The digestion and absorption of protein in man. 2. The form in which digested protein is absorbed.
TL;DR: It is suggested that experiments in which amino acid mixtures, simulating a dietary protein, are fed to experimental animals to determine the rates of amino acid absorption do not present a true picture of the events in the small intestine following the ingestion of a protein meal.
Abstract: 1. Intestinal contents, collected from the human jejunum after a test meal (milk-protein, gelatin or low-protein) were fractionated by centrifugation and gel filtration on G-75Sephadex. The fractions were hydrolysed and the proportion of the total amino acid in each fraction was determined. The amino acids were measured with an EEL Amino Acid Analyser.2. The free amino acid concentrations were determined in samples of the contents of the small intestine collected from various levels after the three types of test meal.3. Intestinal contents collected from two levels of the jejunum after a milk-protein meal, were incubated in vitro at 37° for periods up to 80 min and the rates of release of the individual free amino acids were determined.4. There was a rapid breakdown of the proteins of the test meals to fragments of molecular weight under 5000. The further breakdown (during incubation in vitro) to free amino acids was sufficiently rapid to account for the absorption in the free form of arginine, lysine, tyrosine, valine, phenylalanine, methionine and leucine. It was not rapid enough to account for the absorption of glycine, threonine, serine, the imino acids or the dicarboxylic amino acids in the free form.5. The free amino acid concentrations in the intestinal lumen bore very little relationship to the concentrations in hydrolysates of the test meals or to those in hydrolysates of the intestinal contents. Many of the free amino acids in the intestinal samples were present in approximately equimolar concentrations.6. It is suggested that experiments in which amino acid mixtures, simulating a dietary protein, are fed to experimental animals to determine the rates of amino acid absorption do not present a true picture of the events in the small intestine following the ingestion of a protein meal.
Citations
More filters
TL;DR: The gram-negative bacterial species Salmonella enterica and Escherichia coli are members of the family Enterobacteriaceae that spend a good part of their lives as residents of animal hosts.
Abstract: The gram-negative bacterial species Salmonella enterica and Escherichia coli are members of the family Enterobacteriaceae that spend a good part of their lives as residents of animal hosts. S. enterica is the etiologic agent of gastroenteritis and typhoid fever in humans ([88][1]), whereas E. coli
732 citations
TL;DR: The authors are indebted to Carolyn Brewer and Gaynell Barnes for their excellent clerical assistance.
403 citations
TL;DR: After the ingestion of a test meal containing a substantial amount of protein which is within the usual range of dietary intake, there are greater amounts of amino acids present as small peptides than in the free form in the gut lumen and the ingested protein can be recovered as late as 4 h both in the jejunum and in the ileum.
Abstract: Normal human volunteers were intubated with either aspiration tubes or a biopsy capsule placed in the small intestine. The subjects were then fed a test meal containing 50 g of purified bovine serum albumin which served as the model dietary protein. Electrophoretic analysis of intestinal fluids showed that for at least 4 h the fed albumin was detectable in jejunal and ileal fluids. On separate occasions, subjects were fed the same meal without the protein. No protein was detected in intestinal fluids when the protein-free meal was fed. After the protein-rich meal, total concentrations of measured free and peptide amino acids rose from 3.21 to 29.29, and 15.94 to 117.97 mumol/ml, respectively, (P values < 0.02) in the jejunum. Similarly, total concentrations of measured free and peptide amino acids rose from 5.45 to 19.74, and 13.59 to 65.39, respectively, (P values < 0.05) in the ileum. In contrast, concentrations of free and peptide amino acids in intestinal fluids did not increase after the protein-free meal. While intracellular concentrations of amino acids in the jejunal mucosa did not show significant changes, plasma concentrations of each individual free amino acid were increased after the protein-rich meal and were either decreased or unaltered after the protein-free meal. The amino acid composition of the fed protein was reflected in the increases in intraluminal and plasma concentrations of individual amino acids after the protein-rich meal. It is concluded that after the ingestion of a test meal containing a substantial amount of protein which is within the usual range of dietary intake; (a) the exogenous protein is the principal source of the increased free and peptide amino acids in the intraluminal contents and in the plasma; (b) there are greater amounts of amino acids present as small peptides than in the free form in the gut lumen; (c) the ingested protein can be recovered as late as 4 h both in the jejunum and in the ileum.
