Yes and no. Eating only one is safe if you want to try an MRE for kicks but are worried about its effects. Consistent consumption may cause unpleasant side effects. MREs can cause constipation for a few different causes. This is partly because it is low in fiber and contains a lot of fat and sugar but very few calories. Once again, these foods are intended for serving active military personnel in the field during the war and are not suitable for civilian consumption. These elements are designed to be digested slowly, providing the troops with sustained energy during the long, difficult days ahead.