Why herbicide glyphosate has split scientific community over its cancer-causing links

Some scientists dismiss glyphosate’s carcinogenic links as inaccurate, while others claim that’s due to the studies being paid for by the industry.

A farmer labors on a field in Bana village in the district of Latehar, Jharkhand | PTI

Bengaluru: An RSS affiliate challenged the Modi government this week over what it called misleading claims on the “toxicity” of popular herbicide glyphosate.

In its letter to Agriculture Minister Narendra Singh Tomar, the Swadeshi Jagran Manch insisted that contrary to what the ministry told the Parliament, glyphosate causes cancer and is harmful to the environment.

The row is only the latest addition in a long list of controversies featuring the herbicide, sold by agricultural corporation Monsanto under the brand name Roundup, in which it is the primary ingredient. It is also the primary ingredient in other weedicides such as Glycel and Eraser.

The chemical has generated controversy in recent years for its purported links to cancer and has divided the scientific community. Some scientists dismiss its links to cancer as inaccurate and based on cherry-picked data, while studies claiming a lack of link have been accused of being paid by the industry.

Working and popularity

Glyphosate is traditionally applied to the leaves of a weed to kill it and is used in ecosystems such as farms, lawns and even aquatic environments. It is effective on both grasses and trees. It comes in various forms such as solid salts or amber-coloured liquids. It has been in use for over 40 years.

The chemical works by blocking the ‘shikimate pathway’, a series of steps followed by plants (and even some bacteria and fungi) to produce necessary amino acids such as tryptophan. This pathway does not exist in mammals and humans. The herbicide can be absorbed by leaves and marginally by roots as well. It works on plants that are in the process of growing, but doesn’t interfere with germination of seeds.

When glyphosate is absorbed by a plant, it is transmitted all over the plant’s systems, inhibiting a specific enzyme called 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) which blocks the aforementioned pathway. The buildup of this shikimate eventually causes the plant to die.

In 1996, Monsanto started selling genetically modified seeds whose crops are resistant to glyphosate-containing Roundup. These were called ‘Roundup Ready’ crops, and despite exposure to Roundup, they were unaffected by it.

The introduction of these seeds propelled Monsanto as the world’s biggest seed-selling corporate and their sales of Roundup also went up over 10-fold due to its ease of use and tailored results from usage.

The widely-used herbicide is not persistent in the atmosphere, groundwater, or soil the way DDT and some other chemicals are. It is considered to have a half-life of 47 days. Meta analyses of studies show that it doesn’t affect earthworms in the soil either.

It is also not toxic to humans directly and doesn’t require farmers to wear masks during usage. The only known case of death from glyphosate is when a 37-year old woman purposely overdosed on extremely concentrated quantities.

Carcinogen controversy

Monsanto’s Roundup is currently embroiled in controversy due to three different civil cases in the US where the jury mandated the corporation to pay millions of dollars in sum total for apparent carcinogenic properties of the chemical.

This is in contradiction to results from over 1,000 studies on glyphosate by scientists and food and drug regulation authorities in Europe, Australia, Canada, US, and the World Health Organisation. Europe alone has conducted multiple studies through various agencies which ended up disproving the herbicide’s link to cancer.

It must be noted, though, that one of these studies came under fire for supposed plagiarism of Monsanto text and analysis of unpublished papers.

However, the International Agency for Research on Cancer (IARC), affiliated with the WHO, declared in 2015 that glyphosate is “probably carcinogenic” to humans. The given category implies that there is limited evidence of cancer in humans, mostly from those exposed to glyphosate through farming, but enough evidence of cancer in laboratory animals to warrant this classification.

When it comes to cancer, effects on laboratory animals are not considered a perfect representation of humans, and are seen to be “useful but imperfect surrogates”.

In its report, the IARC also restricted the word ‘cancer’ specifically to a certain kind, called non-Hodgkin lymphoma.

IARC vs regulatory studies

The IARC results are different from other scientific research. While IARC looked at Roundup and fertilisers that contained glyphosate along with other chemicals, studies by regulatory bodies studied glyphosate alone.

There is a key difference in how the data is interpreted and how the conclusions are worded: The IARC findings pointed out if something ‘may’ cause cancer while regulatory bodies decided if something ‘will’. Such a divergence runs common, and counter-intuitively, when it comes to establishing a link to cancer in humans.

For example, tobacco definitely ‘does’ cause cancer, but drinking warm beverages such as tea or even milk ‘may’ cause cancer. Asbestos definitely causes cancer but consumption of alcohol ‘may’ damage DNA and cause cancer.

Glyphosate also does cause damage to the DNA, but its effect as a carcinogen is a lot more nuanced: Even if it does cause cancer, it needs to be consumed in high dosage or high concentrations.

Monsanto, for its part, has funded ample studies that ultimately ended up disproving the link to cancer. And this is where the controversy sets in.

NPR reports that regulatory bodies used peer-reviewed studies that might have been legitimate but funded by Monsanto. As a result, some of the studies that were considered were paywalled. The IARC only used scientific studies that are publicly available. As a result, IARC looked at far fewer studies than regulatory bodies.

Furthermore, Monsanto was revealed to have reportedly engaged in questionable practices. Emails revealed in one of the civil cases showed internal communications between company officials about “ghost-writing a paper” and cozying up with officials from the US Environmental Protection Agency (EPA).

The revelation of these practices and Monsanto’s indifference to the carcinogenic concerns was seemingly what prompted jury members in these cases to award damages to complainants. But it is expected that judges would reduce the amount payable by the multi-billion dollar corporation as there is no evidence of the company having concealed data or influencing the EPA in any way.

For their part, studies that do support the link to cancer are accused of cherry-picking data as well.

Navigating the landscape of glyphosate can be tricky and confusing. But for the moment, at least, glyphosate’s reputation as the drug that is most studied for links to cancer is not going away any time soon.