Is glyphosate, used with some GM crops, dangerously toxic to humans?

Is glyphosate, the active ingredient in Monsanto's Round-up used in conjunction with many GMO crops, dangerous to humans? If your primary source for news is the Internet, you'd almost certainly come away a little frightened. A Google search with the keywords "glyphosate" and "dangers" turns up headlines like "How Glyphosate Worsens Modern Diseases," by Joseph Mercola, the founder of a website that sells unregulated vitamins and supplements, and "Health Hazards of Roundup and Glyphosate," by well-known anti-GMO group Earth Open Source.

These and other groups that oppose GMOs or campaign against chemicals whether they are considered dangerous or not claim that glyphosate has been wrongly marketed as a "safe" herbicide, citing studies that they say "confirm" glyphosate poses serious health hazards. According to Earth Open Source, the effects of glyphosate can be found at low doses as herbicide residues in foods, and the "safe" dose set by regulators is "not based on up-to-date objective evidence.”

One heavily used report was by Stephanie Seneff and Anthony Samsel claiming that glyphosate was supposedly causing all sorts of diseases from inflammatory bowel disease to Parkinson's to depression. Another report by a group of French scientists led by Gilles-Eric Séralini claimed that glyphosate was toxic to human cells. Both reports have been heavily criticized and deconstructed. Seneff and Samsel have no expertise in toxicology or agriculture: Seneff is a computer scientist while Samsel is a retired science consultant. Their report had no supporting data; they basically “made up” their report. Séralini is notorious for authoring a retracted publication that inconclusively linked GMOs to cancer and his report on glyphosate was based on flawed experimental design - glyphosate is not directly exposed to human cells in the real world. Both studies appeared in pay-for-play journals and mainstream scientists have rejected them.

It is easy to find scary information like that on the web, whereas the findings of independent science and regulatory agencies are often buried deep in Google searches. According to the Environmental Protection Agency, glyphosate is “of relatively low oral and dermal acute toxicity.”

Toxicity is all about dosage; this applies to all substances. Some chemicals like aflatoxin and botulin are toxic in small doses, while others like vitamin D and caffeine have low toxicity, becoming dangerous only at higher doses.

Let's take a closer look at glyphosate. Glyphosate is derived from an amino acid, glycine. It acts against plants by suppressing an essential biochemical mechanism commonly found in plants, but not in animals. According to the Extension Toxicology Network, a joint pesticide information project by Cornell University, Michigan State University, Oregon State University and University of California at Davis, and funded by US Department of Agriculture, glyphosate is non-volatile, minimizing exposure through inhalation, and undergoes little metabolism in the human body. If accidentally consumed, glyphosate is excreted mostly unchanged in feces and urine, so it doesn't stay in the body and accumulate.

The EPA has also determined that glyphosate has “minimal” ecological effects. Glyphosate is only slightly toxic to birds and fish, and it binds tightly to the soil, reducing the possibilities of leaching. Microbes in the soil then break glyphosate down so it doesn’t accumulate in the soil. According to plant pathologist Steve Savage, glyphosate has also replaced mechanical tillage to destroy weeds, which is “a substantial positive for the environment because of reduced erosion and retention of soil carbon.”

So how toxic is glyphosate exactly? To examine toxicity, one must look at the LD50 value given to the chemical in question. LD50 is a standard measure of acute toxicity for chemicals, expressed in the amount of chemical (milligrams) per body weight (kg) that it took to kill fifty percent of a population of test animals. Because LD50 is a standard measure, it is used to compare toxicities of compounds; the lower the number, the more toxic it is.

Glyphosate has a LD50 of 5600 mg/kg based on oral ingestions in rats, according to EPA assessments (PDF), placing it in Toxicity Category III. The EPA ranks chemicals in four categories, I being the most toxic and IV being the least. The EPA has also found that glyphosate does not cause cancer. To compare, caffeine has a much lower LD50 of 192 mg/kg based on oral ingestions in rats.toxicity-table4

Caffeine is over ten times more toxic than glyphosate. Is this cause for concern? Should we stop drinking coffee? No, the main reason being that a typical dosage of caffeine is not high enough to cause toxicity. Let's look at the numbers. With LD50 of 192 mg/kg, it would take 12192 mg of caffeine to kill an average 140 lb human being. A typical 8 oz cup of coffee only contains 95 mg of caffeine, much lower than the dose required for acute toxicity. The same reasoning applies to glyphosate. Following the same calculations, it would take 12.5 oz of glyphosate to kill an average 140 lb human being. That means drinking about three gallons of Roundup Original.

But what about long-term exposures to glyphosate? Given its widespread use, there is a good chance that we are eating some residues in our food. The EPA considered this too by setting maximum safe levels of residues called tolerances. The USDA tests crops each year to make sure that herbicide residues do not exceed tolerance levels. If any crops contain residue amounts higher than tolerance levels, the USDA reports the information to the FDA, who has the regulatory power to recall foods, levy fines and take other actions to prevent the foods from reaching consumers. The EPA also made sure that the tolerances were conservative:

EPA conducted a dietary risk assessment for glyphosate based on a worst-case risk scenario, that is, assuming that 100 percent of all possible commodities/acreage were treated, and assuming that tolerance-level residues remained in/on all treated commodities. The Agency concluded that the chronic dietary risk posed by glyphosate food uses is minimal.

A reference dose (RfD), or estimate of daily exposure that would not cause adverse effects throughout a lifetime, of 2 mg/kg/day has been proposed for glyphosate, based on the developmental toxicity studies described above.

Which means that a 140 lb human being can be exposed to as much as 127 mg of glyphosate per day for a lifetime without adverse effects, according to EPA standards. To put this in perspective, the daily upper limit for vitamin D supplements is 4000 IU per day, or 0.1 mg.

The EPA's efforts to ensure that glyphosate is used at safe levels did not put the matter to rest. In an attempt to get out their message that glyphosate is dangerous, anti-GMO activist scientists have turned to pay-for-play journals to publish work that would never be accepted in mainstream science journals, like the two studies by Samsel, Seneff and Séralini. However, uncritical media coverage of these type of studies has lent credibility to them and the anti-GMO activist groups.

For example, Reuters’ Carey Gillam, known for her anti-GMO bias, publicized the Samsel and Seneff work, conveying a false sense of credibility:

Heavy use of the world's most popular herbicide, Roundup, could be linked to a range of health problems and diseases, including Parkinson's, infertility and cancers, according to a new study.

The mainstream science community was left wondering. Andrew Kniss, an agronomist at the University of Wyoming tweeted: “Why are they [Reuters] calling it a ‘study’? There was absolutely no data.” Discover journalist Keith Kloor responded that the study was “so obviously absurd that I was sure it  would be ignored by media,” but Gillam’s coverage gave it "a veneer of legitimacy to readers.”

When GMO critics spread fear and claims of glyphosate's toxicity, one thing is commonly overlooked: today's herbicides and insecticides are far safer than those used in the past. The agrochemicals displaced by glyphosate, such as MCPA, a herbicide for controlling annual and perennial weeds, have much lower LD50 values and bigger problems with bioaccumulation. Savage noted that glyphosate was “probably already the biggest single pesticide product even before biotech crops.” Farmers need to control pests,  “to not control pests to a reasonable degree is problematic for the environment,” Savage wrote. For them, glyphosate is a much safer option compared to many other agrochemicals.

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