Whole oats are generally processed into three different product categories, listed here from least refined to most refined. All oat products have the inedible husk and chaff removed.
- Steel-cut oats are minimally processed. The tough bran is still intact, and the oats have not been heated or pressed. Cooking time is typically 15-30 minutes.
- Rolled oats often have the tough bran removed which makes them easier to cook and eat, but does remove some of the nutritional quality. They are pressed into a flat shape which increases the surface area, allowing them to absorb water faster and thus cook faster. They are pressed with a large roller, hence the name rolled oats.
- Instant oats (or quick oats) are mostly the same as rolled oats, but they are often precooked and pressed even flatter for faster cooking.
According to the USDA Nutrient Database, both steel-cut and rolled oats have identical nutrient contents from a chemical perspective. I have heard that steel-cut oats should have more fibre and trace minerals due to inclusion of the bran, but I had trouble finding source for this and it may vary across different product lines.
From a human digestion perspective, steel cut oats will have the lowest glycemic index because their smaller surface area means digestion happens more slowly. Steel cut oats may be preferable for people who are trying to improve regulation of their blood sugar level.
But really, the healthiest oatmeal is the oatmeal you actually eat. If instant oats makes it much easier for you to eat oats regularly, and if switching to steel-cut oats means you would eat oats less often, then you're better off sticking with what works for you.