People and livestock have been eating GM crops for nearly 30 years without a single documented case of harm to either one of them. Due to the intense debate about these crops and regulatory measures in North America and Europe, there have now been over 500 scientific studies looking for – and failing to find – conclusive risk to human health from GM crops. This isn't surprising: GMO crops are designed with human safety in mind.
For instance, if someone were trying to create a new GM plant by introducing a gene to produce an insect-killing toxin, they'd first comb through the scientific literature, and then perform additional experiments to get a handle on how the gene itself works. They'd ask many, many questions. Does the gene look like any known allergen or toxin? Does it affect any biochemical process that's common in plants, insects, and humans? Does it affect a part of the target insect's biology that doesn't exist in humans? Thanks to the fact that scientists now have access to over 100 million different DNA sequences from a vast variety of organisms, we can check this data to see whether the chosen gene resembles known toxic genes.
While these steps provide a measure of confidence about how a particular gene could affect human biology, scientists perform more conclusive tests prior to releasing the GM plant. These tests include simulating the behavior of GM proteins in the human gut to see whether the proteins degrade during digestion. The most conclusive tests require rats, which are first fed a single meal of the GM crop or protein to test for acute toxicity, i.e. the type of immediate poisoning that happens if, say, someone were to drink something like cyanide. Next, rats are fed repeated meals of GM food for 90 days – and sometimes up to a year –to test for chronic toxicity, which is the type of harm that only appears with repeated use, as with tobacco.
Animal studies that show that GM crops are safe may be reassuring, but a formal study funded by the European Commission in 2013 found that they were superfluous to the safety assessment if the GM crops had been found to be substantially equivalent to their non-GM counterparts. After all, it doesn't make sense to keep testing safe food on rats if we, as a society, also want to reduce the use of animal testing.
Current GM plants use some of the most studied genes in agriculture, and our understanding of their core biology is a major factor in the scientific consensus about GMO safety. Take the example of the Bt toxins, some of the most commonly used genes in GMOs today. Bt toxins are insect-killing proteins produced by a natural bacterium, Bacillus thurigiensis. Dead Bt bacteria were used in organic farming sprays for years, but in some crops, like cotton and corn, insects still penetrated deep within the plant. Sheltered inside the crops, these invading insects are protected from insecticidal sprays, be it an organic Bt-spray or a chemical insecticide.
Hence, scientists transferred some of the genes producing these Bt toxins into the the genome of the crop plants, reasoning that if the plants could produce their own insecticide, farmers would no longer need to spray chemicals to kill damaging pests.
This approach worked beyond all expectations. Bt crops have become one of the fastest-adopted agricultural technologies in history. There are over 600 Bt cotton varieties currently produced and sold in India alone, increasing farmer profits by thousands of rupees annually. At the same time, we have a very good understanding of how Bt toxins work, and why they are harmful to only a small number of insects. In early experiments scientists first injected pure Bt toxins directly into caterpillars – and the caterpillars survived. But caterpillars that had eaten food laced with the toxins died immediately. This hinted at the fact that the toxins need to enter the caterpillar's gut to become active.
Several studies later confirmed that Bt toxins can only work in alkaline environments and require specific enzymes and receptors in the insect gut to cause toxicity. Humans (and other mammals) have very acidic stomachs and lack these enzymes and receptors – the locks to the Bt toxin key – and so are not affected by them.
So far, all commercially available GMO crops, anywhere in the world, have been found to be "substantially equivalent" to their non-GM counterparts. This means that GM plants are just as safe – or un-safe – as non-GM plants of the same species and variety.
Substantial equivalence means that, a GM plant is considered as safe as a non-GM version if it meets the following qualifications:
- It has a few genes added compared to its non-GM counterpart (about 10 of around 30,000)
- It has very similar resulting chemistry
- It doesn't produce any toxic or allergenic compounds
A large part of testing for this standard involves measuring whether the most important nutritional properties of the plant – like dietary fiber, protein, fat, and vitamins – are identical between a GM plant and its non-GM relative.
Another health concern that people often have regarding GMOs is allergenicity, i.e. the likelihood that they could induce an allergic reaction. During the safety testing process, scientists use mice to test both the proteins expressed by the GM plant, and the GM crop itself, for allergenicity, but those results don't necessarily translate to humans. Researchers have also performed tests on patients suffering from allergies, and didn't observe any allergic reactions to GMOs.
No GM food available today has been found to promote or cause allergies. In fact, in a 2008 paper published in Nature Biotechnology, a group of allergy and food experts recommended against the current, excessive, allergenicity testing of GMOs.
The relative non-allergenicity of current GMOs isn't surprising. One of the few shared characteristics of allergy-provoking foods is that the specific protein that causes an allergy is present at very high levels. In peanuts, allergy-causing proteins are found at 1,000-10,000 parts per million. In GM crops on the market, the engineered proteins are present at much much lower concentrations, sometimes as much as 100,000 times lower. These facts – GM proteins are present at low levels, don't look anything like the allergy-causing proteins we know of, and have passed repeated testing – suggest that they are very unlikely to cause or promote allergies.
All of this is not to say that GM foods will always be nutritionally identical to their non-GM counterparts. A major goal of making new GMOs is to improve the nutritional content of our most commonly consumed crops. The Golden Rice project, for example, is an attempt to increase the provitamin-A content in the grains of some of the most commonly used rice varieties in Southeast Asia. Other health-promoting GMOs in the pipeline include the Purple Tomato (rich in antioxidants); aflatoxin-free maize (which is free of a dangerous fungal toxin that contaminates a lot of maize plants); iron and zinc rich rice and wheat plants; vitamin-B9 enriched rice; and vitamin-B6 enriched cassava plants designed for African consumers.
The key message when thinking about GMO safety is that the nutritional properties of a plant depend on what genes a plant has, not how they got there. And the properties of individual genes are often not related to the species from which they originally hail. That is to say that a gene from a peanut moved into corn will not make that corn more "peanut-y," or make it allergenic.
In fact, the broad arc of evolution has ensured that a lot of the genes present in different plants and animals are very similar, or perform similar functions. We humans, for example, share 60 percent of our genes with bananas! Hence, in the Golden Rice project, researchers first took a gene from daffodils and put it into rice to produce provitamin-A, but they were able to reach the same basic result, i.e. provitamin-A rich rice, by using the same gene from maize instead (the maize version of the gene performed a bit better in rice than the daffodil one). It doesn't matter where a gene comes from, we just need to know what it does.