Background: The loss of endogenous (nondietary) amino acids (AAs) from the gastrointestinal tract (GIT) is an important component underlying the adult human dietary requirement for protein and essential AAs (EAAs). Although data with regard to endogenous AA losses to the end of the small intestine have been published, to our knowledge there are no direct measures of colonic endogenous AA losses.
Objective: The objective was to derive quantitative estimates for daily endogenous protein and EAAs lost from the colon of the adult human.
Methods: A factorial model was developed for the prediction of endogenous AA losses across the adult human GIT. Estimates of AAs entering the upper GIT lumen were combined with relative protein synthesis rates in the colon to predict colonic AA losses. The AA composition of human colonic endogenous protein was calculated by estimating the relative contributions of epithelial cell protein and mucin protein on the basis of published data for cell shedding in the pig small intestine, small intestinal protein synthesis rates in pigs and humans, and human upper and lower GIT surface areas. Colonic AA losses were summed with empirical estimates of ileal AA losses in humans to estimate total daily GIT endogenous AA losses.
Results: Colonic AA loss was estimated to total 3.5 g/d in the adult male human, comprising 33% of total GIT endogenous AA loss (10.2 g/d). GIT essential AA losses accounted for 25-97% of the current recommended daily AA requirement for adult humans. For threonine, colonic losses were 54% of total GIT threonine losses, which were 97% of the current recommended daily threonine requirement.
Conclusions: Colonic endogenous AA losses represent a significant fraction of total GIT endogenous AA losses. The requirement of the GIT for EAAs to replace AAs lost via the gut lumen comprises a substantial proportion of the Recommended Daily Intake of AAs.