H. S. Bayley

Digestion of protein in small and large intestine of the pig.

1. Six 45 kg pigs with re-entrant ileal cannulas were used in two 3 × 3 Latin-square design experiments to study the site of absorption of protein and amino acids. Semi-purified diets containing soya-bean meal (SBM), rapeseed meal (RSM) or no protein source (protein-free) were offered at the rate of 1 kg dry matter/d. 2. Flow-rates of ileal contents for 24 h collection periods, corrected for recovery of marker, were 3135, 3127 and 1243 ml (SE 390) for SBM, RSM and protein-free diets respectively. 3. Amounts of dry matter digested in the small intestine were 730, 669 and 809 g/d for SBM, RSM and protein-free diets respectively, all values being significantly different ( P 4. Nitrogen intakes were 32·6, 29·9 and 5·9 g/d, and amounts digested in the small intestine were 25·7, 20·2 and 1·6 g/d for SBM, RSM and protein-free diets respectively, all values being significantly different ( P 5. Total amino acid intakes and amounts collected at the ileum and in the faeces were (g/d): SBM, 177, 24 and 18; RSM, 149, 28 and 22; protein-free 3, 9 and 12. Digestibility in the small intestine was higher for SBM than RSM for seventeen of the eighteen amino acids estimated. Greater quantities of arginine, methionine, cystine and tyrosine were voided in the faeces than passed through the ileal cannulas for pigs receiving the SBM and RSM diets. For those receiving the protein-free diet this was true for each amino acid except proline. 6. Significant differences were found between all diets in the concentration of some amino acids in ileal and faecal amino-N, and endogenous protein secretions did not mask the differences between diets. 7. Differences in digestibility between SBM and RSM were greater at the ileum than in the faeces. Amino acid fermentation in the large intestine obscured or reduced differences between SBM and RSM.

Determination of amino acid requirements of young pigs using an indicator amino acid.

Mixtures of skim milk and free amino acids were compared as diets for pigs which would allow manipulation of dietary amino acid levels. Piglets gained 208 g/d between 3 and 14 d of age on the skim-milk diet, but replacement of 600 g/kg of the dietary nitrogen with free amino acids reduced growth rate to 148 g/d. Supplementation of a lysine-deficient diet with lysine reduced the catabolism of [14C]phenylalanine showing that phenylalanine catabolism could be used as an indicator of the adequacy of diet with respect to another essential amino acid. The dietary level of phenylalanine which would provide an excess for catabolism by the piglet was estimated directly by measuring the influence of dietary phenylalanine level on [14C]phenylalanine oxidation. Reduction of the dietary phenylalanine level below 7 g/kg had no effect on phenylalanine oxidation, whereas increasing the dietary phenylalanine level above 7 g/kg resulted in a linear increase in phenylalanine oxidation. An indirect estimate of histidine requirement was made by examining the influence of dietary histidine level on [14C]phenylalanine oxidation. In diets containing more than 4 g histidine/kg, phenylalanine oxidation was minimal. In diets containing less than 4 g histidine/kg, [14C]phenylalanine oxidation increased as the level of dietary histidine was reduced. This showed that the utilization of the essential amino acid phenylalanine, for protein synthesis, was not limited by histidine supply in diets containing more than 4 g histidine/kg. A direct estimate of histidine requirement was made by examining the influence of dietary histidine level on [14C]histidine oxidation. Diets with more than 4 g histidine/kg contained an excess which was catabolized: there was a linear increase in histidine oxidation in response to dietary histidine levels greater than 4 g/kg. This confirmed the previous indirect estimate of histidine requirement.

Proline as an essential amino acid for the young pig

1. The catabolism of L-[1-14C]phenylalanine was used to indicate the effects of single amino acid supplementation of an inadequate protein diet (200 g crude protein (nitrogen x 6.25)/kg) on the utilization of dietary amino acids in pigs of 2.5 kg body-weight reared on an adequate protein diet (240 g crude protein/kg) containing skim milk and a mixture of free amino acids. 2. The oxidation of phenylalanine was decreased by the addition of proline or arginine to the inadequate protein diet but not by the addition of threonine, methionine, lysine or a mixture of essential amino acids, indicating that proline and arginine were limiting the utilization of dietary amino acids in the inadequate protein diet. 3. Dietary proline concentrations of 13.9 and 14.2 g/kg minimized phenylalanine oxidation in diets containing 200 or 260 g protein/kg. This indicates a dietary proline requirement of 14 g/kg. 4. Increasing the dietary arginine concentration in a diet containing 240 g protein/kg showed that an arginine concentration of 5.1 g/kg minimized phenylalanine oxidation. However, increasing the arginine concentration in a diet containing 200 g protein/kg increased phenylalanine oxidation, suggesting an amino acid imbalance involving arginine at this lower level of protein.

