Night blindness
Night blindness is the difficulty for the eyes to adjust to dim light. Affected individuals have poor vision in the darkness but see normally when adequate light is present. A process called dark adaptation typically causes an increase in photopigment amounts in response to low levels of illumination. This occurs to an enormous magnitude, increasing light sensitivity by up to 100,000 times its sensitivity in normal daylight conditions. VAD affects vision by inhibiting the production of rhodopsin, the photopigment responsible for sensing low-light situations. Rhodopsin is found in the retina and is composed of retinal (an active form of vitamin A) and opsin (a protein). When VAD prevents the body from producing sufficient amounts of retinal, a decreased amount of rhodopsin is produced and night blindness results. Night blindness caused by VAD has been associated with the loss of goblet cells in the conjunctiva, a membrane covering the outer surface of the eye. Goblet cells are responsible for secretion of mucus, and their absence results in xerophthalmia, a condition where the eyes fail to produce tears. Dead epithelial and microbial cells accumulate on the conjunctiva and form debris that can lead to infection and possibly blindness.Decreasing night blindness requires the improvement of vitamin A status in at-risk populations. Supplements and fortification of food have been shown to be effective interventions. Supplement treatment for night blindness includes massive doses of vitamin A (200,000 IU) in the form of retinyl palmitate to be taken by mouth, which is administered two to four times a year.
An estimated 1.25 million deaths due to vitamin A deficiency have been averted in 40 countries since 1998. In 2013, the prevalence of vitamin A deficiency was 29% in low-income and middle-income countries, remaining highest in sub-Saharan Africa and South Asia. A 2017 review (updated in 2022) found that vitamin A supplementation in children five years old and younger in 70 countries was associated with a 12% reduction in mortality rate. The review reported that synthetic vitamin A supplementation may not be the best long‐term solution for vitamin A deficiency, but rather food fortification, improved food distribution programs, and crop improvement, such as for fortified rice or vitamin A-rich sweet potato, may be more effective in eradicating vitamin A deficiency. References
Further reading
UNICEF, Vitamin A Supplementation: A Decade of Progress, UNICEF, New York, 2007. Flour Fortification Initiative, GAIN, Micronutrient Initiative, USAID, The World Bank, UNICEF, Investing in the Future: A United Call to Action on Vitamin and Mineral Deficiencies, 2009. UNICEF, Improving Child Nutrition: The achievable imperative for global progress, UNICEF, New York, 2013. External links
Nutrition International
Global Alliance for Vitamin A
UNICEF Data on Vitamin A Deficiency and Supplementation
Helen Keller International
A2Z
World Health Organization Database on Vitamin A Deficiency
Vitamin A Deficiency on IAPB