintenance of epithelial cell integrity, good vision, reproduction as well as lipid metabolism. Vitamin A is also an important antioxidant, a property shared with vitamins

E and C, respectively [3]. New biological functions of vitamin A such as lipid metabolism, insulin response, energy balance and the nervous system are continuously being discovered.

#### **2.1 Vitamin A and health importance**

Vitamin A has long been known to play a critical role in vision. Night blindness or reduced vision ability under dim light is a very early and purely subjective symptom of vitamin A deficiency (VAD). In the eye, the 11-cis retinal binds to protein, term opsins, to form both the rhodopsins (rods) and iodopsins (cones) visual pigments [10]. Light that enters the eyes will isomerise the bound 11-cis retinal to all-trans form which initiates excitation of the photoreceptor cell. This isomerisation reaction will trigger nervous signal and passes along the cranial 'optic nerve' destined for the visual centre of the cerebral cortex that translates into a picture [11, 12]. A vitamin A metabolite, retinoic acid (RA), is essential for the normal functioning of the immune system [13]. Retinol and its derivatives function as an immune enhancer that potentiates the antibody response; at the same time it maintains and restores the integration of all mucosal cells and their functions. Retinols are also required for the development of leukocytes that play a major role in mounting an immune system. The major site of vitamin A action in the immune response is thought to be the T helper cell and T lymphocytes cell. The retinol derivative '4-hydroxyretinoic acid' rather than retinoic acid is important in this aspect [14].

Along with its role in vision and immune system, vitamin A has also been shown to be actively involved in the production of red blood cells, which are derived from stem cells that depend upon retinoid for their proper differentiation. Vitamin A also appears to facilitate the mobilisation of iron stores to the developing erythrocytes where it is incorporated into haemoglobin, the oxygen carrier complex protein [15]. In addition, vitamin A (retinol, retinoic acid, all-trans retinal) is an important signalling molecule that affects gene expression and is called 'retinoid-controlled genes' which are involved in the differentiation and development of foetal and adult tissues, stem cell differentiation, apoptosis, support of reproductive and immune functions and regulation of lipid metabolism and energy homeostasis [16]. Retinol and retinoic acid also play a vital part in the development of human embryo and differentiation of three germ layers and propagation of the signalling process in the formation of the neural tube, organogenesis and development of limbs during embryogenesis. There are two main types of high-affinity receptor for trans- and cis-retinoic acid isomers within the nucleus cells of vertebrates including mammals. Each set of these receptors has six different domains which are involved in gene expression [17].

In terms of skin health, the isoform retinoic acid will switch on genes that differentiate immature skin cells into mature epidermal cells. Vitamin A and its metabolites have also been shown to improve photo-aged and chronologically aged skin pathologies. They promote the deposition of new collagen fibres and prevent degradation occurring in such skin types [11]. Growth hormone is a peptide hormone that stimulates growth (anabolic metabolite), cell reproduction and cell regeneration in humans and other animals. Growth hormone is a 191-amino acid, single-chain polypeptide that is synthesised, stored and secreted by the somatotropic cells within the lateral wings of the anterior pituitary gland. The availability of vitamin A is necessary for expression of many genes including those human growth hormones [11].

Studies showed that vitamin A in the form of retinol is required for maintenance of adult mammalian spermatogenesis. Spermatogenesis is the production and development of sperm. It is a process which sperm cells undergo a series of cellular changes and divisions in order to fully develop. The cell begins as a spermatogonium and the undeveloped diploid sperm cell and ends as four spermatids. These

*Vitamin A in Health and Disease DOI: http://dx.doi.org/10.5772/intechopen.84460* 

spermatids form fully developed sperm cells that comprise semen. Retinoid acid is an essential regulator of gametogenesis both in male and female gametes, such that they can enter meiosis [18]. Antioxidant activity is another identified vital role, where the presence of vitamin A or β-carotene in small doses showed anticancer effect. It appears to stem from its ability to scavenge for reactive oxygen species (ROS) and can improve immune function in addition to eliciting an anti-proliferative effect through the retinoic acid receptor (RAR) and retinoid X receptor (RXR). ROS are the most important free radical in biological system and harmful byproducts generated during the normal cellular functions. In this way, they can block certain carcinogenic processes and thus inhibit tumour cell growth [11, 19].
