Truth: GM Bt insecticidal crops pose hazards to people and animals that eat them

Myth at a glance

Many GM crops are engineered to produce the insecticide Bt toxin. Regulators have approved GM Bt crops on the assumption that the insecticidal toxin they contain is the same as the natural form of Bt toxin, a substance produced by the soil-dwelling bacterium Bacillus thuringiensis.

Natural Bt is used as an insecticidal spray in chemically-based and organic farming, and is claimed to have a history of safe use and to only affect certain types of insect. Regulators assume that GM Bt crops must also be harmless to humans and other mammals.

But these assumptions are incorrect. Natural Bt toxin is different from the Bt toxins produced in GM crops and behaves differently in the environment. GM Bt plants express the pesticide in every cell throughout their life, so that the plants themselves become a pesticide. Even natural Bt has never intentionally been part of the human diet and cannot be claimed to have a history of safe use.

Animal feeding experiments with GM Bt crops have revealed toxic effects and a laboratory study showed toxic effects on human cells tested in vitro. Bt toxins and Bt crop pollen and debris have toxic effects on non-target and beneficial organisms.

Contrary to claims by the GM industry and regulators, Bt toxin does not reliably break down in the digestive tract. Bt toxin proteins have been found circulating in the blood of pregnant women and in the blood supply to their foetuses.

Regulatory approvals of GM Bt crops worldwide have been granted on the basis of poorly designed and interpreted experiments and false assumptions.

Bacillus thuringiensis (Bt) is a natural soil-dwelling bacterium that produces a protein complex called Bt toxin. Some types of Bt toxin possess selective insecticide properties: that is, they will specifically kill certain crop pests such as caterpillars. Therefore Bt toxin has been used for decades as an insecticidal spray in chemically-based and organic farming. Genetic engineers have engineered Bt toxin into GM crops so that they produce their own form of this insecticide.

Regulators have approved GM Bt crops largely on the assumption that the GM Bt toxin is the same as the natural Bt toxin, which they say has a history of safe use. They conclude that GM crops engineered to contain Bt insecticidal protein must also be harmless. However, this assumption and conclusion are incorrect.

Bt toxin in GM plants is not the same as natural Bt toxin

The Bt toxin expressed by GM Bt plants is different from natural Bt, both in terms of its structure and its mode of action.1 Structurally, there is at least a 40% difference between the toxin in Bt176 maize (formerly commercialized in the EU, now withdrawn) and natural Bt toxin.2 The US Environmental Protection Agency, in its review of the commercialized Monsanto GM maize MON810, said it produced a “truncated” version of the protein – in other words, a much shorter form of the protein that is different from the natural form.3

Such changes in a protein can mean that it has very different environmental and health effects. First, the GM Bt toxin loses its selectivity and can kill non-target insects including beneficial predators. Second, GM Bt toxin can have unsuspected negative health impacts on people or animals that eat a crop containing it. The protein may be more toxic or allergenic than the natural form of the protein.

Even tiny changes in a protein can completely change its properties. For example, soybeans can be genetically engineered to tolerate a herbicide that would normally kill them by changing a gene that gives rise to a protein differing from the natural protein by just two amino acids.4 As researchers at the Centre for Integrated Research in Biosafety in New Zealand pointed out in a submission to the Australia/New Zealand GMO regulator FSANZ on the regulatory assessment of this soybean,5 a change even of a single amino acid can radically change the properties of proteins, which in turn can result in changed behaviour of a plant.6,7

In some cases, not even an amino acid change is necessary to alter the characteristics of a protein. Differences in the sequence of the DNA base units in a gene can change the properties of the resulting protein without altering the amino acid sequence.8Changes in the three-dimensional shape of the protein alone can turn harmless proteins into toxins,9,10 as demonstrated by the prion protein causing the “mad cow disease” BSE.11

Natural Bt toxin also has a very different mode of action from the Bt toxin produced in GM plants. Natural Bt is not a toxin but a protoxin. That means it only becomes toxic when subjected to certain conditions, such as when made into a solution and broken down by enzymes in the gut of the insect that eats it.

