Dietary starch influences growth performance, nutrient utilisation and digestive dynamics of protein and amino acids in broiler chickens offered low-protein diets

1. Introduction

There is considerable interest in the successful development of low protein diets for broiler chickens, which axiomatically contain high inclusions of supplemental amino acids. This is because low protein diets have the potential to generate economic, environmental and bird welfare advantages.

Formulation of low protein diets are usually achieved by decreasing soybean meal inclusions and increasing inclusions of various combinations of essential and non-essential supplemental amino acids. However, an alternative approach was adopted in the present study where dietary protein levels were reduced by the partial replacement of maize grain with maize starch.

The low protein diets (172–192 g/kg protein) were then supplemented with five differing combinations of supplemental amino acids and compared with a positive control, relatively high protein diet (213 g/kg protein). The rationale for this approach was that higher dietary concentrations of maize starch would be more readily and rapidly digested than the starch component of maize grain and that this difference in starch digestive dynamics could influence digestion of protein and intestinal uptakes of both supplemental and protein-bound amino acids.

A number of reports, including Holsheimer and Janssen (1991), indicate that reduced soybean meal inclusions in low protein diets compromise poultry performance despite additions of supplemental amino acids.

The positive control diet contained 465 g/kg maize grain but this was reduced to an average of 100 g/kg in the five low protein dietary treatments which contained an average of 479 g/kg maize starch. Thus, substitution increased the formulated starch concentration from 299 g/kg in the positive control diet to an average of 542 g/kg in the five low protein diets.

The influence of starch digestive dynamics might be an overlooked component in the successful development of low-protein diets. However, the ultimate determinant of the post-enteral availability of glucose and amino acids is their metabolic fates in enterocytes of the gut mucosa. Both glucose and amino acids, perhaps glutamate and glutamine especially, are catabolised within enterocytes to meet the copious energy requirement of the gut (Watford et al., 1979).

For this reason, concentrations of free amino acids in plasma samples taken from the anterior mesenteric vein were determined. Thus, the primary objective of this study was to investigate the impact of dietary starch on growth performance and the digestive dynamics of protein and amino acids in broiler chickens in the context of low protein diets containing elevated inclusions of supplemental amino acids.

In this context, digestive dynamics is defined as digestion of protein, absorption of amino acids and the appearance of amino acids in the portal circulation. It is our contention that the role of digestive dynamics in relation to broiler performance merits further attention.

2. Materials and Methods

2.1. Diet Preparation

A total of six experimental diets were formulated to be iso-energetic (12.80 MJ/kg ME) and on a standardised ileal digestible (SID) amino acid basis as shown in Tables 1 and 2. The positive control (PC) diet (1A) was formulated to contain 213 g/kg protein which

Table 1 Composition of experimental diets.

Dietary starch influences growth performance, nutrient utilisation and digestive dynamics of protein and amino acids in broiler chickens offered low-protein diets - Image 1

1. The vitamin-mineral premix supplied per tonne of feed:
[MIU] retinol 12, cholecalciferol 5;
[g] tocopherol 50, menadione 3, thiamine 3, riboflavin 9, pyridoxine 5, cobalamin 0.025, niacin 50, pantothenate 18, folate 2, biotin 0.2;
[copper (mg)] 20, iron 40, manganese 110, cobalt 0.25, iodine 1, molybdenum 2, zinc 90, selenium 0.3.

Table 2

Nutrient specifications of experimental diets (amino acids expressed as standardised ileal digestibility values).

was derived from soybean meal, maize, and limited quantities of supplemental lysine, methionine, and threonine.

The low protein diets (2B to 6F) were formulated to contain an average of 181 g/kg protein, which was achieved by reducing maize grain and replacing it with maize starch. As a consequence, formulated dietary starch levels increased from 299 g/kg in 1A to a mean of 542 g/kg in 2B to 6F.

Diets 2B to 5E were supplemented with an increasing array of supplemental amino acids as shown in Table 1. Diet 2B was deficient in branched-chain amino acids and contained lower glycine and serine concentrations than the PC diet.

Diet 3C was corrected for the branched-chain amino acid deficiency with isoleucine and valine supplementation. Diet 4D was similar to 3C but with additional glycine, proline, and serine supplementation to meet the levels of the PC diet. Similarly, Diet 5E was supplemented with the remaining non-essential amino acids: alanine, aspartic and glutamic acids.

In contrast, Diet 6F was supplemented with lysine, methionine, threonine, plus two branched-chain amino acids—isoleucine and valine.

Analysed results of starch, protein, and amino acid concentrations in the experimental diets are shown in Table 3.

Maize and soybean meal were ground through a 3.2 mm hammer-mill.

