Fluoride distribution, metabolism and excretion
Once absorbed,
fluoride is rapidly distributed throughout the body via the blood. The short term plasma half-life is normally in the range of 3 to 10 hours.
Fluoride is distributed between the plasma and blood cells, with plasma levels being twice as high as blood cell levels. The saliva
fluoride level is about 65% of the level in plasma (Ekstrand 1977). Plasma
fluoride concentrations are not homeostatically regulated, but rise and fall according to the pattern of
fluoride intake. In adults, plasma
fluoride levels appear to be directly related to the daily exposure of fluoride. Mean plasma levels in individuals living in areas with a water
fluoride concentration of 0.1 mg/L or less are normally 9.5 μg /L, compared to a mean plasma
fluoride level of 19-28.5 μg/L in individuals living in areas with a water
fluoride content of 1.0 mg/L. In addition to the level of chronic
fluoride intake and recent intake, the level of plasma
fluoride is influenced by the rates of bone accretion and dissolution, and by the renal clearance rate of fluoride. Renal excretion is the major route of
fluoride removal from the body. The
fluoride ionis filtered from the plasma by the glomerulus and then partially reabsorbed; there is no tubular secretion of fluoride. Renal clearance rates of
fluoride in humans average at 50 mL/minute. A number of factors, including urinary pH, urinary flow, and glomerular filtration rate, can influence urinary
fluoride excretion. There are no apparent age related differences in renal clearance rates (adjusted for body weight or surface area) between children and adults. However, in older adults (more than 65 years of age), a significant decline in renal clearance of
fluoride has been reported consistent with the age-related decline in glomerular filtration rates.
Approximately 99% of the
fluoride in the human body is found in bones and teeth.
Fluoride is incorporated into tooth and bone by replacing the hydroxyl
ion in hydroxyapatite to form fluorohydroxyapatite. The level of
fluoridein bone is influenced by several factors including age, past and present
fluoride intake, and the rate of bone turnover.
Fluoride is not irreversibly bound to bone and is mobilized from bone through bone remodelling.
Soft tissues do not accumulate fluoride, but a higher concentration has been reported for the kidney due to the partial re-absorption. The blood-brain barrier limits the diffusion of
fluoride into the central nervous system, where the
fluoride level is only about 20% that of plasma. Human studies have shown that
fluoride is transferred across the placenta, and there is a direct relationship between
fluoride levels in maternal and cord blood. In humans,
fluoride is poorly transferred from plasma to milk. The
fluoride concentration in human milk is in the range of 3.8–7.6 μg/L.
