Most animal studies investigating how fluoride effects bone strength have found either a detrimental effect, or no effect. Few animal studies have found a beneficial effect. In fact, one of the few studies that found a beneficial effect was unable to be repeated by the same authors in a later experiment (Turner 1992, 1995). As noted by the lead author of this research team:

“[O]ne cannot help but be alarmed by the negative effects of fluoride on bone strength consistently demonstrated in animal models.”
SOURCE: Turner CH. (1996). Fluoride and the FDA: a curious case. (letter) Journal of Bone and Mineral Research 11(9):1369-71.

Other researchers have made the same observation. As noted by Sogaard, et al:

“[A]n overwhelming majority of the [animal] investigations mentioned found no effect or a negative effect of fluoride on bone strength…”
SOURCE: Sogaard CH, et al. (1995). Effects of fluoride on rat vertebral body biomechanical competence and bone mass. Bone 16: 163-9.

The National Research Council has described the animal studies as follows:

“Fracture risk and bone strength have been studied in animal models. The weight of evidence indicates that, although fluoride might increase bone volume, there is less strength per unit volume.”
SOURCE: National Research Council. (2006). Fluoride in Drinking Water: A Scientific Review of EPA’s Standards. National Academies Press, Washington D.C. p5.

For a discussion of the mechanisms by which fluoride could cause a reduction in bone strength, click here.

Animal Studies on Fluoride & Bone Strength:

“In A/J strain, we found significant decreases in stiffness with increasing fluoride dose treatment. There was a significant difference between the treatment group 0 ppm and 100 ppm… In the A/J strain, there was a decrease in ultimate load with increasing fluoride dose treatment, with significant differences between the treatment group 0 ppm and the treatment group 100 ppm (p=0.017).”
SOURCE: Mousny M, et al. (2006). The genetic influence on bone susceptibility to fluoride. Bone Aug 18; [Epub ahead of print]

“In group treated with NaF both the strength and stiffness were significantly decreased when compared with those in ovariectomized control.”
SOURCE: Czerny B, et al. (2004). The Effect of Tamoxifen and Fluoride on Bone Mineral Density, Biomechanical Properties and Blood Lipids in Ovariectomized Rats. Basic & Clinical Pharmacology & Toxicology 92:162–165.

“The highest fluoride intake (50 mg/L) significantly diminished vertebral strength… This impairment of mineralization by fluoride appeared to be the primary cause of the diminished vertebral strength.”
SOURCE: Turner CH, et al. (2001). Combined effects of diets with reduced calcium and phosphate and increased fluoride intake on vertebral bone strength and histology in rats. Calcified Tissue International 69: 51-57.

“Bending strength of the femoral shaft decreased significantly after fluoride therapy. We conclude that high fluoride intake decreases bone quality of the femoral shaft and neck in young growing rats.”
SOURCE: Bohatyrewicz A. (1999). Effects of fluoride on mechanical properties of femoral bone in growing rats. Fluoride 32: 47-54.

“In this study, despite the observed increased in hardness of both cancellous and cortical bone, the fracture stress and elastic modulus of vertebrae tested in compression and femora tested in three-point bending were decreased by fluoride treatment.”
SOURCE: Chachra D, et al. (1999). The effect of fluoride treatment on bone mineral in rabbits. Calcified Tissue International 64:345-351.

“It is likely that the bone changes induced by fluoride will lead to an impaired biomechanical competence of antlers from deer inhabitating regions with higher levels of environmental fluoride. We, therefore, would expect to find an increased incidence of antler breakage in such populations.”
SOURCE: Kierdorf U, et al. (1997). Fluoride content and mineralization of red deer (Cervus elaphus) antlers and pedicles from fluoride polluted and uncontaminated regions. Archives of Environmental Contamination and Toxicology 32: 222-227.

“Fluoride treatment reduced all biomechanical measurements. The reductions ranged from 5% to 25%. Several of these reductions were statistically significant: the fracture force of the femoral neck was reduced by 25%, the fracture stress of the L-5 vertebra was reduced by 19%, and the bending modulus of the femur was reduced by 21%.”
SOURCE: Turner CH, et al. (1997). Fluoride treatment increased serum IGF-1, bone turnover, and bone mass, but not bone strength, in rabbits. Calcified Tissue International 61:77-83.

