We have always thought that both fats and fibers were useful in reducing the glycemic index of foods and above all the glycemic load of meals.

On the other hand, of the low glycemic index foods, many are high in fiber, such as vegetables and legumes.

Additionally, the fact that fiber slowed digestion made it more plausible to think that it also reduced the effect of carbohydrate foods on blood sugar. Some hesitation about this hypothesis stemmed from the fact that some whole foods have the same glycemic index as their refined variant, such as black bread or spelled (glycemic index 70).

The same thought was made about fats: it was seen that fats did not affect blood sugar, but the insulin response did.

However, it has always been recommended to use fats, for example a drizzle of olive oil on pasta, to reduce the glycemic load of the meal.
Well, now what?

And now a study denies that fibers and fats can reduce the glycemic index of foods, indeed.

FIBERS AND FATS ARE NOT USED TO REDUCE THE GLYCEMIC INDEX OF FOODS

This is a very important study, because it includes four different experiments on the different composition of meals in a sample of adults aged 50 to 80, some of whom are very overweight.
The study examines: pre- and post-meal glycaemia and insulinemia.
Pre- and post-meal blood pressure.
The glycemic and insulin load of meals.

It establishes that postprandial glycemia is not affected by either the extra fibers which therefore only increase the satiating effect of the dish or the extra fats which therefore only increase the calories, while adding carbohydrates worsens both the glycemic index and the glycemic load of the meal.

The only macronutrient among carbohydrates, fats, proteins and fibers that reduces the glycemic load of the meal given by carbohydrates are in fact proteins.