In the environment, natural Bt breaks down rapidly in daylight soon after it is sprayed, so it is unlikely to find its way into animals or people that eat the crop. With GM Bt crops, in contrast, the Bt toxin is present in every cell of the plant in pre-activated form.1,12 The plant itself becomes a pesticide, and people and animals who eat the plant are eating a pesticide.

Bt toxin does not only affect insect pests

GMO proponents claim that the Bt toxin engineered into GM Bt crops only affects the target pests and is harmless to mammals, including people or animals that eat the crops.13 All regulatory approvals of GM Bt crops are based on this assumption and no regulatory body has ever required human toxicity studies to be carried out.

However, these assumptions about the safety of GM Bt crops are constantly being challenged by new evidence.

In an in vitro study (laboratory experiment not carried out in living animals or humans), genetically engineered Bt toxins were found to be toxic to human cells. One type of Bt toxin killed human cells, albeit at the relatively high dose of 100 parts per million. The findings showed that GM Bt toxin is not specific to insect pests and does affect human cells, contrary to claims from the GM lobby and regulators.14

In vitro studies may not accurately reflect what happens in a living human or animal that eats GM Bt crops, so they must be followed up with in vivo studies performed on living animals, and then on humans. However, it is unacceptable that Bt toxins were never even subjected to basic and inexpensive in vitro tests before they were released into the food and feed supply.

Some feeding studies in mammals have been performed with GM Bt crops and have found adverse effects, such as:

  • Toxic effects or signs of toxicity in the small intestine, liver, kidney, spleen, pancreas15,16,17,18,19
  • Disturbances in the functioning of the digestive system17,19
  • Increased or decreased weight gain compared with controls15,20
  • Male reproductive organ damage19
  • Blood biochemistry disturbances20
  • Immune system disturbances.21

Laboratory studies in mice found that genetically engineered Bt toxin produces a potent immune response when delivered into the stomach by intragastric administration (a method considered similar to human dietary exposure), or injected into the abdomen (intraperitoneal immunization).2223 The Bt toxin protein was found to bind to the mucosal surface of the small intestine of the mice, an effect that could lead to changes in the physiological status of the intestine.24The Bt toxin protein also enhanced the immune response of the mice to other substances.25

GM Bt crops and the Bt toxins they are engineered to contain have been found to have toxic effects on butterflies and other non-target insects,26,27,28 beneficial pest predators,29,30,31,32,33,34 bees,35 aquatic organisms,36,37 and beneficial soil organisms38 (see Myths 2.3, 5.3).

Toxic effects associated with GM Bt crops may be due to one or more of the following causes:

  • The Bt toxin as produced in the GM crop
  • New toxins produced in the Bt crop by the GM process, and/or
  • Residues of herbicides or chemical insecticides used on the Bt crop. Many Bt crops have added herbicide-tolerant traits,39 making it likely that herbicide residues will be found on them.

In-depth toxicological research would have to be carried out in order to identify which factors are responsible.

Bt toxin protein may not be broken down harmlessly in the digestive tract

GMO proponents claim that the Bt toxin insecticidal protein in GM plants is broken down in the digestive tract and so cannot get into the blood or body tissues to cause toxic effects beyond the digestive system. But this claim has been shown to be false by several studies:

  • A study in cows found that Bt toxins from GM maize MON810 were not completely broken down in the digestive tract.40
  • A study simulating human digestion found that the Bt toxin protein was highly resistant to being broken down in realistic stomach acidity conditions and still produced an immune response.41
  • A survey conducted in Canada found Bt toxin protein circulating in the blood of pregnant and non-pregnant women and the blood supply to foetuses.42,43 Whether the Bt toxin originated from GM crops or elsewhere is not known. But wherever it came from, it clearly did not break down fully in the digestive tract.

