1. Acute metabolic effects of sodium fluoride therapy were analysed among 41 osteoporotic patients already receiving large calcium supplements, 33 of whom underwent simultaneous metabolic balance studies.

2. Mean serum calcium fell transiently within 24–48 h by 0.03 ±0.07 (sd) mmol/l (P < 0.01) and phosphorus by 0.06±0.08 (sd) mmol/l (P < 0.001). In a subgroup, ionized calcium fell and biologically active parathyroid hormone (bio-PTH) rose more than fivefold (P < 0.01). Urine calcium rose after an insignificant fall.

3. Pretreatment calcium and phosphorus balances were significantly positive and did not change overall during the first 8 days of treatment. However, on analysing balances in two groups relative to serum changes, in patients whose serum levels changed least sodium fluoride increased faecal calcium (P < 0.025) and phosphorus (P < 0.01) and reduced calcium balance (P < 0.01), giving a mean balance difference between the two groups of 2.1 mmol daily (P < 0.001).

4. Very small changes in serum levels therefore indicate well-marked metabolic responses: sodium fluoride acutely stimulates bio-PTH activity and must also enhance mineral uptake from circulation into tissue(s). By separate and opposing action(s) it inhibits intestinal calcium and phosphorus absorption, predominantly in those whose serum levels remain stable. All these effects may be relevant to long-term therapeutic results.

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