A diet containing GM soy and maize fed to rats for 30, 60, and 90 days caused a wide range of toxic effects, including DNA damage, abnormal sperm, blood changes, and damage to liver, kidney, and testes.
The Egyptian team of researchers concluded (see item 1 below) that "there are health hazards linked to the ingestion of diets containing genetically modified components".
Histopathological examinations of various body tissues were carried out, and marked differences in the tissues of the GM-fed animals were found. The images, with explanations, can be seen in the published study.
This study did not use the non-GM isogenic (genetically the same but without the genetic modification) comparator crops in the control diet. Instead the control diet was based on wheat as the major source of protein. This diet was tested for GMO content and, in contrast with standard laboratory rodent diets, it was found to be GMO-free.
Because the non-GM isogenic comparator crops were not used in the control diet, it is not possible to conclude that the genetic modification was the cause of the toxic effects found. The two diets were said to be nutritionally equivalent, so nutritional differences can be ruled out as the cause of the toxic effects found. However, the effects may have arisen from pesticide residues in the GM crops.
What can be concluded with certainty is that the GM soy and maize-based diet was more toxic than the wheat-based control diet.
Although no animals were followed for longer than 90 days, a subchronic (less than long-term) period, it's worth noting that liver damage was found from the earliest time point of 30 days.
Commenting on other studies that found no adverse effects from the GM diet tested, the authors suggested that this may sometimes be due to short study durations and low proportions of GMOs in the diet. Undoubtedly the failure of many other authors to carry out histopathological examinations (microscopic examinations of tissues) has something to do with it, too. Many damaging effects can only be seen in this way.
Some of the same team of researchers have also published a separate study (item 2 below) which showed that ingested fragments from the CaMV-35S promoter used in many GM crops incorporated into blood, liver, and brain tissues of experimental rats. The implications of this finding are unknown, though the authors cite other papers that hypothesise that the CaMV-35S promoter could reactivate dormant viruses, create new viruses, and cause cancer by causing over-expression of normal genes.