To get smaller particles you have to do some work, whether inside your stomach or outside of it. When you eat cold whole oatmeal with milk then bigger colder particles reach your stomach which then will have to perform more work, spend more energy, on breaking that down than when you, say, eat finely-ground porridge that you have cooked for a minute with the same amount of milk. How will that not be different for the body? Of course you can always define 'nutritional effect' such that by definition such effects are excluded but that's probably no what you want to say.