Why is boiled starch such a big deal? It all starts with an understanding of starch itself.
Starch consists of long chains of glucose molecules. Some chains are relatively linear, with few branches (amylose) and others are highly branched (amylopectin). These chains are held in starch granules found in plants, mostly in the seeds (grains and legumes), roots, and tubers (potatoes).
Because starches are such large molecules, they are neither sweet nor soluble in water. In fact, they are mostly indigestible unless they are cooked. So why do we crave them? Why do we like these non-sweet carbohydrates enough to make them the featured foods in the diets of every civilization since before recorded history? Why have we cultivated them wherever climate would allow? The reason is simple. The provide a unique form of energy.
Cooking softens the starch, causing the starch granules to swell to over three times their size when mixed with water. This process called gelatinization allows our digestive enzymes called amylases to attack the starch molecule and split off the sugars in a very orderly fashion from one end of the chain. The fact that starches are digested to release glucose has been taken to mean that starch is just another source of glucose no different from any other and as such should be evaluated based on the rate of sugar release. This is the common and tragic mistake that has led to all the confusion about carbohydrates. Starch plays a unique and irreplaceable role in our diet, so unique that it must be considered an essential nutrient.
Starch functions as a storage molecule for glucose in the plant. In animals, the analogous storage molecule is glycogen, which is built very similar to amylopectin, a highly branched conglomeration of glucose chains. In fact, glycogen is often referred to as animal starch because this structural similarity. It seems only reasonable that we would make use of this unique source of carbohydrate and that it would function differently from simple sugar.
We build glycogen from excess dietary sugar and store it in a few critical places. Glycogen in our liver supplies the blood with needed glucose when we fail to take in sufficient carbohydrates. In the brain, it serves as a critical energy reserve. In muscle, glycogen act as an energy bank account that we draw upon whenever we need energy for movement. In fact, most of what muscles do with glucose from the blood is to make glycogen. Once the glucose in muscle is assembled into muscle glycogen, it's only fate is to supply energy to working muscles.
The unique function of starch is to make cells, particularly muscle cells, more responsive to insulin signaling. Insulin, the hormone secreted by the pancreas in response to dietary carbohydrate, aids cells in pulling glucose out of the blood. For muscle cells, this translates to creating muscle glycogen. That’s why we love grains, beans, and potatoes, and why they have been critical to the balance of human nutrition, since before recorded history. They create a direct link between satiety and energy. But this link depends upon the method of preparation.
Gelatinization occurs when we heat starches in water. Each starchy food has its own unique gelatinization temperature, but in general, gelatinization occurs slightly below the boiling point of water (212 degrees F). Baking and frying occur at temperatures much higher (350 to 600 degrees F and higher), which causes the water to evaporate and the starch chains to fractionate. No longer able to enhance the cellular response to insulin signaling, the broken starch chains, become mere sources of glucose, much like any source of sugar, not at all toxic, but lacking the unique feature that make starch a critical nutrient.
The disassembly of the relatively intact starch chains increases the cells response to insulin. In raw starch, these chains are inaccessible to digestive enzymes. In baked or fried starch, these chains are shattered. Only in boiled starches are the critical starch chains (polymers) both intact and accessible to digestive enzymes. This makes boiling one of the most critical innovations in human history.
Glycemic index is irrelevant. Some boiled starches, like beans and some grains, have a low glycemic index. Some, like potatoes and white rice, have a high glycemic index. All boiled starches have the same effect. In fact, failure to account for the boiled starch effect makes glycemic index a useless measure. Any food with sufficient fat, fructose, and/of fiber will have a low glycemic index, as will foods with no carbohydrates.