The digestion and absorption of protein by normal man

Crane, Chilton; Neuberger, A.

doi:10.1042/bj0740313

Cited by 84 publications

(35 citation statements)

References 23 publications

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“…We have no reason to believe that amino acids are less well absorbed than intact protein; others [3,4] have reported that intact protein and protein hydrolysates are equally well absorbed. Our own studies [13] have consistently shown a reduction in stool nitrogen when infants are shifted from milk to amino acid diets.…”

Section: Discussionmentioning

confidence: 99%

“…Glycine and serine levels were both elevated, with glycine showing the greater elevation. Less striking changes occurred in the concentrations of threonine, of phenylalanine, of ornithine, of butyrine 3 , of cystine and of proline, all of which decreased. Figure 3 represents the results obtained with three other levels of low protein feeding: 1.3, 1.5, and 1.7 g/kg/day; these data are contained in table IV.…”

Section: Effects Of Varying Levels Of Protein Intakementioning

confidence: 99%

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The Plasma Aminogram. I. Influence of the Level of Protein Intake and a Comparison of Whole Protein and Amino Acid Diets

et al. 1968

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ExtractThe changes in plasma aminogram prcduced by variations in level of protein intake provided either as whole protein or a mixture of amino acids were studied. Controls were 29 normal infants, one to three months of age, fed a standard evaporated milk formula providing 3 to 3.5 g/kg/day of protein (fig. 1 and table II). Results obtained with the lowest protein intake (1.1 g/kg/day) are illustrated in figure 2 and presented in table III. Alterations in the plasma aminogram are noted as early as two days after the reduction in intake and become more pronounced as the feeding is prolonged. The most striking changes are depression of the branched chain amino acids (leucine, isoleucine and valine) and of lysine and tyrosine. Glycine and serine are elevated with glycine showing the greater elevation. Less striking changes are decreased concentrations of threonine, phenylalanine, ornithine, butyrine, cystine and proline.The aminograms obtained with feedings of 1.3 and 1.5 g/kg/day are very similar to those obtained with the more extreme degree of protein reduction ( fig. 3 and table IV). The aminogram after feeding 1.7 g/kg/day remains within the range of average ±1 standard deviation except for the elevation of glycine.A high intake of milk protein, 9 g/kg/day, results in an increase in the levels of a majority of the amino acids ( fig. 5 and table V). The sole exception is glycine, which tends to be low. Marked increases occur in the branched chain amino acids, valine being the most affected; there is also a large increase in the level of proline. The greatest elevations occur in the levels of methionine, which also show the most marked individual variation, elevations ranging from 2 to 35 times normal average value. An apparent tendency of the methionine level to return towards normal with prolonged feeding of the high protein diet is evident on the graph. The significance of this finding must be questioned, however, since the subjects who were maintained the longest on this diet were those who originally did not have the very high levels of methionine.There are some differences in the aminogram when infants are fed the equivalent of 3.5 g/kg/day in the form of amino acids instead of intact protein ( fig. 6 and table VI). On the amino acid diets, lower average plasma values are seen for the branched chain amino acids, alanine, proline, asparagine and citrulline; higher values are observed for threonine and serine. The markedly elevated levels of plasma amino acid observed after feeding 9 g/kg/day of whole protein are not seen after feeding the equivalent amount of nitrogen as an amino acid mixture ( fig. 7 and table VII). There is elevation only of valine and serine. When individual subjects were shifted from the high protein to the high amino acid diet, the pattern shifted within 24 hours. SNYDERMAN, HOLT, NORTON, ROITMAN, PHANSALKAR SpeculationA more precise and sensitive indicator of the state of protein nutrition is needed. While the level of the plasma amino acids changes rapidly with alterations in protein...

