—As Drs Rico and Revilla note, the sample of women who were breast-feeding lost in excess of 5% at the lumbar spine and femoral neck. They propose that seasonal variation and weight loss following parturition are viable explanations for the loss rather than breast-feeding.Seasonal variation in sunlight exposure has been observed with seasonal variation in serum vitamin D levels for some time.1,2 This seasonal variation might be associated with bone mass loss under conditions of pregnancy and lactation in absence of a source of vitamin D from sunlight or diet.Several factors argue against seasonal variation in vitamin D as a source of explanation of the observed bone loss in the group with extended lactation. First, the lactating women were compared with controls who had also experienced a pregnancy. Those controls did not experience the transient bone loss of the lactating women. Second, the lactating women
Sowers M. Factors Affecting Bone Mass-Reply. JAMA. 1994;272(13):1003–1004. doi:10.1001/jama.1994.03520130037023
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