The difference between rolled and steel cut oats is that while both contain whole grain oats, they are processed differently. Rolled oats are steamed, rolled, steamed again and toasted, ending up as thin flakes. Steel cut oats are made from oat kernels that have been chopped into thick pieces.
I recommend choosing steel cut (Irish) oats over rolled oats because they digest more slowly than rolled ones. Like all other grains in whole or cracked form, steel cut oats rank lower on the glycemic index than rolled oats. The reason is that it takes longer for digestive enzymes to reach the starch inside the thicker pieces, slowing down its conversion to sugar.
As you probably know, the glycemic index is the measure of how quickly carbohydrate foods affect blood sugar. The higher on the glycemic index a food ranks, the more likely it is to cause spikes in blood sugar that over time can cause genetically susceptible people (many of us) to develop insulin resistance and metabolic syndrome. Insulin resistance is associated with obesity, high blood pressure, elevated blood fats, and an increased risk of type 2 diabetes.
You can be pretty sure you're eating a whole grain with a low GI ranking if you have to chew it or can see the grains or pieces of grains in food products. The more your jaw has to work, the better. But when grains are processed, their surface area expands, allowing digestive enzymes easy access to their starch content.
While I recommend steel cut oats, rolled oats are still preferable to instant oatmeal, some brands of which contain partially hydrogenated oils, artificial flavors and colors, or unnecessary amounts of sugar and salt.
Andrew Weil, M.D.