CHANGES IN BONE-DENSITY WITH LACTATION

Objective.-To test the a priori hypotheses that significant bone loss occurs in lactation of greater than 5 months' duration and that bone m ass returns to baseline levels when breast-feeding ceases. Design.-Pro spective cohort study design of 12 months' duration. Setting.-General community setting with recruitment occurring at birthing education cla sses. Participants.-Volunteer sample of 98 healthy women of white (n=9 5) and Asian (n=3) origin, aged 20 to 40 years, and 0 to 1 parity prio r to parturition, grouped according to lactation duration: 0 through 1 , 2 through 5, and 6 or more months. Main Outcome Measures.-Bone miner al density (BMD) of the proximal femur was measured by dual-energy x-r ay densitometry at 2 weeks (baseline), 2 months, 4 months, 6 months, a nd 12 months following parturition, and BMD of the lumbar spine was me asured at baseline, 6 months, and 12 months after parturition. Results .-Women with lactation duration of 6 months or longer had mean BMD los ses of 5.1% and 4.8% at the lumbar spine and femoral neck, respectivel y, comparing baseline values with those at 6 months post partum. Women who breast-fed 0 through 1 month lost no BMD at either bone site. Bon e loss in women who breast-fed 6 months or longer was not explained by differences in age, diet, body size, or physical activity. Among wome n who breast-fed 6 months or longer, there was evidence of return to b aseline levels of the lumbar spine at 12 months after parturition. The BMD of the lumbar spine of those women who continued to breast-feed m ore than 9 months had increased but was still significantly lower than baseline. Conclusion.-Extended lactation (greater-than-or-equal-to 70 % of energy intake is provided for greater-than-or-equal-to 6 months) is associated with bone loss; however, there is evidence of return to baseline BMD measurement at 12 months after parturition.