[Studies on nitrogen and amino-acid absorption in the swine. Part 2. The course of 15 N labeling in the chyme, urine and blood]

[Determination of the protein quality of food and animal feed]

The estimated value of true digestibility of food and feed proteins is in dependence from the excretion of metabolic faecal nitrogen (MFN). Results of many authors showed that a high fibre content of the diet increase the MFN-excretion and lower the true digestibility of the diet protein. The exact estimation of MFN is only possible with isotopic methods. The labelling of feed proteins with isotopic nitrogen (15N) is possible in experiments with small animals. In big animals (pigs) the utilization of recycled 15N is too high following the long transit time of non digested 15N-labelled feed protein and the parallel 15N-labelling of MFN. The best method for the estimation of true digestibility of proteins is the method with 15N-labelled animals and the differentiation between nonlabelled undigested feed protein and the 15N-labelled MFN in faeces. The estimation of digestibility of essential amino acids at the end of the ileum as a measure for protein quality is not in advantage because the bacterial breakdown and synthesis of amino acids in the small intestine is unknown. The estimation of the biological value (BV) of proteins with the classical method is useful when the exact MFN was determined. The classic formula of BV [formula: see text] is only applicable in experiments with growing animals with nitrogen retention. This result of BV value is in agreement with the method only valid for growing animals with N-retention and not for animals or human beings in maintenance. The measurement of a BV of proteins with animals in maintenance is possible when the animals are labelled with 15N. The 15N-loss of the animal after the feeding of different protein sources is the smallest when the amino acid pattern of the protein is adapted to the need of amino acids for the maintenance metabolism of the animal. It was found that proteins with a high content of glutamic and aspartic acid (proteins of grains) are better proteins for the maintenance metabolism as animal proteins. Measurements with the method of the oxydation rate of essential amino acids showed that the amino acid pattern of wheat protein is in agreement with the need of amino acids for maintenance of the adult men. The evaluation of protein quality in animal nutrition for growing or lactating monogastric animals is in the present time the balance of essential amino acids in the feed protein and the need of this amino acids of the animals.

[The passage, absorption and secretion of nitrogen during the postprandial period using 15N-labeled wheat and marker in growing rats]

After a 9 day preparation period 42 Wistar rats (live weight 100 g) were fed a diet of 15N labelled wheat supplemented with the marker TiO2 (impulse labelling). At 7 time intervals (0.5 to 6 h after feeding) 6 animals were killed and thereafter total N, 15N and TiO2 levels were estimated in the digesta of different intestinal sections. The following results were obtained: The transit rate of the marker amounted to 10.3 +/- 0.62% per hour of the intake. The endogenous part of N increased during passage from stomach (3.5%) to duodenum (38.6%), jejunum (59.1%), ileum (64.8%), large intestine (78.3%) and faeces (87.7%). The apparent N digestibility in the stomach increased with time reaching 26% 6 h after feeding. In the whole small intestine it was 66.3%, in the ileum 78.9% and in the large intestine 90.4%. The true digestibility (6 h after feeding) showed the same course, but was always some units higher (stomach 33.3%, whole small intestine 82.5%, ileum 92.2% and large intestine 93.5%). Apparent and true digestibility values in the ileum correspond best to the data of precaecal digestibility; those of the large intestine correspond to the postileal digestibility. The N disappearance rate in the stomach is the sum of absorption rate (16%/h) and transit rate into the small intestine (12.4 +/- 1.6 mg N/h). Most absorption occurred during passage through the small intestine (2/3 of total absorption). The absorption in the small intestine was about 80% of the N amount flowing from the stomach into the intestine. The amount lay between 18.2 and 26.1 mg N/h and half of this was of endogenous origin. The reabsorption rate of endogenous N for the whole intestinal tract was estimated to be 91.4%. The N secretion into the whole intestine increased during the 6 h after feeding up to 85.5 mg (64% of N intake), for which 77 +/- 5.5% was secreted into the small intestine. Secretion into the stomach was relatively small and up to 4 hours after feeding, amounted only to 3.0 ... 6.7 mg N.

Determining of nitrogen absorption and nitrogen secretion in different sections of the pig's intestine by digesta exchange between 15N labelled and unlabelled animals