363 citations
TL;DR: From this information, it is predicted that E. coli will have at least two phenotypically distinct cell types and that these will have dual molecular control mechanisms of opposite type for the regulation of certain functions.
Abstract: Escherichia coli cycles between two principal habitats-intestines of warm-blooded animals and water, sediment, and soil-that are shown to be quite distinct with respect to physical conditions and the spectrum and level of available nutrients. Elementary calculations suggest that, on the average, an E. coli cell is "born" in the intestine of a warm-blooded animal and spends about half its life there, is excreted onto the surface of the earth where it spends the second half of its life, and then dies or, with a small probability, colonizes a new host. From this information one can predict that E. coli will have at least two phenotypically distinct cell types and that these will have dual molecular control mechanisms of opposite type for the regulation of certain functions. In cases for which the molecular data are available for comparison, they are in agreement with the predictions.
321 citations
Cites background from "The digestion and absorption of pro..."
...Dietary protein (70-100 g/day in man) is rapidly absorbed in the proximal small intestine; approximately 80% is absorbed within the first 40-70 cm of the small intestine (Borgstrom et al. 1957; Nixon and Mawer 1970; Johansson 1975; Chung et al. 1979)....
[...]
...Endogenous protein released into the intestinal tract (35-55 g/day in man) is more resistant to digestion and continues to be autodigested throughout the remainder of the small intestine (Nixon and Mawer 1970; Gibson et al. 1976; Chung et al. 1979)....
[...]
...Indeed, Nixon and Mawer (1970) have shown by direct measurement of intestinal contents that beyond the first 40 cm of the small intestine (140-245 cm from the nose) the profile of protein and free amino acids does not change appreciably, and Chung et al. (1979) have found the same general values…...
[...]
...About 20%-30% of the total amino acids entering the colon are available in free form with an average concentration of about 100 moles per liter of intestinal contents (Nixon and Mawer 1970; Chung et al. 1979)....
[...]
...By contrast, lysine, glutamate, arginine, tyrosine, and tryptophan are estimated to be the most abundant amino acids entering the colon independent of diet (Nixon and Mawer 1970; Savageau 1977)....
[...]
TL;DR: McKnight et al. as discussed by the authors showed that acidified nitrite is bactericidal for a variety of gastrointestinal pathogens such as Yersinia and Salmonella, and that it contributes to the formation of systemic S-nitrosothiols.
Abstract: Based on the premise that dietary nitrate is detrimental to human health, increasingly stringent regulations are being instituted to lower nitrate levels in food and water. Not only does this pose a financial challenge to water boards and a threat to vegetable production in Northern Europe, but also may be eliminating an important non-immune mechanism for host defence. Until recently nitrate was perceived as a purely harmful dietary component which causes infantile methaemoglobinaemia, carcinogenesis and possibly even teratogenesis. Epidemiological studies have failed to substantiate this. It has been shown that dietary nitrate undergoes enterosalivary circulation. It is recirculated in the blood, concentrated by the salivary glands, secreted in the saliva and reduced to nitrite by facultative Gram-positive anaerobes (Staphylococcus sciuri and S. intermedius) on the tongue. Salivary nitrite is swallowed into the acidic stomach where it is reduced to large quantities of NO and other oxides of N and, conceivably, also contributes to the formation of systemic S-nitrosothiols. NO and solutions of acidified nitrite, mimicking gastric conditions, have been shown to have antimicrobial activity against a wide range of organisms. In particular, acidified nitrite is bactericidal for a variety of gastrointestinal pathogens such as Yersinia and Salmonella. NO is known to have vasodilator properties and to modulate platelet function, as are S-nitrosothiols. Thus, nitrate in the diet, which determines reactive nitrogen oxide species production in the stomach (McKnight et al. 1997), is emerging as an effective host defence against gastrointestinal pathogens, as a modulator of platelet activity and possibly even of gastrointestinal motility and microcirculation. Therefore dietary nitrate may have an important therapeutic role to play, not least in the immunocompromised and in refugees who are at particular risk of contracting gastroenteritides.
301 citations
References
More filters
TL;DR: A modified ninhydrin reagent for the photometric determination of amino acids and related compounds and its application in drug discovery is described.
2,851 citations
947 citations
146 citations
114 citations