What Do We Mean By “GMO”?

November 12, 2015


There is a perhaps no more contentious issue in our food system today than the role of genetically modified organisms or GMOs. GMOs come up most commonly in debates about whether they should be labeled on food packages. But many times it appears that people aren't talking about the same thing when they refer to a GMO. So, what exactly are we talking about? In this blog post, I attempt to provide a basic overview of what we mean by "GMO" and why many food and environmental groups advocate labeling them. 

First, let's explain what genetic modification in this context refers to. In the U.S., the majority of corn, soy, sugar beet, cotton, and canola crops are genetically modified in a particular way. These crops have been genetically modified by using a bacteria or virus to insert a foreign gene into the seed’s DNA. This is very different from the traditional means of cross-breeding, where two types of plants cross-pollinate. Cross-pollination occurs naturally in the wild when bees, wind, etc., cause two different plant species to cross and come up with something new. (For example, this is how we get tangelos -- a cross between two citrus varieties, tangerine and grapefruit.) Genetic modification, on the other hand, occurs in a lab, when the bacteria or virus containing the new DNA is "shot" into the DNA of the seed. There are some additional means of genetic modification that involve manipulation through chemicals and radiation, but in the labeling debate, the majority of GMO crops that are discussed involve this type of technology.1 


Second, the genetic modification of these crops is typically for one of two purposes: (1) to be able to withstand herbicides (weedkillers) so that when they are sprayed, only the surrounding weeds and not the seed are killed, or (2) so that the GMO seed itself produces an insecticide that, when eaten, kills pests. 


Both the seeds and the pesticides raise potential issues. Let’s look first at the seed.


As I just mentioned, GMO seeds generally fall into two categories. The first category refers to seeds that are herbicide-resistant. The idea is that the foreign gene that is inserted into the seed's DNA expresses a protein, which gives the seed its resistance to the herbicide. It used to be the case that we believed that one gene expressed one protein. Thanks to the Human Genome Project, our understanding of genetics has changed. We now know that the bacteria or virus that is inserted with the gene can turn on more than one gene, which causes other proteins to express themselves, and those newly expressed proteins can have unanticipated and allergenic effects. Additionally, the insertion of foreign proteins in the seeds' DNA might lead to the production of new, unanticipated proteins in the seed that can also have allergenic effects. 


The second category are those crops, like Bt corn, that produce their own insecticides. Bt corn has been engineered so that it produces a toxin harmful to insects like the corn rootworm. It’s like having a built-in insecticide. Virtually ever cell in the Bt corn produces this insecticide, including the ones we consume. The Bt toxin can survive digestion, as studies have found it in people’s blood. One concern is that this toxin could harm our gut bacteria, thus compromising people’s immune systems. 


Let’s look next at the herbicides. The key ingredient of Monsanto's Roundup, one of the most widely used herbicides with GMOs, is glyphosate. The World Health Organization recently categorized glyphosate as a "probable human carcinogen." In addition to its potential carcinogenic effects, there is concern that, like Bt toxin, glyphosate residues also may contribute to compromising our gut bacteria or intestinal health. 


While manufacturers insisted that weeds would not develop resistance to glyphosate, we know now that is not true. As a result, GMO producers now require additional, more powerful pesticides. For example, in 2014 the EPA approved a new herbicide, Enlist Duo, which is a combination of glyphosate and 2,4-D (one of the chemicals in Agent Orange used in the Vietnam War). 


As a result of the EPA's approval of Enlist Duo, two researchers, in a piece published in the New England Journal of Medicine, concluded that GMOs should be labeled largely because of the herbicides' safety concerns.   See http://www.nejm.org/doi/full/10.1056/NEJMp1505660


This is the nutshell version of what a GMO is. The next time you read about GMOs and labeling in the press and social media, what is generally being discussed are seeds that have had DNA injected into it to resist herbicides or to produce its own insecticide. This system raises various potential concerns for human safety (particularly for the health of our microbiome or intestinal health), and environmental health.  


For more in-depth reading, here are some additional links:








1. There is another class of GMOs, which involves genetically modifying the organism to give it a trait that is desirable: for example, oranges that resist blight, papaya that resist fungus, drought-resistant wheat, "golden rice" that provides more nutrients, salmon that grow quickly, or apples that do not brown. This other category constitutes a very small percentage of GMOs, although manufacturers often cite these types of GMOs as necessary to feed the world. These are an incredibly small percentage of GMOs, and so are not at the center of the labeling debate. In any case, we might be asking ourselves what else we’re doing to our environment that would cause them to be more vulnerable to blight and fungi. And do we need fast-growing salmon or apples that don’t brown? To date, there is no successful “golden rice.” 

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