Why does starch have a higher glycaemic index than sugar?
Starch consists largely of amylopectin, a heavily branched chain that is broken down quickly and therefore causes a higher blood sugar spike than sugar. The preparation method and physical structure of the food also determine how high that spike turns out to be.
Starch is made up of two types of chains: amylose (long, straight chains) and amylopectin (heavily branched chains). Amylopectin offers far more attachment points for digestive enzymes, so it is broken down into glucose extremely quickly. Starch in everyday foods is 70 to 80 percent amylopectin, which is why it causes a rapid rise in blood sugar. Ordinary table sugar (sucrose) is half fructose, and fructose is processed by the liver without causing a direct blood sugar spike. That difference in composition already explains a large part of the difference in glycaemic index.
How you prepare starch also makes quite a difference. Cooking and heating break open the structure of starch granules, a process called gelatinisation. The further this process has gone, the more accessible the starch is to your digestive enzymes and the faster glucose enters your bloodstream. Rice is a good example: depending on the variety and preparation method, its glycaemic index ranges from 48 to 93 on a scale where pure glucose sits at 100.
There are also factors that dampen the blood sugar spike. When cooked starch cools down, part of it re-crystallises, which slows digestion slightly. The same applies to the physical structure of the food: if grains or legumes remain coarse or minimally processed, enzymes have to work harder to reach the starch. Finely milled flour therefore produces a higher spike than coarser versions containing the same amount of starch.
Studies with cells and isolated compounds have also shown that high-amylose starch can form complexes with certain amino acids, meaning part of the starch is never digested at all. This partly explains why foods high in amylose produce a lower blood sugar response. This has only been investigated outside the body, but the mechanism fits well with what is seen in studies in people.
This answer draws mainly on many studies combined looking at the effects of starch composition and preparation method on blood sugar, supplemented by a broad review of studies on rice and one laboratory study on amylose-amino acid complexes. The combined studies provide reasonably strong evidence for the effects of amylose content, gelatinisation and particle size. The evidence for cooling (retrogradation) is somewhat more limited. The laboratory study on amino acid complexes says nothing about what happens inside the human body.