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How does vitamin A deficiency arise?

I am living in a "developed" country, so excuse me if I miss some facts that are real, and threatening, in other parts of our world.

I heard about https://en.wikipedia.org/wiki/Vitamin_A_deficiency , but looking at what foodstuff contains vitamin A or precursors, I'm at loss how anyone can suffer from vitamin A deficiency.

Obviously, I'm lacking some information here.

One thing that looks particularly misleading is when you are presented with vitamin A contents of foods, neglecting precursors like carotenoids that can be easily converted to vitamin A as needed.

As a (sometimes misleading) rule of thumb, I was thinking that most yellow or orange looking food was prone to contain vitamin A or some of its precursors.

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The WHO report "Global prevalence of vitamin A deficiency in populations at risk 1995–2005" (page 12 of 68) puts it like this:

Where animal source or fortified foods are minimally consumed, dietary adequacy must rely heavily on foods providing beta-carotene. However, while nutritious in many ways, a diet with modest amounts of vegetables and fruits as the sole source of vitamin A may not deliver adequate amounts, based on an intestinal carotenoid-to-retinol conversion ratio of 12:1 (2) This ratio reflects a conversion efficiency that is about half that previously thought, leading to greater appreciation for why VAD may coexist in cultures that heavily depend on vegetables and fruits as their sole or main dietary source of vitamin A.

Usually, VAD develops in an environment of ecological, social and economical deprivation, in which a chronically deficient dietary intake of vitamin A coexists with severe infections, such as measles, and frequent infections causing diarrhoea and respiratory diseases that can lower intake through depressed appetite and absorption, and deplete body stores of vitamin A through excessive metabolism and excretion (3, 4). The consequent “synergism” can result in the body’s liver stores becoming depleted and peripheral tissue and serum retinol concentrations decreasing to deficient levels, raising the risks of xerophthalmia, further infection, other VADD and mortality.

Just as a side note: green leaves contain heaps of beta-carotenes as well!

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