Study Shows Effects of Mixed GM Diets on Pig Health

GLOBAL - A long-term study of pigs fed a combination of genetically modified (GM) soybeans and GM corn maize has revealed differences in the animals' reproductive and intestinal tracts and liver compared with those fed a non-GM diet, report researchers based in Australia and the US.
calendar icon 12 June 2013
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In a paper published in Journal of Organic Systems, Dr Judy Carman from the Institute of Health and Environmental Research in Kensington Park, South Australia and co-authors report differences in the reproductive system of gilts and the health of the stomachs and livers of growing pigs fed a diet based on GM maize and GM soybean meal compared with those fed a similar diet based on non-GM ingredients.

At a commercial piggery in the US, the researchers took 168 just-weaned pigs and fed them a typical diet for the piggery, containing soy and corn, for 22.7 weeks (over five months) until the pigs were slaughtered at their usual slaughter age. Half of the pigs were fed widely-used varieties of GM soy and GM corn (the GM-fed group) for this whole period and the other half of the pigs were fed an equivalent non-GM diet (the control group).

According to the researchers, the GM diet contained three GM genes and therefore three GM proteins. One protein made the plant resistant to a herbicide and two proteins were insecticides. They chose a mixed diet instead of a single crop because this is usually what pigs and people eat. Regulators do not require animal feeding studies on mixtures of GM genes and their proteins, regardless of whether the genes are all "stacked" into the one plant or spread across several plants that are eaten in the same meal. The researchers chose pigs because they have a similar digestive system to humans, and because some of the investigators had been observing reproductive and digestive problems in pigs fed GM crops. They took blood from the pigs a few days before they were slaughtered to do standard biochemistry tests. Autopsies were done by qualified veterinarians who did not know if a given pig was fed the GM diet or not, so their observations were completely unbiased.

The researchers report that some of the investigators had previously seen a reduced ability to conceive and higher rates of miscarriage in piggeries where sows were fed a GM diet, and a reduction in the number of piglets born if boars were used for conception rather than artificial insemination. Artificial insemination guarantees the presence of a certain number of viable sperm. Because male pigs were neutered at three days of age in order to provide meat free of boar-taint, the researchers were only able to look at the female reproductive system in these pigs. They found that, on average, the weight of the uterus of pigs fed the GM diet, as a proportion of the weight of the pig, was 25 per cent higher than the control pigs. The researchers found that this biologically significant finding was also statistically significant. They list some of the pathologies that could be occurring in these uteri in the paper.

Some of the investigators had also previously seen higher rates of intestinal problems in pigs fed a GM diet, including inflammation of the stomach and small intestine, stomach ulcers, a thinning of intestinal walls and an increase in haemorrhagic bowel disease, where a pig can rapidly "bleed-out" from their bowel and die.

The researchers were not able to look inside the intestines, due to the amount of food in them, but were able to look inside the stomach. They found that the level of severe inflammation in stomachs was markedly higher in pigs fed the GM diet. Pigs on the GM diet were 2.6 times more likely to get severe stomach inflammation than control pigs. While 22 per cent of male pigs and 42 per cent of female pigs on the GM diet had severe stomach inflammation, when these pigs were compared to pigs on the control diet, it was found that male pigs were actually more strongly affected. While female pigs were 2.2 times more likely to get severe stomach inflammation when on the GM diet, males were four times more likely. These findings are both biologically significant and statistically significant.

The researchers report that these key findings were not reflected in the standard biochemistry tests that are done in GM feeding studies, probably because standard biochemistry tests provide a poor measure of inflammation and matters associated with uterus size. They did, however, find a marginally significant change on a measure of liver health in the blood of GM-fed pigs.


Carman J.A., H.R. Vlieger, L.J. Ver Steeg, V.E. Sneller, G.W. Robinson, C.A. Clinch-Jones, J.I. Haynes and J.W. Edwards. 2013. A long-term toxicology study on pigs fed a combined genetically modified (GM) soy and GM maize diet. J. Organic Sys., 8(1):38-54.

Further Reading

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