Fluoride metabolism and excretion in children.

This paper compares fluoride pharmacokinetics (plasma, renal, and extrarenal clearances) and metabolic balances in healthy infants or children with those in young or middle-aged adults. Regardless of age, the removal of fluoride from the intra- and extracellular body fluids occurs almost exclusively by uptake in calcified tissues and excretion in the urine. While there can be considerable differences among individuals, the rates at which fluoride is cleared from plasma by calcified tissues and the kidneys in adults are approximately equal. The calcified tissue clearance of fluoride from plasma in children is substantially higher than that by the kidneys. This is due to the greater surface area of the loosely organized crystallites in the developing calcified tissues during growth. Thus, the balance of fluoride (total intake minus total excretion) is typically higher in children than in adults, but it can be positive or negative at any age. Positive balance occurs when chronic fluoride intake is sufficient to prevent plasma concentrations from declining. When positive, the fluoride content of the calcified tissues, which contain more than 99 percent of the body's fluoride, tends to gradually increase. Negative balance, which indicates net mobilization of fluoride from calcified tissues, can occur when plasma concentrations decline due to a reduction in the level of fluoride intake.