While most of the time spinach is not genetically modified, this is not always the case. There is still quite a bit of controversy surround the fact that GM foods are still not labeled as such much of the time. The long-term effects of genetically modified foods on the body have still not yet been thoroughly studied, which is mostly why there is such controversy surrounding it.
Sometimes plants and crops are genetically modified in order to resist certain insects and other pests which can destroy them altogether. Although there are currently no known adverse side-effects to consuming genetically modified foods, this may not be the case farther down the line when more research has been on its effects on the human body.