Why do many high-fibre foods still have a high GI value?
Dietary fibre is not one chemical constituent like fat and protein. It is composed of many different sorts of molecules and can be divided into soluble and insoluble types. Soluble fibre is often viscous (thick and jelly-like) in solution and remains viscous even in the small intestine. For this reason it makes it harder for enzymes to move around and digest the food. Foods with more soluble fibre, like apples, oats, and legumes, therefore have low GI values. Insoluble fibre, on the other hand, is not viscous and doesn’t slow digestion unless it’s acting like a fence to inhibit access by enzymes (eg. the bran around intact kernels). When insoluble fibre is finely milled, the enzymes have free reign, allowing rapid digestion. Wholemeal bread and white bread have similar GI values. Brown pasta and brown rice have similar values to their white counterparts.
