What do toxicology studies tell us about glyphosate?
Glyphosate, when ingested, is not metabolized and is rapidly eliminated primarily unchanged from the body. After a single oral, dermal or inhalation administration, glyphosate showed low acute toxicity. Glyphosate produced irritation of the eyes, but no skin irritation, and no sensitization. Comprehensive toxicological studies in animals have demonstrated that glyphosate does not cause cancer, mutagenic effects, birth defects, neurotoxicity or reproductive problems 5, 14, 17, 18.
Is glyphosate detrimental to the reproduction and development of animals and humans?
Based on studies in rats and rabbits, the EU, WHO and US-EPA have come to the conclusion that glyphosate is not detrimental to the reproduction or development of mammals, including humans. According to the regulatory guidelines, developmental toxicity tests must be carried out on two mammalian species (rats and rabbits), and an additional multi-generation reproduction study is conducted with rats. The test substance is normally administered to the test animals orally, although it can be administered through the skin or by inhalation if justified by specific exposure considerations.
Several sets of these standard developmental and reproduction studies have been conducted for regulatory purposes globally, and other studies have been reported in the scientific literature. While key studies have not shown adverse developmental or reproductive problems, some allege that adverse effects occurred. However, such effects were not consistently observed and/or were produced under unrealistic conditions that would not occur in humans. It is important to understand the validity and relevance of these publications, which have themselves been the subject in subsequent expert reviews. Such expert evaluations have all concluded that the toxicology data collectively demonstrate that glyphosate exposure does not pose any increased risk of reproductive problems in humans.
According to international guidelines, cell cultures and isolated chicken embryos are not adequate test models for use in risk assessments of herbicides and other chemical substances (© iStockphoto.com/ Cassandra Tiensivu).
Is there any basis for authorities to affirm that glyphosate is detrimental to reproduction and development based on in vitro studies?
There are some in vitro research publications that have characterised glyphosate-based herbicide formulations as toxic products that cause developmental effects.
One in vitro study that has attracted considerable public attention was conducted by a research group at the University of Buenos Aires in Argentina. The researchers, Paganelli and his colleagues, conducted three in vitro assays, with (i) frog embryos exposed to a glyphosate formulation, (ii) frog embryos directly injected and (iii) fertilized chicken embryos exposed directly to a glyphosate formulation through a hole cut in the egg shell 12.
The method of administering the test substances directly to the fertilized cells, by mixing them into the culture medium or by means of injection is, however, considered to be a highly artificial exposure route and is not in line with standard toxicology testing methods 13. The guidelines for toxicity tests for chemicals prescribe that the test substance must be administered to the females via oral, dermal or inhalation routes, so that exposure of the offspring reflects real world exposure pathway and depends on how much test substance is absorbed and actually passes through the placenta. Other key issues surrounding the research include the lack of blank controls when frog embryos were injected and the excessively high and environmentally unrealistic doses that were applied.
In addition, in vitro studies which link glyphosate to developmental or reproductive disorders often tested glyphosate-based herbicides as a whole. They therefore cannot determine if any effects observed were caused by glyphosate or other compounds, such as surfactants, which are common ingredients of herbicides. Surfactants increase the penetration of the waxy surface of leaves and can also impair the integrity of cellular membranes. The direct application of these test substances at high concentrations in in vitro cell models creates an artificial situation that makes it difficult, if not impossible, to compare these results with the existing in vivo regulatory studies conducted as part of the approval procedure.
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Last update: 03 June 2013