Differences in Bone Density Changes Among African American and Caucasian Women During Lactation
by Anne Eglash MD, IBCLC, FABM
Does the change in bone density related to lactation differ between African American and Caucasian women?
It is well known that bone density declines during lactation. The breast tissue controls calcium metabolism during lactation through the activity of the hormone parathyroid-related protein (PTHrP). PTHrP preferentially moves calcium from the bones to the breastmilk and does not aid in increasing calcium absorption from the maternal gut. Therefore, increasing dietary calcium during lactation does not help to prevent bone density loss during lactation.
The researchers for this week’s study investigated whether similar bone density changes occur among lactating African American (AA) women, as most research thus far has been done among Caucasian (C) and Asian women. There is evidence for differences in bone metabolism among AA and C women. For example, AA women tend to have lower circulating serum vitamin D levels (25 hydroxy vit D) and higher circulating parathyroid hormone (PTH) concentrations.
This was a prospective study, with cohorts of 44 AA and 32 C mothers recruited from the University of Pittsburgh PA who were exclusively breastfeeding. There were several exclusionary criteria that have an impact on bone density such as a history of kidney or pulmonary disease, BMI, history of smoking and use of chronic medications including depo-medroxyprogesterone.
Bone density measurements of the lumbar spine, total hip, femoral neck, and distal radius were measured at 2 weeks, 12 weeks, 24 weeks (or at weaning if earlier), and 6 months after weaning. Several blood and urine tests, along with breastmilk samples, were done at each visit.
They only included women who exclusively breastfed for at least 3 months.
Interestingly, it was harder to recruit AA women for this study, partly because 27% of new AA mothers in the recruitment population received depo-medroxyprogesterone early postpartum compared to 5% of non-AA new mothers.
After controlling for BMI, dietary calcium, activity level, and parity, they found that bone density declined in both AA and C subjects during the first 6 months of lactation. There was no significant difference in the % bone density loss between the 2 groups.
At 6 months post weaning, the bone density of the spine, total hip and femoral neck returned to baseline among AA subjects, but the bone density at the femoral neck was still below baseline among the C subjects.
What else? See the question!
- At baseline, AA subjects had lower vitamin D levels (serum 25-OH) than the C subjects.
- AA subjects had higher baseline bone densities than C subjects, even after controlling for BMI.
- Among both racial groups, the higher the BMI, the lower the serum vitamin D level.
- For both groups, the bone density of the distal radius did not have significant bone loss during lactation as compared to the hip and spine.
See the Answer
During lactation, changes in maternal calcium metabolism are necessary to provide adequate calcium for newborn skeletal development. The calcium in milk is derived from the maternal skeleton through a process thought to be mediated by the actions of parathyroid hormone-related protein (PTHrP) in combination with decreased circulating estrogen concentrations. After weaning, bone lost during lactation is rapidly regained. Most studies of bone metabolism in lactating women have been performed in Caucasian subjects. There are well-documented differences between African American (AA) and Caucasian (C) bone metabolism, including higher bone mineral density (BMD), lower risk of fracture, lower 25-hydroxy-vitamin D (25(OH) D), and higher PTH in AA compared to C. In this prospective paired cohort study, BMD and markers of bone turnover were compared in self-identified AA and C mothers during lactation and after weaning. BMD decreased in both AA and C women during lactation, with similar decreases at the lumbar spine (LS) and greater bone loss in the C group at the femoral neck (FN) and total hip (TH), demonstrating that AA are not resistant to PTHrP during lactation. BMD recovery compared to the 2 week postpartum baseline was observed 6 months after weaning, though the C group did not have complete recovery at the FN. Increases in markers of bone formation and resorption during lactation were similar in AA and C. C-terminal telopeptide (CTX) decreased to 30% below post-pregnancy baseline in both groups 6 months after weaning, while procollagen type 1 N-terminal (P1NP) returned to baseline in the AA group and fell to below baseline in the C group. Further investigation is required to determine impacts on long term bone health for women who do not fully recover BMD before a subsequent pregnancy.
This was a small yet unique study because there is little data on the differences in bone density metabolism during lactation between races. This study validated previous research that baseline bone density is higher in AA women. Estrogen levels have been shown to be higher in AA women, and the researchers in this study demonstrated that the rebound in estrogen levels after weaning was faster among AA subjects than C subjects. This may have played a role in the more complete recovery of bone density 6 months after weaning among AA women as compared to C women.
Bone density loss during lactation has been shown in previous studies to be primarily in the spine and hip with little change in the radius (wrist), which this study confirmed.
As an aside, the authors remind us that depo-medroxyprogesterone for birth control decreases bone density, and they found in their population that AA women were more likely to have received depo than the C women. We don’t know whether lactating women with long term use of depo have appropriate rebound of their bone density after weaning. This is an important consideration for long term contraceptive planning as other birth control methods are not associated with the same degree of bone density loss.