“Fluoride concentrations of 15 and 50 ppm reduced femoral bone strength in renal-deficient animals. Femoral bone strength also was reduced in control animals given 50 ppm fluoride.”
SOURCE: Turner CH, et al. (1996). High fluoride intakes cause osteomalacia and diminished bone strength in rats with renal deficiency. Bone 19:595-601.

“NaF reduced the strength of cancellous bone from the L4 vertebrae, relative to the control animals, and the stiffness (resistance to deformation) of the femora.” Bone strength “did not increase with bone volume, suggesting that for bones with higher volume, there was less strength per unit volume, that is, a deterioration in bone ‘quality.'”
SOURCE: Lafage MH, et al. (1995). Comparison of alendronate and sodium fluoride effects on cancellous and cortical bone in minipigs. A one-year study. Journal of Clinical Investigations 95(5):2127-33.

“Load corrected for ash content, which is a measure of bone quality, decreased significantly after fluoride therapy. It is concluded that the increase in bone mass during fluoride treatment does not translate into an improved bone strength and that the bone quality declines. This investigation thereby supports the hypothesis of a possible negative effect of fluoride on bone quality.”
SOURCE: Sogaard CH, et al. (1995). Effects of fluoride on rat vertebral body biomechanical competence and bone mass. Bone 16(1): 163-9.

“To date, animal studies of fluoride effects on bone have used young and healthy experimental animals exclusively. The effects of fluoride on old animals, that more closely represent people most likely to fracture, have not been studied…. In older rats receiving 50 ppm fluoride, failure stress was decreased by as much as 29%. Such dramatic losses in bone strength only have been shown previously in studies where fluoride intake was accompanied by calcium deficiency, yet, in this study, calcium intake in the older rats was no different from that in the younger rats… [I]t is possible that aging effects and fluoride incorporation in the bone act synergistically to decrease bone strength.”
SOURCE: Turner CH, et al. (1995). Fluoride reduces bone strength in older rats. Journal of Dental Research 74:1475-81.

“[S]everal investigators – including ourselves – have shown that bone strength decreases as bone fluoride levels in the mineral phase increase to beyond about 4500 ppm.”
SOURCE: Turner CH, Dunipace AJ. (1993). On fluoride and bone strength (letter). Calcified Tissue International 53: 289-290.

“The results demonstrate that water fluoride levels of 1 ppm may lead to increased bone strength, while water fluoride levels of 4 ppm would be expected to cause a decrease in bone strength.”
SOURCE: Turner CH, et al. (1992). The effects of fluoridated water on bone strength. Journal of Orthopedic Research 10:581-7. (NOTE: In subsequent studies, Turner was unable to duplicate the beneficial effects on bone strength which he found at low doses in this study. As Turner noted in a more extensive, follow-up study: “the present results showed no evidence of increased bone strength resulting from fluoride levels below 16 ppm.” – Ref:: J Dent Res; 1995; Vol 74: 1475-81.)

“Bone quality seemed to be affected since significant decreases in bone-breaking strength and significant increases in bone mineralization were observed in fluoride-treated kestrels. When the breaking strength (LOAD) was expressed as the maximum load the bone can carry, no significant differences were detected among groups. However, when these figures are used to calculate the maximum stress the bone can resist, bone quality clearly decreased as more fluoride was added to the diet of the growing kestrels.”
SOURCE: Bird DM, Carriere D, Lacombe D. (1992). The effect of dietary sodium fluoride on internal organs, breast muscle, and bones in captive American kestrels (Falco sparverius). Archives of Environmental Contamination and Toxicology 22:242-6.

“[T]he mechanical parameters for the fluorotic animals were unchanged…or decreased…It is concluded that the increased bone mass during the initial stages of fluoride treatment does not necessarily indicate an improved bone quality.”
SOURCE: Mosekilde L, et al. (1987). Compressive strength, ash weight, and volume of vertebral trabecular bone in experimental fluorosis in pigs. Calcified Tissue Research 40: 318-322.

