There are many types of Bt toxin in the wild and in genetically engineered plants; research has raised safety concerns relating to some of them. Declaring them all safe is premature, and here’s why.

Thus far, the Environmental Protection Agency (EPA) has concluded that all Bt endotoxins expressed in genetically engineered (GE) plants meet the basic, statutory safety standard – “reasonable certainty of no harm” – following the expected exposures when people consume foods harvested from, and/or made with, GE, Bt-crops. In reaching this conclusion, the EPA makes three different assessments in order to assure that three basic criteria are satisfied. It describes its approach in assessing Bt crop safety as follows:
“Several types of data are required for the Bt plant-incorporated protectants to provide a reasonable certainty that no harm will result from the aggregate exposure to these proteins. The information is intended to show that the Bt protein behaves as would be expected of a dietary protein, is not structurally related to any known food allergen or protein toxin, and does not display any oral toxicity when administered at high doses. These data consist of an in vitro digestion assay, amino acid sequence homology comparisons and an acute oral toxicity test [italics added].”^[1]
Our review of the scientific literature supports some aspects of the EPA’s assessment, but not all.
First, some Bt proteins appear to enter the mammalian blood stream. From there they would be able to move into various organs and cause harm.
Second, the EPA assumes that Bt proteins break down very rapidly due to the highly acidic conditions in the human stomach and that these fragments are harmless. The agency pays little attention to a number of abnormal but common stomach conditions known to retard the breakdown of Bt endotoxins. It also ignores the portion of Bt toxins that directly enter the bloodstream via the mouth and tongue.
Third, little or no effort has been invested in studying the toxic properties or allergenicity (potential to cause allergies) of the fragments of Bt toxins. The smaller, truncated Bt proteins produced in GE plants could bind to cells lining the human gut and thereby affect a number of physiological functions.
This lax approach in assessing Bt endotoxin risk is in stark contrast with the thorough and rigorous evaluation EPA scientists conduct in tracing the breakdown products of chemical pesticides in the food supply.
Legitimate questions have been raised about the true fate of Bt toxin in vivo (i.e., in the stomach of a live person)^[2] and also about chronic (as opposed to acute) toxicity related to repeated ingestion of Bt toxin. There are also legitimate concerns arising from the fact that the Bt toxins used in the acute oral toxicity tests required by the EPA are not identical to the ones expressed by GE plants.

What Bt toxins are produced in GE plants?

Bt toxins, which are used in agriculture because they are deadly to certain insects, are produced by various strains of a soil bacterium called Bacillus thuringiensis (Bt). There are many strains of this bacterium, and each produces a different toxin.^[3] Each Bt toxin is made up of a so-called “crystal” protein. A Bt toxin may also have a second protein called a “cytotoxic” protein. Through genetic engineering, a gene coding for a crystal (Cry) protein toxin is isolated from a strain of B. thuringiensis; after being manipulated in a laboratory to maximize expression in a GE plant, the gene is inserted into the genome of the recipient plant.
Some have argued that since some Bt toxins are allowed in organic farming, there should be no concern about food derived from plants expressing Bt toxins. However, there are important differences between the Bt toxins sprayed for decades on the leaves of organic and conventional crops to protect them from certain insects and the Bt toxins expressed inside the cells of GE crop plants. These differences have an enormous impact on dietary exposures and, hence, on safety assessments.
Some differences relate to the modifications made to the original bacterial genes that allow the plants to manufacture an activated form of the Bt toxins. These modifications are made in a laboratory prior to transfer into the recipient crop, in part because the original, native bacterial genes are more complex and difficult to move into plants, and also because plants have to expend more energy to produce the untruncated form of Bt emitted by soil bacteria.
Other important differences between GE-Bt crops and non-GE crops sprayed with a liquid Bt bioinsecticide result in a significant change in the degree of risk of dietary exposure:
1) Bt sprayed in liquid form on the leaves of plants breaks down quickly (almost always within 48 hours), and hence rarely even gets onto the harvested portion of the crop. But GE-Bt endotoxins inside corn plant cells – roots, stems, leaves, corn cobs, pollen, and kernels – are protected from the sun, rainfall, and other elements that rapidly break down the Bt in liquid sprays. Bt endotoxins can persist for months inside corn crop residues, as well as harvested kernels.
2) This results in another difference between Bt sprays and Bt GE plants: Spores, sprayed on vegetables can be washed off; Bt toxins inside food cannot.
3) Bt liquid sprays are applied at a very low rate per acre, rarely amounting to more than 0.01 pound of Bt toxin per application. GE corn plants, on the other hand, produce between 0.1 and nearly 4 pounds of 1 to 6 different Bt toxins per acre. (Fruit and vegetable farmers often spray liquid Bt products 3, 5, or even 8 times in a growing season. Accordingly, multiple applications of Bt must be taken into account when comparing the total volume of Bt, by weight, applied per acre on conventional corn crops, versus the volume of Bt endotoxins expressed by the GE-Bt corn plants growing on an acre over a full growing season.)
The significance of these differences between native Bt toxins and the GE-Bt toxins for human health in the long term are not known because safety testing for chronic effects has not been required by the EPA^[4] and very little funding from government science agencies has been made available to independent scientists interested in refining Bt endotoxin exposure and risk assessments.
What is more, there has been almost no safety testing of the Bt toxins expressed in GE plants, neither on one toxin at a time nor (even less so) on the common combinations of Bt endotoxins found in today’s commercial Bt corn and Bt sweetcorn varieties.^[5]
In short, today’s lax regulatory treatment of Bt crops rests on assumptions no longer consistent with well-documented science. The risks arising from modern GE-Bt varieties expressing multiple Bt genes are assumed by the Food and Drug Administration to be no different from risks arising from exposure to one Bt toxin at a time.

