It is a cliche that when the Normans long ago had been in Ireland for several generations they became Hibernicis ipsis Hiberni - more Irish than the Irish themselves. Such seamless integration, however is not always possible when immigrants, invited in or otherwise, are of a different ethnic group. This became evident in Britain some years ago when an anomalous resurgence of rickets had the experts puzzled for a time.
Rickets is a condition caused by a deficiency of vitamin D. The body uses this vitamin to produce calcium, which is in turn required for bones to form and grow. Its importance may be judged from the fact that children who are deficient in the vitamin may sometimes suffer from a softness or deformity of the legs and other bones. Vitamin D is available in many foods, but a prime source as far as humans are concerned is the photochemical action of ultraviolet radiation from the sun on a substance called dehydrochole sterol that is naturally present in the human body.
Now, in order to facilitate the production of the vitamin, it is necessary for sunlight to penetrate to the deeper layers of the skin. Some experts, therefore, believe that people in northern latitudes are light-skinned so that they can optimise for this purpose the small amounts of sunlight available during the long winters. Moreover, it is known that too much vitamin D produces serious toxic effects in the human body. This, the same experts argue, may explain the evolution of dark-skinned peoples in sunny climates - because a translucent skin, combined with constant exposure to the sun, might well induce an over-production of the substance.
Be all that as it may, rickets nowadays in the developed world is relatively rare, so, at first, the British resurgence was a mystery. Then the epidemiologists discovered that its incidence was restricted almost exclusively to children of the Asian community - and the reasons soon were clear.
Many recent Asian immigrants, it seems, were vegans: they avoided meat and dairy products, and so ingested very little vitamin D. Furthermore, many of the Asian women were habitually clothed from head to foot, so in a relatively dull climate the alternative source of vitamin D - that of photochemical conversion using sunlight - was unavailable for them.
Deprived in both ways, pregnant mothers were unable to supply the vitamin to unborn children in the womb, and their babies therefore started life with rickets. Once identified, however, the problem was simply solved by providing the substance artificially in tablet form.