DNA Damage and Glyphosate? Critical Evaluation of a 2007 Report

A question appeared over on GMOanswers.com and I thought I'd take a stab at it.  I remember looking at it briefly awhile ago, but it didn't stick in my brain.  Maybe because it was not worth sticking there.

Last night I took a critical look at this work.  If you take the time to read it you find that even the authors have many mundane explanations for the results.  However, the title becomes a headline and is part of the glyphosate=danger mantra repeated by low-science-resolution readers that seek confirmation of their biases.  Judging by the capitalization in the question, the person with the question even cut-n-pasted the title. Not too many calories being expended to sort out this mystery!  However, education is my goal, so here goes... 

Question on GMOanswers:  Can you comment on this study about DNA damage due to Roundup Evaluation of DNA damage in an Ecuadorian population exposed to glyphosate?

The report you refer to is Paz-y-Miño et al., (2007), a "Short Communication" published in the journal Genetics and Molecular Biology, a small Brazilian journal (Impact factor 0.73, so not a well-recognized journal). So we're starting with a Short Communication in an obscure journal. 

As usual, opinions formed from reading titles are not terribly accurate, and don't match the data within the report.  As evidenced by my analysis here, a conclusion from a title takes a lot of time to address!

Ceasar Paz-y-Mino has an okay publication record and studies a number of regional issues using his expertise as a cell biologist. This report tests assesses "DNA damage" using what's called a "comet assay", an assay where cells are placed into an agar matrix, lysed and subjected to an electric field.  DNA is charged, so large DNA pieces move to the positive pole.  Damaged DNA moves faster because it has greater mobility- that's the basis of the assay. The DNA smears out as a blob with a streak after it, resembling a comet. 

In this report 24 people from an ag intensive area where glyphosate was used were compared to 21 in an area 80km away.  Blood was drawn "between 2 weeks and 2 months" after glyphosate application to the crops. There is one table of data showing that the DNA from those living near the farm (50% tested were 200m-3km). 

The results show consistently higher migration in the "exposed" group, suggesting more damage, according to the authors. 

Before we get too excited about the results:

1.  Glyphosate moves quickly from the body.  After two weeks there would be negligible effect, if any, from acute exposure. The samples could have been from people tested two months after exposure, the authors don't specify. 

2.  The authors say that the "exposed" group had sprays directly over the homes in 50% of cases, and that applications were "20 times the maximum recommended application rate for the formulated product, which may explain our comet assay results". 

If there's one thing we can learn from this paper it is that someone is not reading a label very closely before firing up the crop duster. 

So directly spray the homes of the workers with 20x the normal concentration, and then measure if there's something screwy going on.  Hmmm.  I wonder. 

3.  But all that is likely a moot point.  Glyphosate is rapidly removed from the body and in no cases has it been demonstrated to damage DNA or even carcinogenic (it is classified as "not carcinogenic" even by the strictest standards).  What is happening? 

Here are additional considerations and interpretations: 

First, if these workers were tested 14-60 days after being sprayed with 20x glyphosate, what else are they spraying down there imprecisely and at levels far above recommendations?  Are these chronically ill people from prolonged exposures to 20x ag chemicals sprayed on their homes?  This would be a better explanation, unless the authors knew that the occupational exposures were purely glyphosate.

But the best explanation--  "Blood samples (from the unexposed group) were collected and processed as for the exposed group, but not concomitantly."

Bingo.  The authors counted on one single replicate that was processed at different times. How the blood was handled, how it was prepared... all could easily account for the results seen.  The fact that it was one replicate is also quite telling.  I'd never publish with fewer than three on this kind of test. 

The best thing that could be said is that the data show a potential starting point.  It would have been good to see the data and have the controls and treatments collected and processed blindly and at the same time.  

Conclusion:  Maybe good work, maybe not. Maybe trustworthy data that are a hint of things to study further, maybe not.  The methods and data presented do not rigorously support the authors' conclusions.  It has been seven years since this study and no further evidence to support the DNA damage conclusion.  In a 2011 report by the same authors, glyphosate showed no effect in DNA damage in a larger test with greater resolution in Colobmian/Ecuadaoran populations.

The most likely explanation of the findings is that the cells in one group broke down or had some other damage during handling leading to the results observed.  That's why there has been no additional follow up on this study.