Steel-cut oatmeal is generally considered to be better than rolled oatmeal due to the differences in the ways these two types of oatmeal are processed. Although steel-cut oatmeal is slightly healthier than rolled oatmeal in this regard, all different types of oatmeal have similar protein, whole grain, fiber and nutrient contents.
Compared to rolled oatmeal, steel-cut oatmeal is minimally processed, meaning it retains its nutrient content better than its rolled counterpart. Rolled oats are typically steamed, rolled, steamed again, and then toasted before packaging, whereas steel-cut oats are only coarsely chopped before packaging. Steel-cut oats also contain less sugar than rolled oats, and are lower in calories as well.
Another benefit to the way steel-cut oats are processed is that it results in steel-cut oatmeal having a slightly lower glycemic index than rolled oatmeal. The glycemic index of a food indicates the speed at which the body metabolizes the food and turns it into blood sugar. If a food has a high glycemic index, its digestion results in a higher blood sugar spike. For health's sake, it is preferable to eat foods with a low glycemic index, as over time, regular high spikes in blood sugar can lead to insulin resistance in those who are genetically susceptible.
Although steel-cut oatmeal is the healthiest oatmeal choice, rolled oatmeal is still preferable to instant oatmeal, as this type tends to contain unhealthy additives such as hydrogenated oils, artificial flavors, and excess salt and sugar.