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According to the passage, the high rate of Vitamin D3 deficiency in underdeveloped countries with plenty of sunshine ----.
A deficiency in Vitamin D is known to cause various
diseases due to insufficient calcium or phosphate in the
bones. Vitamin D is actually an umbrella term that covers a
group of steroid molecules. Of these, only Vitamin D3
requires sunlight to synthesize. It is formed in the skin of
all mammals when light energy is absorbed by a precursor
molecule called 7-dehydrocholesterol. A recent study
found that at the height of summer, two minutes' exposure
of the face and arms to the sun, three to four times a week,
could supply enough Vitamin D3. This rose to 15 minutes
in the winter. Ironically, Vitamin D deficiency, which may
also lead to skin cancer, is actually very common in some
of the sunniest but most underdeveloped countries in the
world. This is not because of malnutrition or a lack of
dietary supplements, as most people mistakenly think. In
fact, it is because very dark skin colour slows the rate of
Vitamin D3 production by a factor of six, and people in
these countries are usually heavily veiled when outside, as
well. In addition, wearing sunscreen with a sun protection
factor greater than eight will also block Vitamin D3
production.