The addition of dietary lipids to cooked starch promotes the formation of amylose-lipid complexes, which further reduces enzymatic digestibility beyond the reduction caused by retrogradation alone.

Study: Formation and in vitro starch digestibility of amylose-lipid complex using cooked rice starch and an emulsified formulation.

Adding fat to cooked rice makes the starch harder for your body to digest, even more than just cooling the rice does. The fat wraps around the starch in a way that blocks digestive enzymes.

Study: Physicochemical and Functional Characteristics of RD43 Rice Flour and Its Food Application

This study found that a type of rice with more amylose (a kind of starch) digests slower and releases less sugar — which is what the claim is about. But it didn’t test adding fat to rice, so we can’t say for sure that fat makes it even slower, though the science behind it still makes sense.

Study: Impact of lipid modification on the structural and digestive properties of starch in cooked chestnut paste: A comparative study of butter and soybean oil.

Adding fats like soybean oil to cooked starch makes it form tiny structures that block digestive enzymes, making the starch harder to break down — and soybean oil does this better than butter.

Study: Removal of starch granule-associated lipids from normal and waxy wheat starches: Effects on properties, retrogradation, digestion, and molecular mechanisms.

When fat is removed from starch, it becomes easier for digestive enzymes to break it down—meaning the fat was blocking the enzymes. This shows that adding fat to cooked starch makes it harder to digest, even more than just cooling the starch alone.