As I mentioned in Carbohydrates: Dogs' Doodads or Spawn of Satan?, some breakfast cereals turn into blood glucose faster than table sugar (half glucose bonded with half fructose) even though they're "Wholegrain" cereals.
A whole (i.e. intact) grain consists of a protective outer shell (a.k.a. bran) and innards consisting of starchy/proteiny endosperm and nutritious germ. See Cereal germ.
To turn a grain into a breakfast cereal, it's ground into dust, mixed with water to form a paste and the paste is extruded through holes into whatever shape the manufacturer desires. Technically-speaking, everything that was in the whole grain is in the finished product. However, the form & function have completely changed. Here are a couple of analogies.
1) I want to sell my car. I take it to a scrap-yard and have it shredded into tiny pieces. All of the tiny pieces are put in a skip, which is delivered to my driveway. I place a sign on the skip stating "Whole Mazda MX-5 for sale. Enquire within. £5,000 O.N.O.". What would you offer for my "Whole Car"?
2) An insane person gives me £5,000 (in £50 notes) for my "Whole Car". As I'm on a roll, I take the notes and put them through a cross-cut shredder which turns them into thousands of 2mm x 2mm pieces. I empty the pieces into an attaché case. Who will accept my attaché case full of "Whole £50 notes" as payment for their second-hand car? Any more insane people out there?
Turning grains into dust does four things.
1) It exposes the starchy endosperm.
2) It vastly increases the surface area of the grain resulting in much faster digestion & absorption as blood glucose. Surface area is inversely proportional to particle size. If 3mm grains are reduced to 0.1mm particles, the surface area is increased by a factor of 30. How effective is chewing? Interestingly, once the particle size has fallen below a certain value, reducing it further makes no further difference. See The Effect of Particle Size of Whole-Grain Flour on Plasma Glucose, Insulin, Glucagon and Thyroid-Stimulating Hormone in Humans. Therefore, "wholegrain" bread is just as bad as white bread in terms of its glucose & insulin response.
3) It makes the finished product much more likely to stick to your teeth, resulting in the rapid formation of tartar that damages your teeth and gums.
4) It makes the finished product more energy-dense.
Rolled grains are grains that have been steamed (to cook and make them soft), then put through what's effectively a mangle. They're still intact, if somewhat flat. Puffed grains are grains that have been heated to make the water within boil. As steam takes up a much larger volume than water, the grains are inflated to a much larger size. They're still intact, if somewhat funky-looking!
Don't be conned by breakfast cereal labels. If they look like "O"s or Brillo pads or brake pads, they're not intact grains.
Oats are O.K. even when turned into oatmeal, probably due to their high beta-glucan content, which forms a wallpaper paste-like goo when wet. See Particle size of wheat, maize, and oat test meals: effects on plasma glucose and insulin responses and on the rate of starch digestion in vitro.
See also Anthony Colpo's The Wholegrain Scam.