Table 3

Analysed starch, protein and amino acid concentrations in the experimental diets.

screen prior to being mixed into the diets. The experimental diets were steam-pelleted at a conditioning temperature of 80°C with a 14 s residence time and crumbled after passing through a vertical cooler.

2.2. Bird management

A total of 288 day-old, male Ross 308 chicks were procured from a commercial hatchery, housed in an environmentally controlled facility, and were initially offered a proprietary starter ration. At 7 days post-hatch, birds were individually identified (wing-tags), weighed, and allocated into 48 cages on the basis of body weights so that the mean and variation in each cage was almost identical.

Each of the six dietary treatments was then offered to eight replicate cages (6 birds per cage) from 7 to 28 days post-hatch. Birds had unlimited access to feed and water during the experimental feeding period under a ‘16-h-on’ lighting regime and room temperature was gradually reduced from 32°C at day 1 to 22°C at day 28.

Body weights were again determined at 28 days post-hatch and feed intakes for each cage were recorded over the 21-day feeding period to calculate feed conversion ratios (FCR). These calculations were adjusted by the body weight of any dead or culled birds, which were monitored on a daily basis.

Total excreta collection to determine parameters of nutrient utilisation was completed over a 48 h period from 25 days post-hatch. At 28 days post-hatch, the birds were euthanised (intravenous injection of sodium pentobarbitone) in order to collect various samples for analyses.

2.3. Sample collection and chemical analysis

Feed intake was monitored and total excreta collected from each cage over a 48 h period to determine apparent metabolisable energy (AME), metabolisable energy to gross energy (ME:GE) ratios, nitrogen retention, and N-corrected AME (AMEn).

Excreta were dried in a forced-air oven at 80°C for 24 h and the gross energy (GE) of excreta and diets were determined using an adiabatic bomb calorimeter.

The AME values of the diets on a dry matter basis were calculated from the following equation:

ME:GE ratios were calculated by dividing AME by the gross energy (GE) of the appropriate diets. Nitrogen (N) contents of diets and excreta were determined using a nitrogen determinator (Leco Corporation, St Joseph, MI), and N retentions were calculated from the following equation:

N-corrected AME (AMEn MJ/kg DM) values were calculated by correcting N retention to zero using the factor of 36.54 kJ/g N retained in the body (Hill and Anderson, 1958).

At day 28, three birds were randomly selected from replicate cages of dietary treatments 1A, 3C, and 5E. Immediately following euthanasia, the abdominal cavities were opened and blood samples were drawn from the anterior mesenteric vein. Blood samples were then centrifuged and the decanted plasma samples were kept at −80°C prior to analysis.

Concentrations of twenty proteinogenic amino acids in plasma taken from the brachial and anterior mesenteric veins were determined using pre-column derivatisation amino acid analysis with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC; Waters™ AccQTag Ultra; Waters Australia PL; www.waters.com), followed by separation of the derivatives and quantification by reversed phase ultra-performance liquid chromatography (RP-UPLC; Cohen, 2001). All amino acids were detected by UV absorbance. This procedure is fully described in Selle et al. (2016).

For all birds, pH of gizzard digesta was determined in situ with a pH probe. The pancreas was dissected from the duodenal loop and weighed to determine relative pancreas weights.

Digesta were collected in their entirety from the proximal jejunum (PJ), distal jejunum (DJ), proximal ileum (PI), and distal ileum (DI) and pooled for each cage. The segments were demarcated by the mid-points between the end of the duodenal loop, Meckel’s diverticulum, and the ileo-caecal junction.

Digesta samples were freeze-dried to determine apparent digestibilities of starch, protein, and amino acids, using Celite™ (World Minerals Inc, Lompoc CA), a source of acid insoluble ash (AIA), as the inert dietary marker.

Starch concentrations in diets and digesta were determined by a procedure based on dimethyl sulphoxide, α-amylase, and amyloglucosidase as described by Mahasukhonthachat et al. (2010).

N content was obtained using an FP-428 determinator (Leco Corporation, St Joseph, MI), and AIA concentrations were determined by the method of Siriwan et al. (1993). Crude protein was estimated as N × 6.25.

Amino acid concentrations of diets and digesta were determined following 24 h liquid hydrolysis at 110°C in 6 M HCl, and then 16 amino acids were analysed using the Waters AccQTag Ultra chemistry on a Waters Acquity UPLC. Tryptophan and cysteine cannot be analysed accurately by this procedure.