In an experiment with 3 pigs (initial live weight 30 kg, each fitted with 2 re-entrant fistulas in duodenum and ileum, one labelled with 15N), the duodenal and ileum digesta was exchanged. The N and 15N contents were estimated in faeces, urine, duodenal and ileum digesta of all experimental animals as well as in special organs and in the contents of different tract sections. The 15N excess (15N') of N compounds secreted into the gut lumen was determined using the 15N' in pancreas, gut mucosa and TCA-soluble blood serum. From measuring the digesta passage through the 3 sections of the digestive tract: 1. mouth ... duodenum, 2. duodenum ... ileum, 3. ileum ... after (Krawielitzki et al., 1989) the absorption and secretion rates of nitrogen were calculated. Secretion into the 1st section amounted to 5.3 g N/d (= 15% of intake) and the absorption to approximately 1% of intake. In the 2nd section the corresponding dates were 8.9 resp. 38.6 g N/d (= 25 resp. 110% of N intake), and in the 3rd one 1.9 resp. 8.4 g N/d (= 5.6 resp. 24% of N intake). Total absorption amounted to 134% of N intake and the over all re-absorption of endogenous N compounds secreted into the gut lumen to about 90%. During the passage the amount of endogenous N (g/d) decreased from 5.3 at the duodenum to 3.8 at the ileum to 1.6 in the faeces, but the relative portion increased (13 resp. 35 resp. 39%). An incorporation into body proteins occurred only from N compounds absorbed in the 1st and in the 2nd section. N (or 15N) absorbed in the large intestine was almost quantitatively excreted by urine. The method of digesta exchange between cannulated labelled and unlabelled pigs seems to be a suitable method to estimate absorption and secretion of exogenous and endogenous N portions in various sections of the digestive tract.

[Protein digestibility of 15N-labeled wheat bran and its transit time through the digestive tract of the human]

Six healthy human subjects were given 15N-labelled wheat bran in addition to a normal diet. The apparent digestibility of 15N was 84%. The gastrointestinal transit time varied between 25 and 35 h.

[Nitrogen and amino acid absorption in the small intestine of growing pigs. 2. Apparent and true crude protein digestibility and amino acid absorption to the end of the small intestine]

Studies of the apparent and true ileal digestibility of crude protein and the apparent and true amino acid (AA) absorption were carried out with 7 female pigs in the live weight range between 22 and 34 kg after ileorectostomy. Protein sources of varied quality with regard to the method of treatment (toasted) and untoasted soybean oilmeal) and to the AA composition (vital gluten or vital gluten + L-lysine X HCl) were used. Distinct differences regarding the apparent ileal digestibility of crude protein and the apparent absorption of amino acids were discovered between the protein sources, the cause of which is to be seen in the different amounts of endogenous N. The true ileal digestibility of crude protein and amino acid absorption were partly distinctly higher than the values of the apparent ileal digestibility of crude protein and AA absorption. On the whole one can say that the results of the investigation gained by means of ileorectostomy can be compared to the values gained from cannulated animals. Thus the experimental method used is suitable for studying the absorption process to the end of the small intestine of pigs.

[Effect of type of starch and quantity in rations on apparent and true digestibility of nitrogen and nitrogen balance in sheep]

In a digestibility trial 5 semisynthetic rations were fed in 3 periods to 10 male sheep to examine the effects on N balance, components of faecal nitrogen and N digestibility. The rations contained constant amounts of nitrogen but different contents of cellulose and two different types of starch (untreated and steamflaked). Content and type of starch did not show any noticeable effect neither on excretion of undigested dietary nitrogen nor on true digestibility. There could not be noticed any effects on the apparent N digestibility by changing contents of cellulose or untreated starch. If the rations contained steamflaked corn starch, the animals excreted more faecal nitrogen and therefore showed a lower apparent N digestibility. Especially the water soluble N fraction of the faecal nitrogen was clearly higher. Compensation took place through a lower N excretion with the urine. The reason for increased faecal N excretion may be higher microbial protein synthesis in the rumen. This was accented by the allantoin excretion with the urine.

[Studies with 15N-labeled lysine in colostomized hens. 2. 15N excretion in feces]

Over a period of four days colostomised hens were given 15N-lysine, and the development of 15N-excretion both in the TCA-soluble and the TCA-precipitable fraction of the faeces was measured over eight days. In both fractions the total, lysine, histidine and arginine N and 15N-excess (15N') was determined. The average apparent digestibility of 14N was 81.2% +/- 1.1% and of 15N' 93.2% +/- 0.7%. Labelled N is already excreted in faeces 3 hours after its application. The TCA-precipitable N is less strongly labelled than the TCA-soluble N. During the application of 15N' the labelling in faecal lysine is nearly one power of ten higher than in total N. The atom-% 15N' of the lysine could also be distinctly detected in arginine and histidine. The quotas of the total 15N' in faeces were 3.5% for arginine-15N' and 0.8% for histidine 15N'; 15N' can mainly be detected in the soluble fraction.

