Combined effects of diets with reduced calcium and phosphate and increasedfluoride intake on vertebral bone strength and histology in rats

Ingested fluoride is incorporated into bone apatite and can affect the stru ctural integrity of bone. Fluoride absorption in the gut and incorporation into bone is affected by the presence of other ions, including calcium. We hypothesized that a low calcium phosphate diet combined with hi-h fluoride intake would have independent detrimental effects on vertebral bone strengt h. We measured the vertebral biomechanical properties and bone histology fo r rats given controlled diets with varied fluoride and calcium phosphate in takes for either 16 or 48 weeks. Diets were designed to produce mild calciu m deficiency while keeping Ca:P ratio constant. Hence calcium deficiency wa s accompanied by a proportional phosphate deficiency. Calcium phosphate def iciency (25% of normal) significantly diminished vertebral strength only in younger rats. As the rats Crew older, the effects of calcium phosphate def iciency were mitigated. Calcium phosphate deficiency reduced vertebral stre ngth in young rats primarily by reducing the trabecular bone volume. The hi ghest fluoride intake (50 mg/L) significantly diminished vertebral strength , but there was no effect of lower fluoride intakes on bone strength. In co ntrast to calcium phosphate deficiency, high fluoride intake had no effect on trabecular bone volume but instead increased the amount of unmineralized osteoid, particularly in older rats. This impairment of mineralization by fluoride appeared to be the primary cause of the diminished vertebral stren gth. The detrimental effects of fluoride on vertebral bone strength were no t amplified by calcium phosphate deficiency.