Wheat starches are composed of polymers (repeating chains) of the sugar, glucose. 75% of wheat carbohydrate is the chain of branching glucose units, amylopectin, and 25% is the linear chain of glucose units, amylose.
Both amylopectin and amylose are digested by the salivary and stomach enzyme, amylase, in the human gastrointestinal tract. Amylopectin is more efficiently digested to glucose, while amylose is less efficiently digested, some of it making its way to the colon undigested.
Amylopectin is therefore the “complex carbohydrate” in wheat that is most closely linked to its blood sugar-increasing effect. But not all amylopectin is created equal. The structure of amylopectin varies depending on its source, differing in its branching structure and thereby efficiency of amylase accessibility.
Legumes like kidney beans contain amylopectin C, the least digestible—hence the gas characteristic of beans, since undigested amylopectin fragments make their way to the colon, whereupon colonic bacteria feast on the undigested starches and generate gas, making the sugars unavailable for you to absorb.
Amylopectin B is the form found in bananas and potatoes and, while more digestible than bean amylopectin C, still resists digestion to some degree.
The most digestible is amylopectin A, the form found in wheat. Because it is the most readily digested by amylase, it is the form that most enthusiastically increases blood sugar. This explains why, gram for gram, wheat increases blood sugar to a much greater degree than, say, chickpeas.
The amylopectin A of wheat products, “complex” or no, might be regarded as a super-carbohydrate, a form of highly digestible carbohydrate that is more efficiently converted to blood sugar than nearly all other carbohydrate foods.