A Brazilian study comparing transgenic and conventional soybean feeds for mice found that, at a concentration as small as 10%, the GMO soy compromised their ability to recover from chemical damage and increased the number of random genetic mutations within their cells.
The scientists compared 10% and 20% conventional soybean feeds with 10% and 20% GMO soy mixtures. At the end of the trial all of the mice were injected with a drug called cyclophosphamide, a common chemotherapy drug which stops bone marrow growth. Both groups of mice fed GMO formulas had higher rates of chromosome breakage naturally and in response to the drug. The mice fed the 20% mixture had a higher rate of these mutations and also had the slowest replicating bone marrow in this typically highly active tissue.
I was surprised, then, to read that the authors concluded that BOTH versions of the soy had antimutagenic properties and were nontoxic! This is usually where I get suspicious about conflicts of interest in an article, so I dug a little further into the Brazilian-soy connection.
Most people are familiar with the deforestation of the Amazon and its contribution to climate change. What they may not know is that the largest portion of clear cutting is attributed to ranching and agriculture, with soy dominating the agribusiness market (thanks, primarily, to American Agri-giant Cargill). Obviously this kind of ecological change has its effects. Along with rising temperatures, regionally the shift from trees to soy is getting part of the blame for rising rates of dengue fever. However, the Brazilian government has responded to the crisis appropriately by enforcing a four-year moratorium on further deforestation for soy fields. This doesn't change the fact, though, that there are now farmers with a limited land title who are looking for ways to increase their production (China isn't getting any smaller, after all). Enter the GMO soybean, and the apparent financial incentives for Brazilian researchers to make it look safe.
While researching this article, I also found an interesting, in-depth investigation on the "dark side" of soy. I've read a few articles that suggest too much soy isn't healthy, and Mary Vance's article was the most thorough exposition I've read. She makes some really good points- while we point to lower rates of cancer in soy-consuming eastern societies, they usually only consume 15 grams/day, mostly in fermented versions. In contrast, it's fairly common to see Westerners eat 20 grams of soy in a single serving. No wonder we're still having problems!
Luciana Azevedo, Nathalia R.V. Dragano, Ana P.L. Sabino, Maria Christina C. Resck, Patricia L. Alves de Lima, Cibele M.C.P. Gouvêa. (2010). "In Vivo Antimutagenic Properties of Transgenic and Conventional Soybeans." Journal of Medicinal Food 13(6): 1402-1408. doi:10.1089/jmf.2009.0240