Longitudinal Lactation Bone Density Study

Pregnancy and lactation are both states of altered maternal calcium metabolism. Maternal
calcium mobilization is essential in the provision of an adequate calcium supply to the
developing fetus and infant. During pregnancy approximately 30 grams of calcium is required
for fetal skeletal development. The extra calcium is derived mostly from increased maternal
intestinal absorption, mediated by 1,25 dihydroxyvitamin D (1). During lactation, it is
estimated that 600-1000 ml of milk are produced daily with a net maternal calcium loss of
200-400 mg per day (2,3). This calcium is derived from the maternal skeleton, resulting in
bone loss of as much as 10% of trabecular bone with serial bone density measurements (3).
Demineralization is thought to be mediated predominantly by the actions of parathyroid
hormone-related protein (PTHrP) in combination with a suppressed estrogen state.
Interestingly, this state rapidly reverses itself with weaning (4,5). Several studies have
shown that initial PTHrP measurements are significantly higher in lactating women(3). PTHrP
in lactation is produced in high amounts by the mammary gland. Once in the circulation,
PTHrP increases bone resorption from the maternal skeleton and increases calcium resorption
at the level of the kidney (5,6,27). PTHrP levels have been found to be elevated as much as
10,000 fold in milk as compared with maternal serum, and further increase with suckling
(1,6,28). In mouse models, tissue specific-ablation of the PTHrP gene in lactating mammary
gland results in decreased bone loss(6).

Given the rapid bone loss reported in lactation, an increase in markers of bone resorption
and a decrease in markers of bone formation might be expected. However, a recently published
study by our group showed that both markers of bone formation and bone resorption were
significantly elevated in lactating Caucasian women (7). This seems unlikely as tight
coupling of bone formation and resorption would not result in net calcium mobilization and
rapid bone loss during lactation. A more likely explanation is that while bone resorption
markers accurately reflect robust osteoclastic bone resorption, the increase in markers of
bone formation reflects an increase in immature, partially differentiated pre-osteoblasts
that are arrested in development and therefore unable to effectively form new bone. Thus,
the rate of resorption exceeds that of formation during lactation and results in a temporary
net loss. To support this idea, rat studies have shown that continuous exposure to PTH and
PTHrP recruits and initiates osteoblast differentiation, but stops the program prematurely
at the preosteoblast transition (8,9,10). This would also suggest a mechanism for the rapid
recovery seen after weaning: after withdrawing the PTHrP stimulus, osteoclast-mediated bone
resorption would abruptly cease, and previously recruited osteoblast precursors would
rapidly complete their differentiation program and restore bone lost during lactation.

African-American bone metabolism differs from Caucasian bone metabolism in several ways.
African-Americans display higher bone density and are at lower risk of developing
osteoporosis and osteoporotic fracture compared to Caucasians (15-17,29-31). There are may
factors which may explain these racial differences in bone metabolism, including altered
calcium economy, vitamin D differences, peak attained bone mass, muscle mass and obesity,
remodeling rates, bone micro-architecture, hip axis geometry , and other unknown hereditary
differences. In particular, it is well established that 25-Hydroxyvitamin D (25-OH D) levels
are much lower in African-Americans, due to darker pigmentation resulting in reduced dermal
production of 25-OH D as well as reduced intake of vitamin D (11). This hypovitaminosis
results in a relative secondary hyperparathyroidism, corroborated by higher levels of PTH
and 1,25 dihydroxyvitamin D as well as decreased urinary calcium excretion (12-14).
Paradoxically, these higher PTH levels do not correlate with an increase in bone loss. In
fact, bone turnover is actually reduced compared to Caucasians, suggesting that
African-American bones are more resistant to the effects of PTH, whereas renal sensitivity
is maintained or increased (14,18).

The vast majority of studies examining bone metabolism in lactating women have been in
Caucasian women. Studies on bone turnover and calcitropic hormones in lactating
African-Americans are scarce. Over ten years ago, a study examined markers of bone
metabolism in a population of lactating Gambian women (in Gambia) and compared them with a
similar cohort of British lactating women (in the UK). The study demonstrated significant
increases in older markers of bone formation and resorption during lactation, but also
showed higher levels of PTH, 1,25 Vitamin D, serum phosphate, osteocalcin, and alkaline
phosphatase in the Gambian lactating population compared with the British lactating
population (19). However, Gambian women differ significantly from black women in the
developed world in terms of nutrition, both overall calories and calcium intake. Also in
contrast to African-Americans, black Gambian women displayed lower BMD than Caucasians.
Interestingly, in these studies, Gambian women demonstrated significant losses in whole body
and hip BMD during lactation and showed little evidence of bone mass regain after weaning.
In fact, Gambian women had continued BMD loss at the hip, in contrast to Caucasian controls.

A recent study by our group (unpublished) explored bone turnover markers and calcium
metabolism in African-American lactation. Markers of bone resorption were lower in
African-American than Caucasians in non-lactating young women, but increased 2-3 fold in
both groups during lactation. Baseline bone formation was comparable in African-Americans
and Caucasians, also increasing 2-3 fold in both groups. Fractional excretion of calcium was
lower in African-Americans at baseline and remained constant in both groups during
lactation. It remains unclear whether bone loss occurs at a similar rate during lactation in
the African-American population.

Up to this point, there have been no studies in African-Americans that have utilized bone
densitometry to quantitate bone loss and recovery during lactation. This study aims to
demonstrate changes in bone density in African-American women during the high bone turnover
state of lactation. It will also permit measurements of calcium metabolism, markers of bone
formation and resorption, and calcitropic hormones to be compared to bone density changes.
We will also look for racial differences in lactating bone metabolism by directly comparing
a cohort of African-American lactating women with a similar Caucasian cohort. No prior
studies have been done comparing bone mineral density changes in lactating American black
and Caucasian women.

Inclusion Criteria:

- 21-45 years old

- Post-partum after a singleton pregnancy

- Exclusively breast-feeding (not more than one supplemental bottle of formula per day)

- African-American or Caucasian by self-identification

Exclusion Criteria:

- Subjects with cardiac, hypertensive, vascular, renal (serum creatinine of >1.5),
pulmonary, endocrine, musculoskeletal, hepatic, hematologic, malignant or
rheumatologic disease

- Fractures or bone surgery within the past 12 months

- Smokers and subjects with history of significant alcohol or drug use

- Pregnant women

- Women who achieved pregnancies with IVF or other hormonal manipulation

- Women who had significant complications with the most recent pregnancy or who are
unable to exclusively breastfeed beginning at birth

- Subjects on chronic medications other than

- stable doses of thyroid hormone

- oral contraceptives

- vitamin supplements

- Women on Depo-Provera will be excluded

- Receiving an investigational drug within 90 days

- Weight greater than 130 kg

- Z-score -3.0 or less (hip or spine) on initial DXA