How do Bt toxins kill insects?

Cry proteins work by targeting a receptor in the gut of certain insect larva.^[6] After binding to the toxin, the intestinal wall becomes damaged, causing liquids in the stomach to leak into the body cavity. Impacted insects become dehydrated, stop feeding, and die.
Because different Cry toxins bind to different receptors on the surface of cells in the insect gut, and target different species of insects, crops can be engineered to poison the insects most likely to damage that particular crop. For example, one set of Cry proteins introduced into corn plants helps control the European corn borer and other insects attacking corn stalks, while another class of Bt toxins targets the corn rootworm and other soil-dwelling insects that attack corn roots.
Since there is no required safety testing, it is unknown whether they bind to receptors on the surface of human cells, in particular those of the gut, or affect them in any other way.

Bt toxins are not selective enough

In 1998 a team of researchers from Egypt reported the results of tests of the Bt toxin produced by Bacillus thuringiensis var. kurstaki on mice.^[7] They divided mice into three groups: one (the control group) was fed non-GE potatoes, the second was fed genetically altered potatoes expressing the toxin, and the third ate non-GE potatoes sprayed with the toxin.
The three groups were sacrificed and their intestines were examined using an electron microscope. Remember, Bt toxins target insect larva guts – mice are mammals and therefore should be unaffected. The mice who were fed unmodified potatoes sprayed with the toxin were clearly the most severely impacted. Their intestines showed changes that would impair the nutrient absorption. Mice who were fed the genetically modified potato that expressed the Cry protein also had a few harmful cellular changes (damaged mitochondria, for example). Although a statistically significant conclusion was not reached for that specific finding, these changes signal the need for further study.
This variety of potato is not available on the market, but the study makes it clear that we cannot be reassured that Cry proteins have effects only on insects.
The results of this study are consistent with the conclusion that this particular Bt toxin can be harmful to mammals. We must study each new version of Bt toxin that is applied to our food crops to determine if it affects the human intestines and pay particular attention to those Bt crops that express relatively high levels of multiple Bt toxins in the harvested part of the crop that is eaten in various forms by people, farm animals, and pets.

