Naturally,the bulk of your calories would come from grains. The body is very well adapted, if not specialized to consuming grains.
However, most of flour is "fortified" with toxic amounts of iron. One pound may easily contain your whole monthly requirement of iron.
Most people will naturally feel revulsion against eating such toxic food, and won't eat enough carbs to sustain their needs. (Some will, which makes them develop celiac disease or crohns from the iron irritating the gut)
However, since the body needs a certain amount of carbohydrates, they will consume other sources, such as potatobased snacks, which are not poisoned, but contain fat that will be eaten with the carbs and cause obesity.
Another problem is that the iron makes it impossible for the body to absorb manganese. This is especoally a problem, as grains would normally be the primary source of manganese. The deficiency in manganese causes diabetes.