Growing evidence indicates serious health problems can be caused by the consumption of certain genetically modified corn varieties.  Monsanto tends to do brief, narrow studies.  This lets them put their products on the market without necessarily proving that they are safe, because many problems do not show up in a brief 90-day study on only one species.  Some countries have banned the use of GMOs and will not accept imported foods that contain them.  In America, alas, your best bet is to eat organic — because there is no labeling requirement for GMOs.