Soundharya. S, BSc. Clinical Nutrition and Dietetics
MSc. Clinical Nutrition (student), MMM College of Health Sciences A Unit of Madras Medical Mission
Introduction
Vitamin-A is a fat-soluble vitamin discovered in 1909 by McCollum and Davis1. The chemical name of vitamin-A is retinol (vitamin-A1). In the body, vitamin-A exists in three oxidation states (retinol- the alcohol; retinal-the aldehyde; and retinoic acid). Retinol and retinal can be readily inter converted however, retinoic acid cannot be converted back either into retinol or retinal. The other two members of the vitamin-A family of compounds are retinyl esters and β-carotene2.
Role of vitamin-A
Vitamin-A plays a critical role in the retina for vision in a dimly lit environment. According to Wald (1935), the pigments of the retinal rods and cones are rhodopsin and isodopsin. They differ only in respect of the protein moieties (scotopsin and photopsin). The specific pigment common to both is a cis- isomer of retinal2.
Process of vision cycle
- Retinol is oxidised to retinal in the epithelium of the rods by an enzyme alcohol dehydrogenase in the presence of Nicotinamide Adenine Dinucleotide (NAD). The protein opsin (scotopsin) reacts with retinal and form rhodopsin. Rhodopsin, also known as visual purple is located in the light sensitive rod cells of the retina2.
- When light strikes the retina, the rod cells are bleached as the rhodopsin splits to form retinal and opsin. As this occurs a nerve stimulus is transmitted through the optic nerve fibres to the visual centre of the brain, where the sensation of vision is created2.
- Most of the retinal is rapidly converted to retinol. The retinol can then be converted to 11-cis-retinal that combines with opsin to produce rhodopsin, ready to undergo the cycle again. At each ‘turn’ of this cycle, however, a small amount of retinol is converted to retinoic acid or another inactive compound and is lost from the rod cells. This lost retinol must be continually replaced by fresh supplies of retinol brought by the blood2.
- The amount of retinol in the blood determines the rate at which rhodopsin is regenerated. The speed with which the eye recovers its full powers after exposure to bright light is directly related to the amount of vitamin-A available to form rhodopsin. The recovery process is dark adaptation because it allows the eye to adapt to vision in dim light after exposure to bright light2.
Sources of vitamin-A
- Retinol- liver, meats, eggs, milk products, and fatty fish3.
- β-Carotene/Pro-vitamin-A (soluble pigments) – carrots, sweet potatoes, spinach, kale, collard greens, papaya, bell peppers, and tomatoes3.
Recommended daily allowances
- Men – 1000 mg of Retinol equivalents (RE)3.
- Women – 800 mg of Retinol equivalents( RE)3.
References
- McCollum E. V. Davis The necessity of certain lipids in the diet during growth. J. Biol. Chem.15 1913 167 175.
- Srilakshmi, B. (2006). Nutrition Science. New Age International (Chapter 14, page no:264-267).
- Nieves, J. W. (2013). Alternative Therapy through Nutrients and Nutraceuticals. In Osteoporosis(pp. 1739-1749). Academic Press.