The kidneys are responsible for ridding the body of ingested fluoride, and thereby preventing the buildup of toxic levels of fluoride in the body.
In healthy adults, the kidneys are able to excrete approximately 50% of an ingested dose of fluoride.
However, in adults with kidney disease the kidneys may excrete as little as 10 to 20% of an ingested dose - thus increasing the body burden of fluoride and increasing an individual's susceptibility to fluoride poisoning (e.g. renal osteodystrophy).
The bone changes commonly found among patients with advanced kidney disease closely resemble the bone changes found among individuals with the osteomalacic-type of skeletal fluorosis. This raises the possibility that some individuals with kidney disease are suffering from undiagnosed skeletal fluorosis.
As noted by Dr. Edward Groth, a veteran Senior Scientist at Consumers Union:
"It seems probable that some people with severe or long-term renal disease, which might not be advanced enough to require hemodialysis, can still experience reduced fluoride excretion to an extent that can lead to fluorosis, or aggravate skeletal complications associated with kidney disease... It has been estimated that one in every 25 Americans may have some form of kidney disease; it would seem imperative that the magnitude of risk to such a large sub-segment of the population be determined through extensive and careful study. To date, however, no studies of this sort have been carried out, and none is planned" (Groth 1973; Doctoral Thesis; Stanford University).
Because the kidney accumulates more fluoride than all other soft tissues (with the exception of the pineal gland), there is concern that excess fluoride exposure may contribute to kidney disease - thus initiating a "vicious cycle" where the damaged kidneys increase the accumulation of fluoride, causing in turn further damage to the kidney, bone, and other organs.
The possibility that fluoride exposure can cause direct damage to kidney tissue is supported by a long line of animal and human studies.
In studies on fluoride-exposed animals, kidney damage has been reported at levels as low as 1 ppm if the animals consume the water for long periods of time.
In humans, elevated rates of kidney damage are frequently encountered among populations with skeletal fluorosis. In addition, several case reports suggest that some individuals with kidney disease can experience significant recovery in their clinical signs and symptoms following the provision of fluoride-free water.