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.