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

Ingested fluoride is incorporated into bone apatite and can affect the structural 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 high fluoride intake would have independent detrimental effects on vertebral bone strength. We measured the vertebral biomechanical properties and bone histology for rats given controlled diets with varied fluoride and calcium phosphate intakes for either 16 or 48 weeks. Diets were designed to produce mild calcium deficiency while keeping Ca:P ratio constant. Hence calcium deficiency was accompanied by a proportional phosphate deficiency. Calcium phosphate deficiency (25% of normal) significantly diminished vertebral strength only in younger rats. As the rats grew older, the effects of calcium phosphate deficiency were mitigated. Calcium phosphate deficiency reduced vertebral strength in young rats primarily by reducing the trabecular bone volume. The highest fluoride intake (50 mg/L) significantly diminished vertebral strength, but there was no effect of lower fluoride intakes on bone strength. In contrast 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 strength. The detrimental effects of fluoride on vertebral bone strength were not amplified by calcium phosphate deficiency.