“The data reported herein suggested that levels of dietary F greater than 7 ppm are detrimental to bone integrity. Breaking stress and modulus of elasticity were reduced significantly at each level of added dietary F in both experiments. Similar observations have been made with nearly all species that have been subjected to F ingestion.”
SOURCE: Burnell TW, et al. (1986). Effect of dietary fluorine on growth, blood and bone characteristics of growing-finishing pigs. Journal of Animal Science 63(6):2053-67.

“Thirty-six young rats were used to determine the effect of the fluoride on collagen synthesis in healing of fracture… Collagen synthesis of the callus was examined histochemically and histologically. In the fluoride-treated group, collagen synthesis was found to be defective, while it was normal in the controls.”
SOURCE: Uslu B. (1983). Effect of fluoride on collagen synthesis in the rat. Research and Experimental Medicine 182:7-12.

“In the present study high levels of fluoride in the drinking water did not prevent osteoporosis, but in some experiments, by certain criteria, tended to increase it.”
SOURCE: Robin JC, et al. (1980). Studies on osteoporosis III. Effect of estrogens and fluoride. Journal of Medicine 11(1):1-14.

“F at high levels, tended to decrease bone ash, cortical thickness, and mechanical strength parameters.”
SOURCE: Guggenheim K, et al. (1976). The effect of fluoride on bone of rats fed diets deficient in calcium or phosphorus. Calcified Tissue Research 22: 9-17.

“The strength of osteopenic bone from calcium deprived rats, quail and roosters was significantly reduced after fluoride supplementation…This detrimental effect on bone strength must be considered in any therapeutic attempt to use fluoride ion to stimulate bone formation in osteopenic bone disorders.”
SOURCE: Riggins RS, et al. (1976). The effect of fluoride supplementation on the strength of osteopenic bone. Clinical Orthopedics (114):352-7.

“The administration of sodium fluoride increased bone diameter, indicating stimulation of periosteal bone formation, but bone strength was reduced or not affected by fluoride ingestion.”
SOURCE: Riggins RS, et al. (1974). The Effects of Sodium Fluoride on Bone Breaking Strength. Calcified Tissue Research 14: 283-289.

“Our observations corroborate the findings that, in general, elevated dietary fluoride results in an acceleration of bone mineralization. Uniquely, however, the increase in mineralization was accompanied by a decrease in bone strength… the changes in bone that occur with prolonged and excessive fluoride ingestion may result in a reduction of bone strength.”
SOURCE: Chan MM, et al. (1973). Effect of Fluoride on Bone Formation and Strength in Japanese Quail. Journal of Nutrition 103: 1431-1440.

“Femurs of fluoride-treated rats exhibited a decrease in mechanical strength as manifested by a decrease in ultimate stress to breaking as well as decrease in limit and modulus of elasticity.”
SOURCE: Wolinsky I, et al. (1972). Effects of fluoride on metabolism and mechanical properties of rat bone. American Journal of Physiology 223: 46-50.

“In the low calcium group a similar significant increase in flexibility appeared at the 10.0 ppm dosage level as well as the 45.0 ppm, but a significant decrease in strength at the two dosage levels were observed. These were in direct relation to the amount of fluoride given.”
SOURCE: Beary DF. (1969). The effects of fluoride and low calcium on the physical properties of the rat femur. Anatatomical Record 164: 305-316.

“[T]he heavily fluorinated bone tended to break under less stress than did bone from any other group. These findings suggest that the heavily fluorinated bone was not as strong as the bone from normal rats or from rats fed low-calcium diets without fluoride.”
SOURCE: Daley R, et al. (1967). The Effects of Sodium Fluoride on Osteoporotic Rats. The Journal of Bone and Joint Surgery (Abstract). 49A:796.

“[T]he decrease in the mean breaking strength was significant statistically” among the fluoride-treated rats, and “is in agreement with the known fact that the breaking strength of bone decreases with increased fluoride intake.”
SOURCE: Gedalia I, et al. (1964). Effects of Estrogen on Bone Composition in Rats at Low and High Fluoride Intake. Endocrinology 75: 201-205.

“Cristiani working with guinea pigs found that the fragility of the bones was increased about 20 per cent in the fluorized animals.”
SOURCE: Dean HT. (1936). Chronic endemic dental fluorosis. Journal of the American Medical Association 107: 1269-1273.