How selective are the Bt toxins in GM crops?

Monsanto argues that Bt toxins only affect a certain class of insects and are non-toxic to mammals, including humans.44 However, Bt toxins have been found to have toxic effects on non-target organisms other than insect pests – including mammals.

For example, in one study, Bt toxins were found to be toxic to the blood of mice.45 This was not a feeding study with Bt crops, so the findings do not tell us whether GM Bt crops are toxic to the blood of mice. Instead the Bt toxins were fed to the mice in the form of spore crystals containing individual Bt toxins Cry1Aa, Cry1Ab, Cry1Ac, and Cry2A obtained from genetically engineered Bt bacteria. Different GM Bt crops are engineered to express these Bt toxins. The Bt toxins caused red blood cells of the mice to rupture, albeit they were fed at high doses.45

This is of concern because Bt toxins exercise their toxic effects in target pests in a similar fashion, by rupturing the cells of the gut, causing the insect to die from starvation or septicaemia due to the gut contents, including pathogenic bacteria, leaking out into the body. This study showed that the assumption that Bt toxins are non-toxic to mammals is questionable, as the Bt toxins in the genetically engineered spore crystal form tested were toxic to the blood of mice, a species of mammal.45

Also, a wide range of external factors can influence the selectivity and toxicity of Bt toxin proteins. These include interaction with infectious disease agents, nematodes (roundworms, many of which are parasitic), gut bacteria, and other Bt toxins.46

It cannot even be assumed that the natural Bt toxin used in insecticidal sprays is safe for those applying it or exposed to it immediately after spraying. In farm workers, exposure to Bt sprays was found to lead to allergic skin sensitization and immune responses.47 An immune response to Bt toxin was found in the blood serum of 23–29% of Danish greenhouse workers in a respiratory health study.48

Regulatory assessment of Bt crops flawed

Some of the safety tests carried out for regulatory approvals of Bt crops, such as investigation of allergenic, nutritional, and immunological properties, are not carried out with the Bt toxin protein as expressed in the GM plant. Instead, tests are carried out on a “surrogate” Bt toxin protein derived from genetically engineered E. coli bacteria,49 as GM companies find it too difficult and expensive to extract enough Bt toxin from the GM crop itself.

The problem with this is that the protein that is expressed in a plant will be different in structure, conformation and stability from the protein expressed in a bacterium. Thus it is scientifically invalid to draw conclusions about the safety or digestibility of a protein in a GM plant on the basis of experiments on a protein produced in E. colibacteria, even if the two proteins are coded for by the same gene.49

This fundamental flaw in the regulatory process could partly be addressed by long-term animal feeding trials with the whole GM plant, which would contain the actual protein that people and animals eat. Although the 90-day animal feeding trials that are routinely carried out by GM developer companies are not long enough to identify the full range of potential toxic effects from GM crops, studies of even this short duration and less performed by both industry and independent scientists have revealed worrying health effects.15,16,18,50,19,20 Unfortunately, these effects are routinely dismissed.

Another problem is that the 90-day animal feeding trial required in Europe for single-trait GM crops does not apply to stacked-trait GM crops, many of which incorporate multiple Bt insecticidal traits. Instead, regulators assess the safety of the stacked-trait crop on the basis of the company’s animal feeding trials on the single-trait varieties that were cross-bred to create the stacked-trait crop. This process is scientifically invalid, as unintended changes can result from the process of combining the traits into the stacked-trait crop and the total Bt toxin content will be higher than in single-trait Bt crops.


Studies on GM Bt crops show that Bt toxin is not specific to a narrow range of insect pests but can affect a wide variety of non-target organisms. Taken together, the studies on GM Bt crops and natural Bt toxin raise the possibility that eating GM crops containing Bt toxin may cause toxic effects to multiple organ systems or allergic reactions and/or sensitize people to other food substances.


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