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 mass returns to baseline levels when breast-feeding ceases.


METHODS
Prospective cohort study design of 12 months' duration.


METHODS
General community setting with recruitment occurring at birthing education classes.


METHODS
Volunteer sample 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, grouped according to lactation duration: 0 through 1, 2 through 5, and 6 or more months.


METHODS
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.


RESULTS
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 breast-fed 0 through 1 month lost no BMD at either bone site. Bone loss in women who breast-fed 6 months or longer was not explained by differences in age, diet, body size, or physical activity. Among women who breast-fed 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.


CONCLUSIONS
Extended lactation (> or = 70% of energy intake is provided for > or = 6 months) is associated with bone loss; however, there is evidence of return to baseline BMD measurement at 12 months after parturition.
Department of Epidemiology, University of Michigan, Ann Arbor 48109-2029.
M
M
Sowers
G
G
Corton
B
B
Shapiro
M L
ML
Jannausch
M
M
Crutchfield
M L
ML
Smith
J F
JF
Randolph
B
B
Hollis
D015502:Absorptiometry, Photon; D000328:Adult; D000704:Analysis of Variance; D015519:Bone Density / Q000502:physiology*; D005260:Female; D005269:Femur / Q000000981:diagnostic imaging / Q000502:physiology; D006801:Humans; D007774:Lactation / Q000097:blood / Q000502:physiology*; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging / Q000502:physiology; D009752:Nutritional Status; D011446:Prospective Studies; D013997:Time Factors; D014886:Weaning