This study set out to test the a prior hypotheses that significant bone loss occurs in lactation of greater than 5 months' duration and that bone mass returns to baseline levels when breastfeeding ceases.
A total of 98 healthy women of white (n = 95) and Asian (n = 3) origin, aged 20 to 40 years, and 0 to 1 parity prior to parturition were grouped according to lactation duration: 0 through 1, 2 through 5, and 6 or more months. Bone mineral density (BMD) of the proximal femur was measured by dual-energy x-ray densitometry at 2 weeks (baseline), 2 months, 4 months, 6 months, and 12 months following parturition, and BMD of the lumbar spine was measured at baseline, 6 months, and 12 months after parturition.
Women with lactation duration of 6 months or longer had mean BMD losses of 5.1% and 4.8% at the lumbar spine and femoral neck, respectively, comparing baseline values with those at 6 months post partum. Women who breastfed 0 through 1 month lost no BMD at either bone site. Bone loss in women who breastfed 6 months or longer was not explained by differences in age, diet, body size, or physical activity. Among women who breastfed 6 months or longer, there was evidence of return to baseline levels of the lumbar spine at 12 months after parturition. The BMD of the lumbar spine of those women who continued to breast-feed more than 9 months had increased but was still significantly lower than baseline.
The authors concluded that extended lactation is associated with bone loss; however, there is evidence of return to baseline BMD measurement at 12 months after parturition.
Sowers M et al (1993) Changes in bone density with lactation. JAMA 269: 3130-5 [Abstract]