Influence of dietary protein concentration on the oxidation of phenylalanine by the young pig.

1. Piglets were weaned at 3 d of age and reared to 2.5 kg on a liquid diet in which the protein was supplied by dried skim milk and a mixture of free amino acids. The oxidation of L-[1-14C]phenylalanine was measured as an indication of the partition of amino acids between retention and catabolism in pigs (2.5 kg) offered meals containing varied concentrations of crude protein (nitrogen x 6.25). 2. The dietary protein concentration was varied either by increasing the inclusion of a mixture of free amino acids in a series of diets containing 100 g protein/kg from skim milk, or by increasing the level of inclusion of the skim milk in a series of diets containing the equivalent of 100 g protein/kg from the free amino acid mixture. 3. The oxidation of phenylalanine was minimized by dietary protein concentrations of 240 and 258 g/kg for the diets containing increasing concentrations of free amino acids or skim milk respectively. 4. These results show that a mixture of free amino acids is used more effectively than intact protein for promoting retention of essential amino acids. 5. The recovery of radioactivity in expired carbon dioxide was inversely related to the recovery of radioactivity in liver tissue when the concentration of dietary crude protein was increased from deficient to adequate, demonstrating that the fractional oxidation of the indicator amino acid was inversely related to protein synthesis.

Oxidation of an indicator amino acid by young pigs receiving diets with varying levels of lysine or threonine, and an assessment of amino acid requirements

The catabolism of [14C]phenylalanine was used to indicate the effects of varying the dietary level of lysine and threonine on the retention of dietary amino acids by 2-week-old pigs receiving diets containing skim milk and a mixture of free amino acids. Reducing the dietary level of lysine from 16 to 12 g/kg had no influence on phenylalanine oxidation, reducing the lysine level from 12 to 11 then to 10 g/kg caused an almost linear increase in phenylalanine oxidation whereas further reduction to 9 or 8 g/kg resulted in a less-marked increase in phenylalanine oxidation. This showed that 12 g lysine/kg was required to maximize amino acid retention and indicated that lysine was conserved more effectively at low dietary concentrations than at dietary concentrations approaching the requirement. Reducing the dietary level of threonine from 8 to 6 g/kg had no influence on phenylalanine oxidation, whereas further reduction to 4 g/kg caused a linear increase in phenylalanine catabolism showing that 6 g threonine/kg was required to maximize amino acid retention. Reduction of the levels of lysine, threonine and methionine from the generous levels characteristic of a diet containing 240 g protein from skim milk/kg, to the requirement levels determined separately in the presence of the generous levels of all the other amino acids, resulted in a twofold increase in phenylalanine catabolism. This shows that the pig seems able to conserve limiting intakes of a single amino acid, but not if the intakes of two or three amino acids are limiting.

Amino acid bioavailability.

Publisher Summary This chapter discusses the amino acid composition of a feedstuff or a diet that can be determined by chemical procedures, usually acid hydrolysis followed by ion-exchange chromatography with colorimetric or fluorimetric detection of the amino acids. The goal of in vitro procedures is to predict from laboratory tests of feedstuffs the amounts of amino acids that will be available to animals. Most chemical tests of availability are specific for lysine. Measurement of amino acid bioavailability in the target species is obviously more desirable than tests performed solely in the laboratory. However, direct measurements with animals are both expensive and time-consuming. The digestibility of feeds and their proximate components is an important determinant of their relative worth as components of mixed diets for animals. On the assumption that amino acids that disappear from the digestive tract in the hindgut are of no value to the animal, the flux of amino acids from the ileum must be measured. The purpose of determining amino acid digestibilities is to optimize the use of feed resources for animal production. Comprehensive reviews of digestibility data in poultry and swine have been published by Nutri-Quest, Inc. Assays of bioavailability that involve some measurement of animal growth are usually considered as the ultimate standards against which other methods are judged. Most chick assays now consist of 7- to 10-day growth period that is begun approximately one week post-hatching. When the basal diet contains intact proteins, assumptions have to be made about the bioavailability of the test amino acid in these proteins.

