Addition of monofluorophosphate to estrogen therapy in postmenopausal osteoporosis: a randomized controlled trial

Treatment of osteoporosis with high-dose fluoride alone does not reduce fracture risk. We hypothesized that the antifracture efficacy of fluoride could be optimized by its use in low doses combined with an antiresorptive agent.

Subjects included 80 women with postmenopausal osteoporosis who had been taking estrogen for at least 1 yr.

Subjects were randomized to receive monofluorophosphate (MFP) (fluoride content of 20 mg/d) or placebo over 4 yr in a double-blind trial.

There were progressive increases in lumbar spine bone density over the duration of the study (MFP, 22%; placebo, 6%; P < 0.0001). In the trabecular bone of L3, these increases were even greater (MFP, 49%; placebo, 2.5%; P < 0.0001). In the proximal femur, there were smaller but significant treatment effects (P = 0.015). Total body scans and their subregions also showed significantly greater increases in the MFP group. Bone formation markers increased significantly in the MFP group at yr 1. Hyperosteoidosis was present in biopsies from five of seven MFP subjects, with osteomalacia in two of seven. The hazards ratio for vertebral fractures was 0.20 (95% confidence interval, 0.05-1.30), and the incidence rate ratio was 0.12 (95% confidence interval, 0.06-0.23; P < 0.01). The hazards ratio for nonvertebral fractures was 3.3 (95% confidence interval, 0.8-12.0).

We conclude that fluoride at 20 mg/d produces substantial increases in bone mineral density but still interferes with bone mineralization. This indicates that most previous studies with this ion have used toxic doses and that much lower doses should be assessed to find a safe dose window for the use of this powerful anabolic agent.