Foods that contain carbohydrates (including starchy foods such as oats and other cereals) are broken down during digestion into sugar (mainly glucose but also fructose and galactose).
As the foods are broken down, the levels of sugar in the bloodstream begin to rise. This is a normal but important process – the sugar provides us with immediate energy or is stored by the muscles and liver cells for energy later.
Some foods take longer to digest, which means they spend more time in the stomach before reaching the intestines. As such, they will cause a smaller but more sustained rise in blood sugar.
The glycemic index shows that the sugars in oatmeal and porridge are absorbed at about two-thirds the rate of sugar from white bread. This means oats are considered a medium glycemic index food, similar to pasta but absorbed more slowly than many other breakfast cereals.
Generally, it is a good thing for food to be absorbed more slowly, as it is thought this helps with appetite control. So, while your blood sugar may rise after eating oats, this rise is a normal part of the digestive process.
But while the glycemic index tells us how quickly sugars are absorbed by the body, it doesn’t really look at the portion size of the food.