Vitamin A & Eye Health

A philosopher once said, “The eyes are the mirror of the soul and reflect everything that seems to be hidden.” But what if the mirror is hazy or even worse broken. What exactly happens when there is Vitamin A deficiency in the body. According to Sommer (2008) and Checkley et al., (2010) vitamin A deficiency is a serious condition specially in malnourished children and women of developing countries like India. In 2006, WHO released new global estimates that approx. 250,000 to 500,000 malnourished children from a deficiency of vitamin A turn blind each year and around half of them loose their lives within a year post blindness. The most recent survey data by WHO suggests that in India 62% of pre-schoolers are vitamin A deficient, with a serum retinol concentration of lower than 20 μg/dL.

Classification

The different signs of vitamin A deficiency (VAD) as graded by the WHO are:

  • Night Blindness (XN)
  • Conjunctival Xerosis (X1A)
  • Bitot’s Spots (X1B)
  • Corneal Xerosis (X2)
  • Corneal ulcer over less than 1/3 of the cornea (X3A)
  • Corneal ulcer over at least 1/3 of the cornea, known as keratomalacia (X3B)
  • Corneal scarring (XS)

The Visual Cycle & Vitamin A

When light enters the eye it passes through the cornea (a transparent membrane) and hits the retina (a paper thin tissues) that lines the back of the eye. The retina has  millions of light-sensitive cells called cones and rods. The rod reacts to dim light and processes black and white images. Within both rods and cones a cascade of reactions help convert light into a nerve signal that the brain is able to process so we experience sight.

Retinol through the blood is carried to the retina, where it gets converted to retinal. Retinal combines with the protein opsin forming a pigment called rhodopsin. Rhodopsin has a number of rod cells that makes it possible to see in dim light. When light strikes the retina then the rod cells undergo a bleaching process, this causes the colour of the rod cells to fade. In this transformation, retinal gets separated from the opsin and undergoes a structural shift, from a ‘bent’ or cis configuration to a ‘straightened’ or trans configuration. As the retinal detaches, the opsin also changes shape, disrupting the activities within the cell membrane and generating an electrical impulse. This impulse goes to the brain and forms a ‘black and white’ image. Most of the retinal released in this process is quickly recovered back to trans-retinol, and then to cis-retinal, which spontaneously again combines with opsin. The re-formed rhodopsin can respond to light again and begin another cycle.

We may experience a sudden lack of site while stepping into a dark room from a well-lit area, this happens because the pupils need time to adjust with the amount of light in the surroundings. Our eyes produce rhodopsin to increase the sensitivity to light this mechanism is called dark adaptation. The speed of adjustment in dim light is directly related to the amount of vitamin A available to regenerate rhodopsin. People with vitamin A deficiency experience night blindness. Although the eye contains only 0.01 % of the body’s vitamin A, they are highly sensitive to vitamin A levels.

Vitamin A plays a crucial role in colour vision, as a part of the pigment iodopsin in cone cells. In case of colour vision, iodopsin undergoes a transformation cycle similar to the rhodopsin. A lack of vitamin A affects rod cells before affecting the cone cells, so as vitamin A deficiency worsens, night blindness emerges with colour blindness.

Sources of Vitamin A

Almost half of the dietary intake of vitamin A is from animal food source as preformed vitamin A. The rest intake comes from fruits and vegetables in the form of provitamin A carotenoids, especially beta-carotene.

Dietary fortification & Supplementation

Though a wide variety of vitamin A rich foods are available but due to the faulty cooking practices or ignorance natural vitamin A is not available to the body. In such cases food fortification with vitamin A  like milk with fortified vitamin A, or vitamin A supplementation is essential. In general multivitamin supplementation contains 2,500 –10,000 IU vitamin A, often in the form of both retinol and beta-carotene.

Conclusion

In India, due to illiteracy, lack of knowledge, ignorance or poor handling of food causes loss of vitamin A in the diet. This burden multiplies when poverty comes into the picture. Altogether they cause a serious problems like night blindness. To prevent this, a number of national programmes have been adapted along with awareness campaigns.

Reference

  1. Sommer, A. (2008) Vitamin A Deficiency and Clinical Disease: An Historical Overview. The Journal of Nutrition. 138(10), 1835-1839
  2. Checkley, W., West, KP Jr, Wise, RA.  et al. (2010) Maternal vitamin A supplementation and lung function in offspring. N Engl J Med. 362(19), 1784-1794
  3. World Health Organization.(2010) WHO global database on Vitamin A deficiency. Retrieved from http://who.int/vmnis/vitamina/data/database/countries/ind_vita.pdf

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