Whole grain cereals contain starch with a little protein. They are digested comparatively slowly, but efficiently by the starch digestion process, but there are enough protein digesting enzymes in the small intestine to ensure the proteins as well as the starches are digested. The sugars that result from the digestion of starch are absorbed by the villi, and passed into the bloodstream for several hours. The liver is able to convert sugars not immediately required into glycogen for storage. If more sugar is absorbed than can be converted into glycogen, the surplus will be changed into fat.
White flour is almost pure starch. It mixes easily with saliva in the mouth and breaks down rapidly into sugars. As soon as the digested starch reaches the small intestine, a huge surge of sugars passes into the bloodstream. Insulin is produced rapidly to control it and the liver is put under stress to process the surplus. The body works flat out to remove the surplus from the bloodstream, and usually overshoots, leaving the bloodstream with too low sugar levels, and the person feeling tired and lacking in energy. These fluctuating blood sugar levels can put the body under too much stress and the system controlling blood sugar starts to break down. The result can be diabetes.
Refined sugar is even worse because it requires no further digestion but enters the bloodstream in a dangerous surge.