Question: If GM foods become commonplace, will our digestive systems need to adapt at all, to process patterns of DNA that have hitherto not been part of creation? If so, what sort of stresses might this adaptation impose on the human body?

Ricarda Steinbrecher answered on 3 Jul 2012:

Currently, the modified DNA present in GM organisms and food is made up of the same building blocks as any other DNA (which might change in future with certain applications of synthetic biology). Digestion should thus – in principle – be the same. DNA is a long molecule made up of two strands with solid bridges across, somewhat like a ladder. It is thus quite stable and resilient. What happens during digestion in the gut is that DNA gets partially broken down, with fewer and fewer long pieces remaining. It is important to remember that genes of higher organisms, such as plants and animals, are usually quite long and made up of stretches of coding sequences interspersed with non-coding sequences (coding means they carry the information required to build a protein). GM genes are noticeably shorter. They are made up of coding sequences only (as well as regulatory elements), like genes from bacteria. There has been concern that due to their short length a percentage of the GM genes will survive digestion, ie remain intact, and become available to bacteria in the gut.

The gut flora consist mainly of several hundreds of species of bacteria, mainly beneficial but some pathogenic. Their total number and exact species composition are still unknown but for a rough idea: there are more bacteria in the gut then there are cells in the human body and they play a crucial role in our ability to digest food, but can also cause illness if out of balance.

Bacteria are very good at taking up bits of DNA from their environment and integrating them into their own genome: that’s their way of evolving, of adapting to changing environments or simply becoming fitter than other bacteria. This is called ‘horizontal gene transfer’ (HGT).

Whilst (gut) bacteria would never have been able to pick up a gene from a plant, they can – at least in theory – pick up GM genes, as these are short and don’t have any non-coding sequences (called introns). This is made much more likely by the fact that a number of DNA sequences used in genetic engineering originally come from bacteria so there are fewer barriers to our gut bacteria taking them up if they encounter them in the food.

Very often a bacterial gene for antibiotic resistance is used as an additional genetic modification of many GMOs – not because the GM plant will need it but because it makes it easier for the researchers in the lab to distinguish between modified and non-modified cells and kill off the un-modified ones. After this they usually remain in the GMO.

This use of antibiotic resistance ‘marker’ genes in most of the GM crops is of particular concern, as gut bacteria may be able to pick them up, multiply them and pass them on to pathogenic bacteria (via horizontal gene transfer). There have been arguments that those genes only give resistance to ‘old’ antibiotics no longer frequently used – and thus it would not matter if the gut bacteria were to pick them up and pass them on. Others argue that the so called ‘old’ antibiotics are often the last line of defence against serious infectious diseases, such as tuberculosis, when modern more specific antibiotics are overcome by the pathogens. I share this concern with many other scientists, including many health professionals. A problem might also arise were bacteria to pick up other GM genes, such as some of the herbicide tolerance genes, eg glufosinate resistance gene, which is able to reverse the deactivation of the herbicide.

Moreover there is a further concern, investigated by different groups of scientists. Can the GM gene be taken up by the organisms that eat (are being fed on) GM food? For example by chicken, cattle, cows, mice, fish, humans?

To only look at the DNA and its digestibility is not sufficient, as it might be the proteins produced due to the newly introduced gene(s) or other metabolites present in the GM food that might not otherwise be there. Such products may be derived directly from the introduced gene (eg a modified Bt-toxin gene), its interaction with other genes (of the host plant), interaction of the gene product with other compounds of the plant, or may be due to the genetic engineering process itself and resulting changes.

Such proteins and or metabolites may be potential allergens, toxins or anti-nutrients, which needs to be tested both through feeding trials and compositional analysis. How these tests are being performed is still a controversial issue. Many scientists and consumer organisations would like to see more rigorous tests and analysis being performed than currently required or undertaken, in order to safeguard human health.

Digestive systems will commonly not be able to adapt that quickly as to overcome particular negative effects of particular GM foods. If adaptation was possible it would likely take many generations. And not everybody’s digestive system is the same.