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Layinka Swinburne (JRSM August 20061) provides fascinating documentation that rickets was well-established in London by the early 17th century. However, the widespread prevalence of rickets is commonly ascribed to the industrial revolution of the 19th century, with increased pollution and closely crowded urban dwellings reducing the exposure of children to sunshine.2 The origin of rickets before the industrial revolution may relate to the widespread practice of wet-nursing alluded to in the article. Infants from wealthier parishes were sent out of London to wet-nurses in country parishes.3 Nursed children of wealthy parents apparently had the greatest risk of rickets. The wetnurse engaged in continuous breast feeding, even nursing several children simultaneously. Nursed children were included in the country family of the wet-nurse and not necessarily deprived of sunshine.
As the duration of lactation increases, the concentration of calcium in breast milk declines.4 Consequently, the calcium intake of very young infants who were wet-nursed was probably considerably less than required for adequate bone mineralization. Although calcium in breast milk is more bioavailable than that of cow's milk, the concentration of calcium in breast milk is only about one quarter the concentration of calcium in cow's milk. As Swinburne points out, non-dairy weaning paps would further aggravate marginal calcium intake. The appearance of nutritional rickets in the 17th century probably resulted primarily from restricted calcium intake rather than from vitamin D deficiency, coinciding with the increased adoption of wetnursing.
Nutritional rickets exists along a spectrum ranging from isolated vitamin D deficiency to isolated calcium deficiency.5 Nutritional rickets may most often result from relative calcium and vitamin D deficiencies interacting with genetic and environmental factors. Although calcium deficiency as a cause of rickets has been relatively recently described, it may have been the primary cause of nutritional rickets prior to the industrial revolution.
Competing interests None declared.