Professor Anthony Trewavas's letter regarding Prof GE Seralini's study, which found that a Monsanto GM maize and tiny amounts of the Roundup herbicide it is grown with caused serious health effects in rats, misrepresents the study as well as OECD guidelines on safety studies (Flawed study fails to prove harm, Letters, September 29).

Trewavas's claim that there are "many hundreds" of lifetime feeding experiments with GMOs is false. A search by the French food agency ANSES in the wake of the Seralini controversy uncovered just two comparable studies performed over the natural lifetime of the animal. One found health problems in the GM-fed animals and the other was only available in Japanese.

Trewavas writes that 75%-80% of the type of rats in Seralini's experiment develop tumours spontaneously. He fails to mention that, in Seralini's experiment, the rats that ate the GM maize and Roundup developed more tumours at an earlier stage of life, and that those tumours grew more aggressively than the control rats. It is these trends, not the total number of rats with tumours at the end of their life, that are important and worrying.

Contrary to Trewavas's implication, the OECD has never set guidelines for chronic toxicity studies on GM foods like Seralini's. But it has for chronic toxicity studies for chemicals - and stipulates analysis of the same number of animals that Seralini used. The OECD requires larger numbers of animals to be used in cancer studies for industry safety tests on chemicals to protect against "false negative" findings, where a toxic effect exists but is missed. Seralini did find toxic effects, so the number of animals used is not an issue.

Finally, Seralini's control group had the same number of animals as each treatment group. Each treatment group was compared separately to the control group, according to standard scientific practice and OECD guidelines. We address the misleading claims about the Seralini study made by Trewavas and other GM supporters in detail online at:

Claire Robinson