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Does it make a difference how and when you take your calcium? The acute effects of calcium on calcium and bone metabolism^1,2

¹ From the Calcium Research Unit, the Department of Applied Chemistry and Microbiology, University of Helsinki, and the Helsinki University Central Hospital.

Background: Calcium supplements are widely used to prevent osteoporosis. However, little is known about the metabolic effects of different dosages and of the timing of the dosages.

Objective: The aim was to study the effects of the timing of the dose (study 1), the effects of the size of the dose (study 2), and the effects of small repetitive doses (study 3) of calcium on calcium and bone metabolism in women.

Design: The investigation was conducted in 3 parts, each with 10 participants. In study 1, calcium loads (0 and 25 mg/kg body wt) were taken at 0900 and 2100. In study 2, calcium loads of 0, 250, and 1000 mg were taken at 0900. In study 3, calcium loads of 0 and 200 mg were taken 4 times/d. Markers of calcium and bone metabolism were followed.

Results: There was no significant difference in the response of serum parathyroid hormone (PTH) to the calcium load taken at 0900 and that at 2100. There was a significant dose-response effect of the calcium load on serum ionized calcium (P = 0.00005) and serum PTH (P = 0.0003). Small calcium doses (200 mg) taken 4 times/d kept the PTH secrection at a lower level than during the control day (P = 0.016). None of the doses caused significant changes in the markers of bone formation and resorption measured.

Conclusions: The calcium loads had no significant effect on the markers of bone formation and resorption measured, although even small calcium doses decreased serum PTH and increased serum ionized calcium concentrations rapidly. The effect was similar whether calcium was taken in the morning or in the evening.

Key Words: Parathyroid hormone • calcium intake • bone metabolism • timing of supplementation • women

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