Agriculture Reference
In-Depth Information
ticularly that of the cornea. In developing countries, these comprise the single most
common preventable cause of blindness in children (Geissler and Powers 2006).
Numerous other functions of vitamin A have also become apparent. Vitamin A
deficiency increases susceptibility and reduces immunity to infections. The former
can be related to the need for vitamin A for epithelial integrity. Deficiency results in
mucosal atrophy with loss of goblet cells and mucus (Wolbach and Howe 1925).
Although several studies of vitamin A supplementation in children with infec-
tions have not met with the expected results, such as reduction in incidence, morbid-
ity, or mortality, it seems that this has been at least in part due to selecting children
who were not vitamin A deficient. Indeed, one study showed a slight increase in
respiratory infections in supplemented children (Grotto et al. 2003). It is clear yet
again that more is not necessarily better.
Infection itself increases vitamin A loss in urine and may precipitate deficiency
signs. Thus, in poverty in developing countries, measles is well known to precipi-
tate vitamin A deficiency and, of interest, edematous malnutrition. In malnutrition,
however, retinol's transport protein, retinol-binding protein, is low, in part due to
zinc deficiency. This traps retinol in the liver, thereby reducing its tissue bioavail-
ability. Thus, malnutrition, zinc deficiency, and infection each exacerbate vitamin A
deficiency. In malnutrition, especially when complicated by infection, both zinc and
vitamin A supplementation are indicated (WHO 1999a).
Finally, an association between vitamin A deficiency and anemia has been dem-
onstrated repeatedly. Vitamin A supplementation alone has often resulted in a rise
in hemoglobin. The cause is still unclear. There is evidence, however, that vitamin
A deficiency modulates iron metabolism in a similar way to the effect of infection,
which of course often coexists. Thus, when vitamin A deficiency is present, iron, like
zinc, is trapped in the liver and spleen, rendering it unavailable for erythropoiesis
(Semba and Bloem 2002).
Although retinol has not been shown to have any influence on the risk of malig-
nancy, carotenoids appear to have separate effects as antioxidants and, perhaps, as
protection against some cancers (Geissler and Powers 2006).
Vitamin A deficiency is common; WHO estimated that nearly 3 million children
under 4 years were clinically affected in the 1990s, and around 250 million were
subclinically affected (WHO 1995). The highest prevalence is in Southeast Asia
in association with poverty and malnutrition. Breastfeeding protects, provided the
mother is replete, but weaning and future diets, especially in rural areas, often con-
tain little in the way of plant carotenoids, which still require conversion to retinol,
and even less in the way of animal sources of preformed retinol. Vitamin A defi-
ciency rates tend to fluctuate with season because the plants that are good sources of
carotenoids are vulnerable to drought and heat (WHO 1995).
Vitamin A toxicity is not uncommon in industrialized countries, due mainly to
easy access to supplements, which are advertised widely. Acute intoxication can be
fatal. Chronic overdosing damages tissues, among them the central nervous system,
bones, liver, and skin. Liver, the richest natural source of vitamin A, is contraindi-
cated in pregnancy; however, in poverty in the developing world, vitamin A defi-
ciency is a greater risk. Synthetic retinoids are useful, both orally and topically, for
certain severe skin conditions; they are teratogenic.
Search WWH ::




Custom Search