Panel finds fluoride-cancer link 'equivocal.'

Panel finds fluoride-cancer link 'equivocal'

A review panel of nongovernment scientists last week affirmed the findings of a recent federal study that weakly links sodium fluoride -- a compound widely used in public drinking water to discourage tooth decay -- to bone cancer in male rats. Both the panel and the study's authors caution, however, that the results are "equivocal," meaning the excess bone cancers have only marginal statistical significance and might have occurred even in the absence of fluoride.

No excess bone cancers appeared in groups of female rats and in mice of both sexes, notes study director John R. Bucher of the Public Health Service's National Toxicology Program (NTP) in Research Triangle Park, N.C. The NTP, which commissioned the review panel, released the research results on April 2.

"The findings are much ado about nothing," asserts Joseph A. Cotruvo of EPA's office of toxicology. EPA is required by law to review fluoridation safety every three years, and Cotruvo estimates it will complete its current review sometime next year. "There is an extremely minimal amount of information [in the NTP study] to apply to human health considerations," he says.

Pathologist John A. Yiamouyiannis of Delaware, Ohio, strongly disagrees. A longtime opponent of water fluoridation, Yiamouyiannis coauthored a controversial epidemiologic report in 1977 that claimed a yearly excess of 10,000 human cancer deaths due to U.S. water fluoridation, prompting Congress to request the NTP animal tests. Yiamouyiannis calls for an immediate half to fluoride use, citing the NTP tests and other reports, which he contends "provide clear evidence that fluoride is a carcinogen."

Still other researchers see no evidence at all in the two-year NTP study. John W. Stamm of the University of North Carolina at Chapel Hill, a dental epidemiologist and spokesman for the American Dental Association, maintains that NTP and the review panel took an overly cautious route by labeling the findings equivocal rather than statistically insignificant. Stamm notes that NTP scientists who conducted a similar animal study eight years ago, examining the toxicity of stannous chloride in drinking water, found a cancer incidence slightly higher than that of the recent fluoride study, yet they concluded there was no statistically significant cancer association. NTP's Bucher responds, "That was in 1982. We're allowed to change our thinking."

The disagreement centers on NTP's recent finding of a small excess rate of bone cancer among 180 male rats given sodium fluoride in their drinking water. Three cancer cases appeared among the 80 males receiving the study's highest fluoride dose -- 79 parts per million (ppm), or about 80 times the usual concentration in fluoridated drinking water -- and one cancer arose among the 50 rats exposed to a 45-ppm concentration. In contrast, no bone cancers developed in a group of 50 males receiving a dose of 11 ppm or in a control group of 50 males drinking fluoride-free water.

Bucher and his colleagues performed two types of analyses. Simple comparisons between the control group and each group with excess bone cancer indicated the cancers have no statistical significance. But a more sensitive analysis known as the trend test, which accounts for varying disease rates among animals exposed to different doses of a chemical, indicated the excess cancers have marginal statistical significance, Bucher says.

While emphasizing that the data are far from conclusive, Bucher maintains that a link between fluoride and the excess bone cancers "is plausible." He notes that fluoride binds to bone and can stimulate the proliferation and activity of osteoblasts, cells that increase bone mass.

A Public Health Service subcommittee is now comparing the NTP findings with other animal and human studies, and will likely issue its recommendations on fluoridation in July.