June 12, 2013

expert reaction to new study on GM pig feed and stomach inflammation

A study published in the Journal of Organic Systems appeared to show that pigs fed on genetically modified soy and corn developed stomach inflammation at greater rates than those fed a conventional diet. A before the headlines analysis accompanied this roundup.

Prof Tom Sanders, Head of the Nutritional Sciences Research Division at Kings College London, said:

“It does not look like a convincing adverse effect as it was a minor incidental finding.  There were no differences in growth and mortality rates and pigs at sacrifice appeared in similar health.  The dietary groups were also not identical as different supplements had been used.  If you do not specify outcomes at least one in 20 will come up as being statistically significant by play of chance.

“The probabilities for the abnormal findings were low.  Gastric inflammation is often due to infections and uterine weight can be influenced by factors such as rate of weight gain.  The number of pigs with mild to severe inflammation did not different between groups with 69/73 showing inflammation in the control group and 64/79.  The authors have unwisely highlighted differences in the number with severe inflammation.  It seems unlikely that the effects observed were treatment related.”

Prof David Spiegelhalter, Winton Professor of the Public Understanding of Risk at the University of Cambridge, said:

“The study’s conclusions don’t really stand up to statistical scrutiny.   The authors focus on ‘severe’ stomach inflammation but all the other inflammation categories actually favour the GM-diet.  So this selective focus is scientifically inappropriate.  

“When analysed using appropriate methods, the stomach inflammation data does not show a statistically statistical association with diet.  There are also 19 other reported statistical tests, which means we would expect one significant association just by chance: and so the apparent difference in uterus weight is likely to be a false positive.”

Prof Patrick Wolfe, Professor of Statistics at University College London, said:

“I am not an expert on animal health, husbandry, toxicology etc, and therefore I cannot comment on these aspects of the study.  As a statistical methodologist I can however comment on the data analysis undertaken and presented in the article.

“The biggest issue is that the study was not conducted to test any specific hypothesis.  This means that the same sample (in this case nearly 150 pigs) is, in effect, being continually tested over and over for different findings.  

“The statistical tests employed assume that a single test is done to test a single, pre-stated hypothesis; otherwise the significance levels stemming from the tests are just plain wrong, and can be vastly over-interpreted.  

“Thus there is a higher-than-reported likelihood that the results are due purely to chance.  The number of pigs being in the low hundreds (instead of, say, the thousands, as is often the case in large medical studies) can make this effect even more prominent.

“Bottom line: a better-designed study would have hypothesized a particular effect (such as changes in stomach size), and then applied a statistical test solely to check this hypothesis.   Perhaps another independent team of researchers will go down this path.  Until then, this study definitely does not show that GM-fed pigs are at any greater risks than non-GM fed pigs.”

‘A long-term toxicology study on pigs fed a combined genetically modified (GM) soy and GM maize diet’ by Judy Carman et al, published in the Journal of Organic Systems.