Vitamin A

Vitamin A

   Deficiency:

        Globally, the most important consequence of vitamin A deficiency is blindness. Each year, approximately 500 000 new cases of blindness occur in young childre
n, mostly in Asia. Adults are not usually at risk because liver stores can supply vitamin A when foods containing vitamin A are unavailable.

      Early deficiency causes impaired adaptation to the dark (night blindness). Keratinisation of the cornea (xerophthalmia) causes characteristic Bitot's spots, which progresses to keratomalacia, with corneal ulceration, scarring and irreversible blindness.

                  

                   In countries where vitamin A deficiency is endemic, pregnant women should be advised to eat dark green leafy vegetables and yellow fruits (to build up stores of retinol in the fetal liver), and infants should be fed the same.

      

      WHO is giving high priority to prevention in communities where xerophthalmia occurs, with single prophylactic oral doses of 60 mg retinyl palmitate (providing 200 000 U retinol) given to pre-school children. This also reduces mortality from gastroenteritis and respiratory infections.

Toxicity

       Repeated moderate or high doses of retinol can cause liver damage, hyperostosis and teratogenicity. Women in countries where deficiency is not endemic are therefore advised not to take vitamin A supplements in pregnancy. Retinol intake may also be restricted in those at risk of osteoporosis. Acute overdose leads to nausea and headache, increased intracranial pressure and skin desquamation. Excessive intake of carotene can cause pigmentation of the skin (hypercarotenosis); this gradually fades when intake is reduced.