Evaluation of markers of bone turnover during lactation in African-Americans: a comparison with Caucasian lactation

Abstract

Context: The African-American skeleton is resistant to PTH; whether it is also resistant to PTHrP and the hormonal milieu of lactation is unknown.

Objectives: The objective of the study was to assess bone turnover markers in African-Americans during lactation vs Caucasians.

Design and participants: A prospective cohort study with repeated measures of markers of bone turnover in 60 African-American women (3 groups of 20: lactating, bottle feeding, and healthy controls), compared with historic Caucasian women.

Setting: The study was conducted at a university medical center.

Outcome measures: Biochemical markers of bone turnover and calcium metabolism were measured.

Results: 25-Hydroxyvitamin D (25-OHD) and PTH were similar among all 3 African-American groups, but 25-OHD was 30%-50% lower and PTH 2-fold higher compared with Caucasians (P < .001, P < .002), with similar 1,25 dihydroxyvitamin D [1,25(OH)(2)D] values. Formation markers [amino-terminal telopeptide of procollagen-1 (P1NP) and bone-specific alkaline phosphatase (BSAP)] increased significantly (2- to 3-fold) in lactating and bottle-feeding African-American women (P1NP, P < .001; BSAP, P < .001), as did resorption [carboxy-terminal telopeptide of collagen-1 (CTX) and serum amino-terminal telopeptide of collagen 1 (NTX), both P < .001]. P1NP and BSAP were comparable in African-American and Caucasian controls, but CTX and NTX were lower in African-American vs Caucasian controls. African-American lactating mothers displayed quantitatively similar increases in markers of bone formation but slightly lower increases in markers of resorption vs Caucasians (P = .036).

Conclusions: Despite reported resistance to PTH, lactating African-American women have a significant increase in markers of bone resorption and formation in response the hormonal milieu of lactation. This response is similar to that reported in Caucasian women despite racial differences in 25-OHD and PTH. Whether this is associated with similar bone loss in African-Americans as in Caucasians during lactation is unknown and requires further study.

Trial registration: ClinicalTrials.gov NCT00785824.

Figure 1.

Serum total calcium (A), serum phosphorus (B), FECa (C), and the TmP/GFR (D) in lactating, bottle-feeding, and control African-American and Caucasian women. The first bar (dark gray) of each study group represents the mean and SE of the 2 time points studied for African-American women. The second bar (light gray) of each group represents the same data for Caucasian women. A, There were no differences in total serum calcium among the 3 African-American groups. There was a slight statistical difference in serum total calcium between African-American and Caucasian bottle-feeding women (P = .049). B, Serum phosphorus was higher in bottle-feeding and lactating African-American women as compared with African-American controls (P = .038 and P = .001 respectively). In addition, serum phosphorus was higher in bottle-feeding and lactating African-American women as compared with their Caucasian counterparts (P = .019 and P = .001 respectively). C, FECa was similar among the 3 groups of African-American women but was lower in both control and lactating African-American women compared with Caucasian women (P = .002 and P = .005, respectively). D, There was no difference between the African-American and Caucasian control groups for TmP/GFR. TmP/GFR was slightly higher in the African-American lactating and bottle-feeding mothers both compared with African-American controls (P = .001 and P = .038, respectively) and compared the same groups of Caucasian women (P < .001 and P = .019, respectively).

Figure 2.

PTH, 25-OHD, and 1,25(OH)₂D in lactating, bottle-feeding, and control African-American and Caucasian women. The 2 bars for each group represent the mean and SE of the 2 time points studied in African-Americans and Caucasians as previously described for Fig. 1. There were no differences in PTH, plasma 25-OHD, or 1,25(OH)₂D between study groups in African-American women. A, PTH was strikingly higher in the 3 African-American as compared with Caucasian groups (P = .002). B, Plasma 25-OHD was markedly lower in the three African-American as compared with Caucasian groups (P < .001, control; P = .009, bottle; P < .001, lactating). C, African-Americans and Caucasians displayed similar 1,25(OH)₂D in all 3 study groups, with no racial differences.

Figure 3.

Markers of bone formation in lactating, bottle-feeding, and control African-American and Caucasian women. The 2 bars for each group represent the mean and SE of the 2 time points studied in African-Americans and Caucasians as previously described in Fig. 1. A and B, P1NP and BSAP, bone formation markers, were significantly higher in the African-American lactating and bottle-feeding mothers as compared with the African-American controls (P = .001 both for P1NP, and P < .001 and P = .006 for BSAP). However, there were no racial differences among the groups.

Figure 4.

Markers of bone resorption in lactating, bottle-feeding, and control African-American and Caucasian women. The 2 bars for each group represent the mean and SE of the 2 time points studied in African-American and Caucasians as previously described in Fig. 1. A, Baseline CTX was lower in African-American controls as compared with Caucasian controls (P = .003). In addition, CTX was 2- to 3-fold higher in African-American lactating and bottle-feeding mothers as compared with the African-American controls (P < .001 and P = .002). Finally, CTX was higher in lactating Caucasian as compared with African-American women (P = .036). B, Baseline NTX was also lower in African-American controls as compared with Caucasian controls (P < .01). In addition, NTX was higher in lactating and bottle-feeding African-American women as compared with African-American controls (P < .001 for both).