Unexpected long-term effects of calcium supplementation in pregnancy on maternal bone outcomes in women with a low calcium intake: a follow-up study

Background: Calcium supplementation of pregnant Gambian women with a low calcium intake results in lower maternal bone mineral content in the subsequent lactation. Objective: The objective was to investigate whether the lower bone mineral content persists long term. Design: All women in the calcium supplementation trial (International Trial Registry ISRCTN96502494) who had been scanned with dual-energy X-ray absorptiometry at 52 wk of lactation (L52; n = 79) were invited for follow-up when neither pregnant nor lactating for >= 3 mo (NPNL) or at 52 wk postpartum in a future lactation (F52). Bone scans and anthropometric and dietary assessments were conducted. Results: Sixty-eight women participated (35 at both NPNL and F52 and 33 at only one time point): n = 59 NPNL (n = 31 calcium, a = 28 placebo) and n = 44 F52 = 24 calcium, a = 20 placebo). The mean (+/- SD) time from L52 was 4.9 +/- 1.9 y for NPNL and 5.0 +/- 1.3 y for F52. Size-adjusted bone mineral content (SA-BMC) was greater at NPNL than at L52 in the placebo group (P <= 0.001) but not in the calcium group (P for time-by-group interaction: lumbar spine, 0.002; total hip, 0.03; whole body, 0.03). No significant changes in SA-BMC from L52 to F52 were observed in either group. Consequently, the lower SA-BMC in the calcium group at L52 persisted at NPNL and F52 (P 0.001): NPNL (lumbar spine, -7.5 +/- 0.7%; total hip, -10.5 +/- 1.0%; whole body, -3.6 +/- 0.5%) and F52 (lumbar spine, -6.2 +/- 0.9%; total hip, -10.3 +/- 1.4%; whole body, -3.2 +/- 0.6%). Conclusion: In rural Gambian women with a low-calcium diet, a calcium supplement of 1500 mg/d during pregnancy resulted in lower maternal bone mineral content in the subsequent lactation that persisted long term. This trial was registered at www/controlled-trials.com/mrct/ as ISRCTN96502494.