African-American Bone Metabolism and Lactation Study
Condition: Lactation; Bone Diseases, Endocrine
MeSH terms: Bone Diseases; Bone Diseases, Endocrine; Endocrine System Diseases
Study type: Observational

Summary: 
      The primary aim of this study is to obtain measures of amino-terminal telopeptides of
      procollagen 1 (P1NP), a marker of bone formation, in lactating and non-lactating post-partum
      African-American women both at 6-8 and at 12-14 weeks post-partum, and to compare these
      values to those of normal controls. The secondary aim is to obtain at the same time points,
      measurements of Parathyroid Hormone-related Protein (PTHrP), additional markers of bone
      turnover [e.g. N-telopeptide of collagen cross-links (NTx), C-telopeptide of collagen
      cross-links (Ctx),bone specific alkaline phosphatase (BSAP) and osteocalcin (OC)], calcium
      and vitamin D metabolism in these subjects. These results will be compared with a
      non-African-American cohort of post-partum women and normal controls.

      The investigators hypothesize that African-American lactating women will have increased bone
      turnover when compared to non-lactating postpartum women and normal controls. The
      investigators further hypothesize that bone turnover is increased in lactating women
      independent of race.
    

Detailed description: 
      Pregnancy and lactation are both states of altered maternal calcium and bone metabolism which
      may have a significant impact on the development of peak bone mass. While these two states
      are characterized by different hormonal environments, both have been associated with
      significant bone loss. The maternal hormonal mechanism for providing calcium to meet the
      needs of the developing fetus appear different from those that meet the needs of lactation.
      During pregnancy, the 30 gm of calcium required by the fetus comes predominantly from an
      increase in maternal intestinal calcium absorption which is mediated by 1,25 dihydroxy
      vitamin D and other factors. Several studies have measured total and free 1,25 dihydroxy
      vitamin D through pregnancy and find the values nearly double. Serum Parathyroid Hormone
      (PTH) levels fall to about 10-30% of the mean non-pregnant value in the first trimester and
      then increase to the mid-normal range by term, while ionized calcium remains normal
      throughout pregnancy. Parathyroid Hormone-related Protein (PTHrP) levels gradually increase
      throughout pregnancy although the source (maternal, fetal, or placental) remains unclear.
      Most studies of bone metabolism in humans during pregnancy have measured changes in markers
      of bone turnover rather than bone density to avoid radiation exposure to the fetus. These
      studies have been confounded by several variables such as the effects of hemodilution in
      pregnancy, altered glomerular filtration rates (GFR), degradation and clearance of markers by
      the placenta, which may cloud the results. Some of these studies report an increase in
      urinary markers of bone resorption from early to mid pregnancy while bone formation markers
      decrease and then rise before term. Importantly, no one has assessed state-of-the-art markers
      of bone formation such as type 1 Procollagen N-terminal Propeptide (P1NP) in pregnancy or
      lactation.

      During lactation in humans, it is estimated that 600 to 1000 ml of milk are produced a day
      with daily calcium loss of 200 to 400 mg. In contrast to pregnancy, a majority of this
      calcium comes from demineralization of the maternal skeleton, and is probably predominately
      mediated by PTHrP in the setting of low estrogen. PTHrP levels are significantly higher in
      lactating women than non-lactating controls while intact PTH is reduced by approximately 50%
      during the first several months of lactation. The source of the PTHrP is likely the mammary
      gland, as PTHrP levels are elevated 10,000 fold in milk and circulating maternal PTHrP levels
      are increased further with suckling. This is also supported by a mouse model in which the
      tissue-specific ablation of the PTHrP gene in the lactating mammary gland resulted is a
      decrease in bone loss during lactation. When PTHrP enters the maternal circulation, it
      stimulates maternal bone resorption from the skeleton and renal tubular resorption of
      calcium. PTHrP indirectly suppresses PTH as ionized calcium rises to upper levels of normal.
      1,25 dihydroxyvitamin D levels fall to within the normal range during lactation, although
      they have been reported to be higher in lactating than non-lactating postpartum women.
      Intestinal absorption of calcium also returns to normal during the post-partum period. Serial
      bone density measurement (BMD)obtained during lactation show a fall of 3-10% in trabecular
      bone (spine, hip, femur) with a smaller 1-2% loss at cortical bone. Both losses are far
      greater that than that seen in early postmenopausal women, or in women receiving gonadotropin
      releasing hormone (GnRH) agonist therapy. This implies that it is not only the fall in
      estrogen that mediates bone loss during lactation. The bone loss during lactation seems to be
      transient as there is rapid recovery of bone density in postpartum women with weaning and the
      resumption of menses.

