SUMMARY: The urinary fluoride (F) concentration is recognized as an indication of F intake of preceding days. Since F is found naturally in water supplies, plants and animals, urinary F is readily affected by various food intakes. In this study, the influence of dietary F intake on urinary F concentrations was investigated, as well as the validity of spot urine samples as indices of F exposure.
Subjects were eight healthy female adults (age: 21 + 1 year) who stayed for 18 days, for the analysis of F metabolism, in a human experimental facility of the National Institute of Nutrition. F concentrations in the diets and urine were analyzed by the F ion specific electrode method, or its combination with the steam distillation method after ashing procedure.
F intake during 24 hrs from foods usually consumed in Japan, exclusive of green tea which contains a high amount of F (1.5 ppm), ranged from 0.79 to 2.74 mg/day. The levels were the same as have been reported in fluoridated communities. The excretion ratio of ingested F in the urine during 24 hrs was 19 to 35%. With 2.73 mgF/day intake, urinary F concentration reached 1.17 ppm at 3.5 hrs after dinner intake, and maintained that level to the next morning. The results showed that F excretion continued for several hrs after the intake from the diets. The amount of F in the diets and F excretion in 24-hr urine were well correlated (r = +0.95). Correlation coefficients with spot urine, with or without corrections for specific gravity or creatinine, were over +0.82.
The results show that, when estimating the F body burden, particularly due to low F exposure, it is necessary to monitor accurately F intake from diets. It is also suggested that the use of spot urine F concentration, with or without correction, is a valid procedure.