“ ... more than 150 scientific studies have been done on animals fed biotech crops and to date, there is no scientific evidence of any detrimental impact” (biotech representative, CropLife International).
With a bit of cherry-picking of what constitutes appropriate scientific evidence this may be true. However, it's easy to see why no 'detrimental impact' has emerged. The 'scientific studies' has been largely based on animal models with little relevance to humans, on diets restricted to single GMOs, on very short time-scales, and on superficial data relevant only to commercial livestock production.
When a team of Australian and American scientists, led by Dr. Judy Carman, carried out an experiment which avoided the above short-comings, it raised concerns.