      Markers of bone resorption have been measured in urine in several prospective studies of
      lactation in humans where they have been reported to be elevated 2-3 fold. However, these
      results may be confounded by a decrease in GFR and volume contraction that may occur during
      lactation compared to pregnancy. Surprisingly, more reliable markers of bone resorption
      measured in serum (CTX and NTX) have not been measured in a controlled lactation study.
      Markers of bone formation as measured by osteocalcin (Oc) and bone specific alkaline
      phosphatase (BSAP) have generally been reported to be higher during lactation. However, these
      results are difficult to interpret as BSAP is not a very sensitive marker of bone formation.
      Recent data has emerged suggesting that Oc may measure bone resorption as well as formation.
      The current most accurate measure of bone formation is serum amino-terminal telopeptides of
      procollagen 1 (P1NP), which until recently had not been measured in a control study of
      lactating women. We are currently analyzing data from a controlled study of lactation in
      Caucasian, Hispanic, and Asian women.

      Most of the studies on bone metabolism during pregnancy and lactation have been performed in
      Caucasian females. Most studies have excluded African-Americans due to the unique differences
      in bone metabolism known to occur in this population which may create wider statistical
      variation in the study results.

      It is well established that bone metabolism in African American subjects differs from the
      Caucasians in several aspects. The mean 25-hydroxyvitamin D levels are lower in
      African-Americans primarily due to reduced vitamin D production in the skin associated with
      increased pigmentation as well as reduced intake of vitamin D. As a consequence,
      African-Americans have relative secondary hyperparathyroidism and resultant higher levels of
      PTH, and 1, 25 dihydroxyvitamin D, and lower urinary calcium excretion. One would expect that
      patients with secondary hyperparathyroidism would result in increased bone turnover and
      decreased bone mass. This is not what is observed in African Americans though. African
      Americans have higher bone density and lower fracture risk as compared to Caucasians.
      Moreover, biochemical markers of bone turnover in African-Americans are in general lower than
      in Caucasians, particularly for bone formation, as measured by osteocalcin. On the other
      hand, there is normal or increased renal absorption of calcium. Based on these, Dr. Normal
      Bell proposed that the black skeleton is resistant to the effects of PTH, while renal
      sensitivity is maintained or even enhanced. This hypothesis is further supported by the
      observation of significantly lower bone resorption (as measured by cross-linked N-telopeptide
      of procollagen I (NTx), cross-linked C-telopeptide of type I collagen (CTx), and free
      deoxypyridinoline (DPD) and normal to low renal excretion of calcium in response to PTH
      infusion in African American as compared with Caucasian women.

      This is a prospective cohort study of post-partum lactating, non-lactating, and healthy
      control African-American women who are not currently or have not recently been pregnant. The
      investigators hope to estimate the measurable differences in bone formation and resorption by
      comparing blood and urine samples from African-American lactating women to African-American
      non-lactating postpartum women and normal controls. 100 female volunteers between the ages of
      21-45 years will be recruited to achieve 75 evaluable subjects or 25 in each of the three
      groups. There are two out-patient visits, each 6-8 weeks apart. The visits will take place at
      the University of Pittsburgh Medical Center (UPMC) - Clinical & Translational Research Center
      (CTRC) located on 6 North East (6 NE) in Montefiore Hospital. The post-partum subjects will
      be recruited during their last trimester of pregnancy or at their 6-week post-partum visit.
      Healthy controls will be recruited and matched to a breast-feeding postpartum woman who has
      already completed the study. Visit 1 for all postpartum women will take place at 6-8 weeks
      after delivery and Visit 2 at 14 weeks after delivery. Healthy control subjects will have two
      study visits at similar intervals to their matched breast-feeding subject (1.5 to 2 months).
      For all subjects, an informed consent will be obtained at the first visit, before any study
      procedures are done: including a medical history and measurement of vital signs, an intake
      history about menstrual cycles, pregnancy, and if applicable, delivery and breast or bottle
      feeding events. Subjects will complete a dietary calcium intake questionnaire. Blood
      collections are obtained at each visit for measurements of the following: serum ionized and
      total calcium, phosphorus, creatinine, albumin, PTH(1-34), PTH(1-84), PTHrP (1-36),
      25-hydroxy Vitamin D by High Performance Liquid Chromatography (HPLC), including D2 and D3,
      1,25 (OH)2 vitamin D, estrogen status, Luteinizing Hormone (LH), Follicle-stimulating Hormone
      (FSH), Estradiol, and markers of bone metabolism (e.g. NTX, CTX, P1NP, Osteocalcin (OC),
      BSAP, Insulin-like growth factor-1 (IGF-1), and selected cytokines of the interleukin-6
      (IL-6). Urine samples will be obtained for calcium, creatinine and phosphorus. A urine
      pregnancy test will be done at each visit for normal controls. Post-partum women will have a
      urine pregnancy checked at Visit 2 only, to avoid false positives that are likely at Visit 1
      due to the recently completed pregnancy. Thyroid Stimulating Hormone (TSH) will be checked at
      Visit 1 to exclude those women who have significant alterations in thyroid functioning that
      could interfere with bone metabolism.
    

Observational model: Cohort
Time perspective: Prospective
Sampling method: Non-Probability Sample
Study population: 
        New African-American mothers who are either almost exclusively breast-feeding or
        bottle-feeding; normal controls to match the lactating mothers
      
Biospecimen description: 
      archival blood serum and plasma
    
Keywords: Lactation; Endocrine System Diseases; Musculoskeletal System Disease; Hormone; Physiological Properties