Factors affecting bone mass

Abstract

<h3>In Reply.</h3> —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.^1,2This 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