Viewpoint: Contradictory evidence exposes critical flaws in International Agency for Research on Cancer glyphosate assessment

Credit: IARC

This commentary does not concern itself with the need for, or the value of glyphosate as an agrochemical: rather it examines the scientific basis for the various conclusions reached at different times by a number of regulatory authorities and by the International Agency on Research on Cancer (IARC). Why do they differ? An appropriate regulatory stance depends critically on the application of good science and consistency in the application of established criteria. Constant reappraisal of the methodology used is essential.

Glyphosate has been assessed for carcinogenicity in several studies in rats, some of which were excluded from consideration in the IARC monograph. Across these eight studies, rats were exposed to 24 different dietary exposure concentrations that ranged from control (0) to 30,000 mg glyphosate/kg diet.

In males, the incidences of hepatocellular adenomas showed no dose-response relationship and varied within the same range as observed in the controls. This observation was also evident for hepatocellular carcinomas and provided confirmation that glyphosate was not carcinogenic in the liver of rats.

[T]he extensive reviews of the large number of studies on the genotoxicity of glyphosate, its degradate aminomethylphophonic acid (AMPA), and glyphosate-based formulations (GBFs) that were available prior to the publication of the IARC Glyphosate Monograph all support a conclusion that glyphosate (and related materials) are not genotoxic.

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[T]he evidence of an oxidative stress mechanism of carcinogenicity is largely unconvincing. The expert panel concluded that there is no new, valid evidence presented in the IARC Monograph that would provide a basis for altering these conclusions, an opinion which is shared by regulatory agencies around the globe.

Critical to a realistic evaluation of rodent toxicity data is an understanding of the natural history of neoplasms in rats and mice and of the variation seen in aged animals of different strains. It is important to consider the weight of evidence, not to place excessive attention on individual studies (notably if these are not repeatable).

Finally, the power of new techniques of analysis of human tumors has made it increasingly clear that great caution should be applied to analyses that “read across” from rodent to human tumors. Overall, there is no evidence in the animals studies to support the conclusion of IARC that glyphosate is a probable human carcinogen. Likewise, a similar conclusion was recently reached for the epidemiology studies.

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