Flow of endogenous and exogenous amino acids along the gut of pigs

Digesta were collected from 5 pigs of 33 kg live weight fitted with re-entrant cannulas in the duodenum (within 20-30 cm of the pylorus) and terminal ileum. The pigs received a diet of barley, soya bean oilmeal and a vitamin and mineral mixture. The flow rates of digesta, total nitrogen and the individual amino acids were measured at different time after feeding and during two 24 h periods. A marked increase in the flow of digesta, nitrogen and amino acids was seen in the duodenum after feeding. Total flow during 24 h of nitrogen and amino acids except His, Val, Leu, Phe and Met exceeded intake. Output of nitrogen and amino acids from the duodenal cannula was 117 and 108% of intake, respectively. A method to calculate the ratio of endogenous amino acids in digesta based on the amino acid composition of digesta, diet and endogenous secretions was developed. The calculated amounts of endogenous amino acids passing the proximal duodenum and terminal ileum were 32.2 and 21.9 g per 24 h, respectively. The greatest amount of endogenous amino acids passed through the duodenal cannula in the first two hours after feeding (2-3 g/h) and then gradually decreased to 1 g per hour. The results are discussed in relation to other studies on the secretion of endogenous protein and its amino acid composition.

[Absorption of protein-linked N and synthetic lysine in the small intestine of pigs]

The absorption and absorbility of the protein-linked N was investigated with 3 pigs in the live weight range between 15 and 27 kg, which had been provided with simple cannulae at the end of the ileum and with portal vene catheters. After receiving a semi-synthetic ration with 15N-labelled curds as sole protein source, supplemented with 0,3% 35S-DL-methionine, the course of absorption could be observed both for 15N and 35S with the help of the chyme samples. Considering the endogenous 15N-quota in the chyme in the small intestines, it was possible to acquire data concerning the apparent and true absorption resp. absorbility of the feed-N. It could be ascertained that the supplemented synthetic amino acid is better absorbed than the protein-linked amino acids. So the absorption of 35S amounted to 91,2% of the intake whereas only 84,2% of the labelled N was observed.

[Endogenous N-secretion in the small intestine of pigs]

The method of isotope dilution for the determination of endogenous N-secretion was used with three pigs of a live weight of between 15 and 27 kg, which were provided with simple cannulae at the end of the small intestines. The test animals received a single dose of a semi-synthetic ration with 15N-labelled dried curds as sole source of protein. The passage rate to the ileum ascertained with the help of 51Cr2O3 was 70% in 24 hours. During the test period of 24 hours the endogeneous N-amount in the ileum chyme was 1.11 g resp. 56.6 mg/kg live weight. Of that, 60% were allowed to be absorbed in the chyme at the end of the small intestine.

[Protein digestibility and amino acid absorption in various segments of the digestive tract of pigs. 5. Results of the application of 35S-methionine]

The intake, excretion and the rate of flow of protein, methionine and 35S were measured at four pigs (live weight 63 kg) with re-entrant cannulae in the duodenum and the ileocecal region after the oral application of 35S-methionine and the feeding with a wheat + wheat gluten + lysine resp. a protein-free diet over a period of 7 days. From the measured values one pattern each for the N- and the methionine metabolism was laid down with the help of the compartment model, and the endogenous and exogenous levels of protein resp. methionine in the duodenum, the ileum and in feces were estimated.

[Protein digestibility and absorption of amino acids in various segments of the digestive tract of pigs. 2. Protein and amino acid balances at the end of the small intestine and of the whole digestive tract (apparent and true protein and amino acid digestibility)]

Growing pigs with ileum and ileocecal re-entrant canulae were given 5 different rations (pig fattening feed 1 and 2, rations with dried skim milk, wheat gluten + lysine and wheat + wheat gluten + lysine). The crude protein and amino acid excretion was quantitatively determined with ileum chyme and feces. The calculation of the amino acid balances (apparent and true digestibility) at the end of the small intestine, at the end of the wholedigestive tract and the isolated colon showed characteristic differences for the individual amino acids. On ileum level the amino acids with relatively high endogenous quotas, threonine, tryptophane, alanine, aspartic acid and glycine showed a distinctly lower, the amino acids arginine, methionine, phenylalanine, glutamic acid, proline and tyrosine a distinctly higher digestibilityhan crude protein. In comparison of the values on the feces level, including the true digestibility, these differences are largely balanced out. At the end of the small intestine the absorption of the amino acids is in the main completed. The disappearance rate of amino acids from the colon shows large differences with reference to the different rations as well as the individual amino acids. They were particularly high for various rations as far as proline, tryptophane, glycine and cystine are concerned. If wheat gluten + lysine rations were given, a net synthesis of methionine in the colon could be proved.

[Nitrogen and amino acid absorption in swine. 3. Endogenous secretion and absorption of nitrogen]

The endogenous nitrogen in the duodenum and the distal segment of the small intestine of 3 fistulated pigs fed a 15-N-labelled feed protein was determined using the isotope dilution technique. Over 24 hours, the endogenous N secretion was found at least 12.5 g at the end of the duodenum. For the upper segment of the small intestine, a statistically secured correlation between endogenous nitrogen and chyme amount was found to exist. It was possible to determine the true and apparent absorption of the nitrogen for both individual segments of and the whole digestive tract.