Cry1Ab and Cry1Ac

In a study published in 2012, rats that had been fed for 3 months on GE corn expressing a specific crystal protein called Cry1Ab were found to have damaged intestinal surfaces, as well as damage in major organs including the liver, kidneys, and testes.^[8]
Another recent study on offspring of sows fed GE corn expressing Cry1Ab showed changes in the microscopic appearance of the gut.^[9] While the authors of this study state in the abstract of this paper that this GE corn is not harmful, the body of their paper does in fact document statistically significant differences in gut health, liver and spleen size, and weight gain. (Please note that claims of safety are often made in the abstracts of peer-reviewed scientific papers, despite the presence of worrisome results in the main body of the text.)
A study of mice fed GE corn expressing Cry1Ab documented changes in the immune function of the gut.^[10] As a large portion of the human immune system operates in the gut and the gut is critical to the proper development of the immune system, we would expect these changes to affect health.^[11] 
GE food crops expressing Cry1Ac have also been implicated in altering immune function. The toxin Cry1Ac was found to bind to receptors in the mouse intestinal mucosa,^[12] raising the possibility that this Bt toxin may have a clinically relevant effect on mammals. Cry1Ac was also found to cause a stimulation of the immune system in mice similar to that caused by the cholera toxin.^[13] The EPA noted these findings in their document declaring that these toxins were safe for humans. However, they had a different interpretation of the findings and also did not remark on later research by the same authors suggesting that Cry1Ac co-administered with another food protein increases the chance of allergy to the other food protein.^[14]
These results raise concerns for physicians. A substance that injures the intestinal mucosa and modifies the activity of the immune system would be expected to play a role, likely negative, in health and disease.
The studies mentioned above contradict the notion that GE crops expressing Bt toxins have been decisively shown to be safe for human or other animal consumption. Our conclusion, based on these studies, is that the Bt toxins that are produced in various GE food crops can, at least in some cases, cause harm to the intestines, which could adversely affect the immune systems of mammals.
Given the damage seen in mice fed GE potatoes, and in rats, mice, and pigs fed GE-Bt corn, it would be prudent to test the intestines of mammals fed Bt crops for extended periods. There has been nowhere near enough high-quality independent science focused on the effects of Bt toxins on the mammalian gastrointestinal tract and immune system, yet several next-generation Bt fruit and vegetable crops are under development around the world.

Bt toxins are likely not completely broken down in mammalian guts

In a 2010 in vitro study, a group of French scientists showed that Cry1Ab proteins are extensively degraded only at an extremely low pH typically used in toxicology experiments (pH 1.2), that they are only slightly degraded at pH 2.0, and are stable at the slightly higher pH that is not uncommon for the human stomach.^[15] Thus, Cry1Ab toxins would be expected to be stable (not degraded) in the stomachs of many people taking medications that lower stomach acid to a gastric pH above 4.^[16] This could adversely affect their intestines and could even result in Bt toxins circulating in the bloodstream.
In 2011, a study by Canadian scientists documented the presence of herbicide metabolites and Cry1Ab in the blood of pregnant women and the umbilical cord blood of their newborns.^[17] These results were unexpected and controversial, because it was previously believed that these compounds remain in the gut, are excreted in the stool, and are not absorbed into the bloodstream. The study was criticized in part because the probe the researchers used to identify Cry1Ab may have identified only a fragment of Cry1Ab, not necessarily the intact form of the protein present in Bt sprays or GE crops. This one study does not prove that functional forms of Bt toxin are typically present in the human bloodstream, but it does raise questions that need to be explored using more sensitive and rigorous study designs.

Combinations of chemicals

Lastly, the fact that Bt toxins have not been studied in combination with other chemicals commonly found in or applied to GE plants also raises concerns. For example, corn varieties are frequently genetically engineered to be both Roundup Ready® and express multiple Bt toxins. In one study in which this combination was analyzed, results indicated that applying Roundup® herbicide on GE-Bt corn plants modified the effects of Cry1Ab toxins, but not those of Cry1Ac.^[18] Evidently, unintended interactions can occur between proteins expressed in GE plants and the chemicals applied to them.
Conclusion: Certain Bt toxins such as Cry1Ab and Cry1Ac have been found to adversely impact mammals. The claim that all Bt toxins are safe for humans and other animals is therefore not based on a comprehensive review of the available scientific evidence. Furthermore, breakdown products of Cry1Ab – at the very least – have been identified in human blood, indicating that we cannot assume that these proteins are completely degraded during digestion or that they are benign once they enter the bloodstream.
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