The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program. The findings may not have been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by NTP in March 2009. For more information, see the Explanation of Levels of Evidence for Developmental Toxicity. The findings and conclusions for this study should not be construed to represent the views of NTP or the U.S. Government.
This study was conducted to assess the potential for orally administered sodium fluoride to cause developmental toxicity in rabbits. The most common human exposure to NaF occurs as a result of fluoridation of municipal drinking water supplies. The lack of pertinent developmental toxicity data in the published literature prompted the initiation of this study. NaF (CAS No. 7681-49-4) was administered ad libitum in drinking water to mated NZW rabbits (26/group) on gestation days 6 through 19 at levels of 0, 100, 200, or 400 ppm (0.1, 0.2, or 0.4 mg/ml). Drinking water (vehicle) contained less than 0.6 ppm of sodium fluoride (the detectable limit). Animals were observed daily for clinical signs of toxicity. Food, water, and body weights were recorded for the animals in each group on gd 0 and every two days thereafter through gd 30. Blood samples were collected from 5 animals per group per replicate on gd 20; serum was delivered to the sponsor for determination of fluoride concentration. All animals were killed on gd 30 and examined for maternal body and organ weights, implant status, fetal weight, sex, and morphological development.
Based on measurement of water intake, animals in the low, mid and high concentration groups ingested an average of 10, 18 or 29 mg NaF/kg body weight/day, respectively. However, samples of rabbit chow contained an average of 15.6 ppm fluoride (range 14.6-16.6 ppm) and therefore feed served as a secondary source of fluoride exposure. The average measured fluoride intake from both sources (food and water) was 3, 21, 34 and 52 mg fluoride per animal/day (or 0.8, 6, 9 and 14 mg fluoride/kg body weight/day) for the control through high concentration groups. Water intake provided approximately 84%, 91 % and 95% of the total F consumed for the low through high concentration groups in this study.
No maternal mortality occurred in this study. Pregnancy rates were 84%, 87%, 78%, and 83% in the control to high exposure groups, respectively. Maternal body weight change for the animals receiving 400 ppm NaF was significantly lower than that of control animals for the period from gd 6 to 8 (14 grams average weight gain for controls vs. 112 grams weight loss for the 400 ppm group); this difference probably resulted from significantly decreased food and water consumption during the same period. Maternal body weight change was significantly increased from gd 10 to 12 (22 grams average weight gain for controls vs. 71 grams weight gain for the 400 ppm group), but did not differ among groups for the treatment period as a whole, indicating that animals in the 400 ppm group recovered from the weight change effects observed during the first few days of exposure to NaF in the drinking water. Maternal water consumption (g/kg/day) during exposure was significantly decreased in the animals exposed to 400 ppm NaF. Post-exposure water consumption was normal in these animals indicating the probability of dereased palatability of the 400 ppm solution. Maternal food consumption was decreased compared to control during the first four days of treatment (g/day on gd 6 to 8 and 8 to 10; g/kg/day on gd 6 to 8), but was normal thereafter. No clear clinical signs of toxicity were observed. Determination of serum fluoride levels by the sponsor, in 7-8 pregnant animals per group, revealed levels of 0.06 ± 0.04, 0.24 ± 0.10, 0.39 ± 0.14, and 0.70 ± 0.33 ppm at the end of the exposure period for the control through high dose groups, respectively. Necropsy of the maternal animals revealed no effects on kidney or liver weights. In utero sodium fluoride exposure did not affect the frequency of post-implantation loss, mean fetal body weight per litter, or external, visceral, or skeletal malformations.
In summary, there was evidence of minimal maternal toxicity but no definitive evidence of developmental toxicity with levels of sodium fluoride in drinking water as high as 400 ppm (resulting in an average exposure of 29 mg/kg/day) although the palatabillity of a 400 ppm sodium fluoride solution apparently reduced water consumption. This study established a no observed adverse effect level for maternal toxicity at 200 ppm NaF in drinking water (approximately 18 mg/kg/day) and a NOAEL for developmental toxicity of 400 ppm NaF in drinking water (approximately 29 mg/kg/day) administered to pregnant NZW rabbits during organogenesis.