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Section: Discussionmentioning

confidence: 99%

Section: Effects Of Varying Levels Of Protein Intakementioning

confidence: 99%

The Plasma Aminogram. I. Influence of the Level of Protein Intake and a Comparison of Whole Protein and Amino Acid Diets

et al. 1968

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“…The results with the low protein diet (Table II) support this conclusion and show that hydroxyproline excretion is not influenced by the same factors that influence the excretion of other amino acids. Since it has been shown that only minimal amounts of peptides are absorbed during protein digestion (20), the increase of bound hydroxyproline excretion after the ingestion of gelatin (Figure 1) raises the possibility that all of the hydroxyproline peptides absorbed were excreted in the urine. These exogenous peptides may be altered before excretion (12), but the results suggest that they are metabolized slowly, if at all.…”

Section: Discussionmentioning

confidence: 99%

Significance of Urinary Hydroxyproline in Man

Prockop¹,

Sjoerdsma²

1961

J. Clin. Invest.

168

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Since nearly all of the hydroxyproline of the body is found in collagen, it has been suggested (1, 2) that the urinary excretion of this imino acid may be an important index of collagen metabolism. The origin of urinary hydroxyproline, however, is not definitely established. The isotopic studies of Stetten (3) in rats indirectly suggested that most of the free and peptide hydroxyproline in the body arises from the breakdown of collagen, since she found that hydroxyproline-N'5 was not significantly incorporated into collagen. Ziff, Kibrick, Dresner and Gribetz (1), on the other hand, observed an increased excretion of hydroxyproline when it was added to the diet of human subjects, in the form of gelatin. Although Ziff and co-workers did not emphasize this point, their results suggested the possibility that under normal conditions much of urinary hydroxyproline may originate from dietary proteins.The present study was undertaken to investigate further the factors that influence hydroxyproline excretion in man. Although ingestion of large amounts of gelatin was found to increase urinary hydroxyproline, its urinary excretion did not decrease when hydroxyproline was eliminated from the normal diet, or even if subjects were placed on a protein-free diet. No diurnal variation in urinary hydroxyproline was found, and hydration or dehydration did not alter excretion. Ingestion of large amounts of hydroxyproline as the free imino acid resulted in an increased excretion of free, but not of bound hydroxyproline, the peptide form which normally accounts for nearly all of the hydroxyproline in urine.These findings indicate that the daily excretion of hydroxyproline is largely independent of factors that influence the excretion of other amino acids, and they support the suggestion that urinary hydroxyproline arises from the breakdown of body collagen. The results are consistent with the hypothesis that the large hydroxyproline excretions found in children (1) and in some cases of Marfan's syndrome (2) reflect a rapid rate of collagen degradation.An incidental discovery in the study was that the increase in urinary hydroxyproline after ingestion of gelatin represents an increased excretion of hydroxyproline peptides. This appears to be the first demonstration that significant amounts of peptides can be excreted following ingestion of a protein. MATERIALS AND METHODSThe 8 subjects utilized in the study were hospitalized for periods of 3 to 12 weeks; 3 were patients with Marfan's syndrome, 2 of whom were previously shown to have elevated excretions of hydroxyproline (2). The remaining 5 were normal adult volunteers-2 males and 3 females.Procedures. Urine specimens were collected daily under 30 ml toluene and stored at 0°C for periods up to 1 year.-It was found that, in the absence of preservative, hydroxyproline was converted from the peptidebound to the free form in normal urine, and that hydroxyproline disappeared rapidly in urines containing large amounts of the free imino acid. For the first 5 days after admission, the subjec...

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“…The previous studies have concluded that protein digestion is quite rapid (20) and is completed in the upper jejunum (1). The differences in the design and methodology of the experiments may account for the varied conclusions; for example, the intraluminal fate of ingested protein was not directly examined in the previous studies.…”

Section: Methodsmentioning

confidence: 99%

“…Healthy male volunteers closely matched in weight (71-78 kg), height (172-181 cm), and age (19)(20)(21)(22)(23)(24) yr old) were utilized. All the studies to be presented in this report were approved by the Committee for the Protection of Human Subjects in Experiments.…”

Section: Methodsmentioning

confidence: 99%

Protein Digestion in Human Intestine as Reflected in Luminal, Mucosal, and Plasma Amino Acid Concentrations after Meals

Adibi¹,

Mercer²

1973

J. Clin. Invest.

357

162

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A B S T R A C T 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 ,umol/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 proteinfree 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

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The digestion and absorption of protein by normal man

Cited by 84 publications

References 23 publications

The Plasma Aminogram. I. Influence of the Level of Protein Intake and a Comparison of Whole Protein and Amino Acid Diets

The Plasma Aminogram. I. Influence of the Level of Protein Intake and a Comparison of Whole Protein and Amino Acid Diets

Significance of Urinary Hydroxyproline in Man

Protein Digestion in Human Intestine as Reflected in Luminal, Mucosal, and Plasma Amino Acid Concentrations after Meals

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