Why reassurances on Bt toxins don't add up
Since the paper's publication, the biotech industry and its supports, and regulators, have done their best to offer reassurances. The item below is the answer to a Parliamentary Question by Anne Milton, a UK Government Minister - the no. 2 at the Dept of Health.
COMMENT by Bill Freese, who's a Science Policy Analyst with the Center for Food Safety:
Her remarks [below] make clear that Bt proteins in GM crops have not really been subject to independent safety testing; rather, EU and US regulators have relied on prior assessments of Bt proteins used in Bt sprays.
1) rapid degradation of externally applied Bt spray proteins in UV light (within days to a week or two), which does not happen with Bt proteins embedded in Bt crops;
2) Bt proteins engineered into crops are active toxins, while Bt insecticides in sprays are predominantly in the "protoxin" form that requires processing to become active toxins; and
3) In most cases the amount of Bt toxin in Bt crops will be higher than the amount that is sprayed on, making it more likely that the Bt toxins detected in the study originated from Bt crops, not Bt sprays.
Second, the in vitro digestibility tests she mentions are a joke. They involve putting a bacterial surrogate of the Bt toxin actually found in the crop in a test tube with highly acidic pH (normally 1.2) and high concentrations of a single digestive enzyme (pepsin). Actual gut conditions can be much milder (higher pH), particularly after ingestion of food, meaning less rapid breakdown. To the extent these tests are worth anything, they have been carried out under conditions quite a bit harsher (fostering rapid breakdown) than those recommended by experts who wrote a 2001 FAO-WHO report on testing transgenic proteins for allergenic potential.
Third, while it is true that the study did not find a health risk, the fact is that the authors did not set out to test for health impacts. They performed instead a crucial first step, to discover whether or not there is systemic exposure (i.e. passage from gut into the bloodstream) to Bt insecticides. And their conclusions - assuming no errors in their detection method demonstrated precisely the systemic exposure that the largely worthless in vitro digestibility assays were supposed to preclude.
For a critique of the "in vitro" digestibility assays and other points re: safety testing as applies in particular to Bt crops, see http://www.centerforfoodsafety.org/wp-content/uploads/2011/05/BGER-PAPER.pdf
13 Jun 2011 : Column 638W
Food: Genetically Modified Organisms
Hansard: House of Commons Written Answers
Zac Goldsmith: To ask the Secretary of State for Health what recent reports he has received on the discovery in the human blood stream of Bt pesticide derived from genetically-modified components attributable to the consumption of meat and dairy from animals fed on genetically-modified feed. 
Anne Milton: In the European Union, genetically modified (GM) foods undergo a rigorous pre-market evaluation that includes the safety of components introduced into GM crops, such as Bt proteins. The safety of Bt proteins has also been assessed during the authorisation of pesticides that contain the bacterium Bacillus thuringiensis. The assessment of the Bt proteins that are present in authorised GM foods is based on tests in animals and does not include human trials to examine whether the proteins pass into the blood supply. However, the available data from in vitro tests have shown that Bt proteins are rapidly broken down by digestive enzymes in the same way as most other proteins in the diet.
A Canadian publication has recently suggested that small amounts of one Bt protein may be present in human blood, at around one part per billion, although it is not clear whether the analytical methods that were used in this study were suitable for identifying and quantifying such low levels in human samples. If the finding is correct, the design of this study did not provide any evidence of the source of the protein, which is present in commercial pesticide preparations as well as in some GM crop varieties. The detection of trace levels of Bt proteins in people does not necessarily imply any risk to health.