Amino acid oxidation by young pigs receiving diets with varying levels of sulphur amino acids

Piglets were weaned at 3 d of age and were introduced to liquid diets in which 400 g/kg protein was supplied as skim-milk powder and the balance as a mixture of free amino acids. The skim milk contributed 2.3 g methionine and 1.4 g cystine/kg diet; the experimental diets were made by supplementing these levels with free amino acids. The adequacy of the test level of the amino acid in the diet was assessed by measuring the oxidation of [1-14C]phenylalanine as an indicator of the partition of the essential amino acids between incorporation into protein and degradation. Radioactivity recovered as carbon dioxide was used as a measure of catabolism. Addition of 0.2, 0.4, 0.7, 1.2 and 1.7 g L-methionine/kg to a diet containing 5 g cystine and 3 g choline chloride/kg showed that phenylalanine catabolism was minimal for the diet with 2.7 g methionine/kg indicating that this is the dietary requirement for methionine to serve as a source of methionine residues for protein synthesis. Addition of D-methionine to produce a series of diets with graded levels of methionine showed that the D-isomer was less effective than the L-isomer in reducing phenylalanine catabolism: the addition of 0.8 g D-methionine/kg diet was needed to produce the same effect as 0.4 g L-methionine/kg diet showing that the replacement value of D-methionine for L-methionine was 50% in the young pig.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of frequency of feeding of diets containing free or protein-bound lysine on the oxidation of [C]lysine or [C]phenylalanine by growing pigs

The effect of frequency of feeding (once daily or six equal portions at intervals of 3 h) of diets containing lysine (6.4 g/kg) supplied as either protein-bound or free lysine on the metabolism of essential amino acids was investigated in pigs over the 23-34 kg weight range. The amounts of radioactivity from 14CO2 produced by the pigs given oral doses of [14C]lysine or [14C]phenylalanine were determined. [14C]Lysine was confirmed to be unsuitable presumably because the oral dose was diluted by the dietary free lysine or was absorbed ahead of the protein-bound lysine, or both. With [14C]phenylalanine addition, there was a significantly (P less than 0.01) greater production of 14CO2 by pigs fed once daily on the diet containing free lysine. However, with frequent feeding, there was no significant difference (P less than 0.05) in the amount of 14CO2 produced by pigs from [14C]phenylalanine for either diet. Growth responses of the pigs were lower when the diet containing free lysine was given once daily (P less than 0.01) but were similar when the diets were given frequently. It is concluded that the more rapid absorption of free lysine by growing pigs fed once daily results in an imbalance of amino acids at the sites of metabolism. This leads to greater oxidation of essential amino acids than occurs when all the amino acids are supplied in the protein-bound form. With frequent feeding, a better balanced supply of amino acids is absorbed, leading to similar rates of oxidation of excess essential amino acids from diets containing either free or protein-bound lysine.

The influence of level of grain intake on protein digestion in the intestine of cattle

1. Protein digestion in the gastro-intestinal tract of cattle receiving diets with either low or high content of maize was investigated. Digesta obtained from the duodenum and ileum were separated into three fractions; soluble, microbial and particulate, and the amino acid composition of the three fractions and that of the total digesta were determined. The proportion of particulate material digested in the intestine was less from the high-maize than from the low-maize diet, but more of the particulate fraction entered the duodenum from the high-maize than from the low-maize diet, so the absolute amount of particulate fraction which was digested was greater for the high-maize diet. 2. More protein was digested in the rumen from the low-maize diet (high-roughage) than from the other diet. Total digestion of protein from the low-maize diet was also higher. 3. Although more protein from the high-maize than from the low-maize diet reached the duodenum, the protein from the low-maize diet was more digestible in the intestine, so the total amount of dietary protein digested in the intestine was the same for both rations.

Body Composition Analysis by Dual Energy X-Ray Absorptiometry Compared to Chemical Analysis of Fat, Lean and Bone Mass in Small Piglets

The influence of nutrient intake on body composition in growing low birthweight infants is essential knowledge in order to optimize nutritional management for the support of growth and development. Until recently, indirect methods of measuring body composition were inappropriate; but the latest generation of whole body densitometers, dual-energy X-ray absorptiometry (DXA) may be a useful tool for this age group. The application of DXA for quantitative assessment of fat, lean and bone mass should provide measures with precision and accuracy while being safe and noninvasive for use in infants and small children. Our objective was to establish the precision and accuracy of DXA measurements of whole body bone mineral content (BMC), fat and lean tissue in young piglets which are similar in body composition to infants1,2.