Biological rat assay for total fluoride availability in foods

A fluoride (F) bioassay procedure based on the method of standard additions and using weanling rats was designed and tested. Rats were distributed between a control (C) group fed MIT 200 diet (F conc. less than 0.5 ppm) and an experimental (E) group fed this diet plus 1% dehydrated instant tea used as a model test food. Both groups were further divided into subgroups fed the C or E diets, but supplemented with 0.5, 10, 20 or 40 ppm F as NaF, and offered deionized water for a period of 10 days. Rats were then killed and femur (fm) and tibia (tb) dissected. Bone samples were treated in a closed system with HCl (6 N) and later with sodium citrate (30%) to determine F concentration using a specific ion electrode. F values (y) for fm and tb were plotted against F amounts added to the diets (xt). Weighted regression equations describing the linear relationship between the two variables were used to calculate the available F content of dehydrated tea which was 830 +/- 43 ppm F (using fm data) and 836 +/- 67 ppm (using tb data). Chemical assay of the dehydrated tea used as a test food gave a F concentration of 887 +/- 16 or 93.5 +/- 5.1% availability. This rat bioassay to determine total available fluoride in a dried food can be combined with a chemical assay to obtain a measure of percent availability of F.