Acute and 3-month effects of microcrystalline hydroxyapatite, calcium citrate and calcium carbonate on serum calcium and markers of bone turnover: a randomised controlled trial in postmenopausal women.

Randomized controlled trial in postmenopausal women at least 5 years after menopause, without cardiovascular disease or major systemic illness, recruited in New Zealand from volunteers for other osteoporosis studies. Active-arm sample sizes were calcium citrate N=18, calcium carbonate N=20, MCHA N=20, and MCHB N=19, with a placebo group also included.

Participants received oral calcium supplements as encapsulated powders, providing 1 g elemental calcium per day in two divided doses for 3 months. Active regimens were calcium citrate, calcium carbonate, and two microcrystalline hydroxyapatite preparations (MCHA and MCHB), compared with placebo.

Overall, calcium supplementation lowered bone turnover, and microcrystalline hydroxyapatite appeared to be as effective as conventional calcium salts over 3 months. After the first dose, ionised and total calcium increased in the citrate and carbonate groups and in the microcrystalline hydroxyapatite groups versus control, but the ionised calcium rise and AUC were greater with citrate-carbonate than with MCH. After 3 months, CTX and PINP were significantly lower than baseline in the citrate-carbonate and MCH groups, with no difference in the magnitude of reduction between them. MCH produced a smaller increase in ionised calcium but a larger increase in phosphate and the calcium-phosphate product, so it may not be a safer alternative to standard calcium supplements despite similar antiresorptive effects.

Small per-arm sample sizes, a short 3-month follow-up, and reliance on biochemical surrogate outcomes rather than fractures limit interpretation. Generalizability is also restricted because the trial enrolled relatively healthy older postmenopausal women from New Zealand, and the two microcrystalline hydroxyapatite preparations complicate direct product-level comparison.