In the middle of the last century, it was believed that in higher organisms food is almost completely broken down to elements capable of absorption in the cavity of the small intestine under the action of pancreatic and intestinal enzymes. This type of digestion was designated by the term cavity (luminal) digestion. In this case, the wall of the small intestine was considered simply as a semipermeable barrier between the lumen of the intestine and the internal environment of the body. Initially, Alexander Mikhailovich himself was a supporter of this concept. The inconsistencies in this classic two-link scheme of food assimilation were explained within the framework of another concept that was popular at that time — intracellular digestion.

Therefore, in the early 1960s, the concept began to lead, combining the two previous ones. According to her, the initial and intermediate stages of food hydrolysis occur in the cavity of the gastrointestinal tract, and the subsequent stages are intracellular.

However, unresolved issues remained. Ugolev began to study one of them in 1957: what is the reason for such large differences in the rates of starch hydrolysis in the small intestine in vivo and in vitro?

The experimental model of Ugolev differed from the generally accepted ones. In his model, the digestion of food was investigated not only under the action of digestive juices, but also in the presence of a piece of the small intestine. The scientist compared the effect of the presence of pieces of the mucous membrane of the esophagus, stomach, small and large intestine, on an increase in the rate of starch hydrolysis. He found that membrane digestion is more pronounced in the small intestine.

Before the research of Alexander Mikhailovich, it was believed that the epithelium of the small intestine performs two functions: absorption and secretion. The effect of the surface of the small intestine on the enzymatic processes occurring in its cavity has not been practically studied experimentally. Ugolev concluded that the surface of the small intestine performs the function of a living porous adsorbent.

In 1960, an article on membrane digestion was published in the reputable scientific journal Nature. Foreign scientists were at first skeptical about the discovery, but later were forced to admit its importance.

In the course of studies conducted under the leadership of Ugolev, the levels of organization of membrane digestion, its spatial topography along the small intestine and in the crypt-villus system, organization and regulation of polysubstrate processes in the small intestine were characterized.

The discovery of membrane digestion led to a radical revision of many classical concepts and resolved serious contradictions in the physiology of digestion.

Thanks to the discovery of membrane digestion, it became possible to explain the mechanisms of development of some types of pathology of the gastrointestinal tract. Chapters on membrane digestion have been included in many textbooks and manuals. The history of research into various aspects of membrane digestion continues today.