The absorption of large breakdown products of dietary proteins into the body tissues including brain

Abstract

For many decades it has been accepted doctrine that dietary protein is broken down to amino acids in the gut lumen, before absorption of these amino acids by the gut wall. Thus Bayliss (1918), Hammarsten-Mendel (1909) and Fisher (1954) held to this view. Only recently has it been accepted that small peptides may be absorbed through the gut wall faster than amino acids (Matthews, 1975). Yet since the first decade of this century it has been known that immunologically significant amounts of protein evade this mechanism and reach the circulation as intact native protein. This finding has lain in the literature and has been rediscovered anew in each decade, but has had little effect on our thought on the digestive process. Recently from my laboratory has come evidence that certain proteins seem to be absorbed in very high degree in almost intact form, or at least in the form of large cleavage products. These products pass the gut wall, permeate the body and enter the cells, and, what is particularly relevant to the present topic, they enter the brain and its cells apparently passing freely across the blood-brain barrier, whatever that is (Brightman and Reese, 1969).