The apparent digestibility coefficients for starch and protein in four small intestinal sites and the apparent digestibility coefficients of sixteen amino acids in the three posterior small intestinal segments (insufficient digesta in proximal jejunum) were calculated from the following equation:

Starch, protein and amino acid disappearance rates (g/bird/day) were deduced from feed intakes over the final phase of the feeding period using the following equation:

Table 4

Effect of dietary treatments on growth performance (weight gain, feed intake, FCR, mortality/cull rates) from 7 to 28 days post-hatch, pH of gizzard contents and relative pancreas weight.

3. Results

The effects of dietary treatments on growth performance from 7 to 28 days post-hatch, pH of gizzard contents and relative pancreas weights are shown in Table 4. Dietary treatments did not influence weight gain (P> 0.25). However, dietary treatments did influence feed intakes (P< 0.20) where the average feed intake of birds offered four low protein diets (2 B to 5E inclusive) exceeded that of the positive control diet by 6.95% (2109 versus 1972 g/bird). Treatments also influenced FCR (P< 0.005) where the average FCR of birds offered the five low protein diets was less efficient that the PC diet by 6.02% (1.462 versus 1.379). The mortality/cull rate of 5.6% was not influenced by treatment (P > 0.175). The transition from the positive control to low protein diets increased gizzard pH (3.90 versus 2.57) and decreased relative pancreas weights (1.72 versus 2.36 g/kg) to significant (P < 0.001) extents.

The highly significant (P < 0.001) effects of dietary treatments on nutrient utilisation are shown in Table 5. On average, the transition from positive control to low protein diets improved AME by 0.67 MJ (12.87 versus 12.20 MJ/kg), ME:GE ratios by 14.2% (0.806 versus 0.706), N retention by 5.0 percentage units (65.3 versus 60.3%) and AMEn by 0.64 MJ (12.30 versus 11.66 MJ/kg). Excreta dry matter was reduced from 30.3% to an average of 22.8%.

Table 6 shows the highly significant (P < 0.001) dietary treatment effects on apparent starch digestibility coefficients and disappearance rates in four small intestinal segments. The transition from positive control to low protein diets increased average starch digestibility coefficients by 23.5% (0.908 versus 0.735) in PJ, by 30.3% (0.951 versus 0.730) in DJ, by 15.9% (0.964 versus 0.832) in PI and by 10.9% (0.968 versus 0.873) in DI. Increases in starch disappearance rates followed a similar pattern. In birds offered the positive control diet, 84.0% of starch (18.54 ex 22.07 g/bird/day) disappeared in the proximal jejunum; in contrast an

Table 5

Effect of dietary treatments on nutrient utilisation (AME, ME:GE ratios, N retention, AMEn) and excreta dry matter.

Table 6

Effect of dietary treatments on apparent digestibility coefficients of starch and starch disappearance rates (g/bird/day) in four small intestinal segments (PJ: proximal jejunum, DJ: distal jejunum, PI: proximal ileum, DI: distal ileum).

average of 93.7% of starch (39.60 ex 42.25 g/bird/day) that disappeared in PJ in birds offered the low protein diets.

The effects of dietary treatments on apparent protein digestibility coefficients and disappearance rates in four small intestinal segments are shown in Table 7 where significant effects were not observed in PJ. However, the transition from positive control to low protein diets significantly decreased average protein digestibility coefficients by 9.91% (0.591 versus 0.656) in DJ, by 9.03% (0.675 versus 0.742) in PI and by 6.36% (0.721 versus 0.770) in DI. Also, the transition retarded (P < 0.001) average protein disappearance rates by 12.9% (11.76 versus 13.50 g/bird/day) in DI, by 11.5% (13.48 versus 15.24 g/bird/day) in PI and by 9.36% (14.33 versus
15.81 g/bird/day) in DI.

Effects of dietary treatments on apparent proximal jejunal digestibility coefficients and disappearance rates of essential amino acids are shown in Table 8. Significant effects were observed for digestibility coefficients of methionine (P < 0.02) and threonine (P < 0.03). On the basis of a pair-wise comparison, methionine digestibility was significantly higher in birds offered diet 2 B than those offered the positive control diet 1A by 14.4% (0.716 versus 0.626; P=0.024). In the same comparison, threonine digestibility was higher by 22.8% (0.668 versus 0.544; P=0.002) and diets 5E and 6F also supported significantly higher threonine digestibility coefficients than diet 1A. Dietary treatments significantly influenced the disappearance rates of arginine, histidine, leucine, methionine, phenylalanine and threonine. In comparison to diet 1A, diet 2 B accelerated disappearance rates of methionine by 35.5% (0.42 versus 0.31 g/bird/day) and threonine by 26.7% (0.57 versus 0.45 g/bird/day) but retarded rates of leucine by 35.8% (0.43 versus 0.67 g/bird/day) and phenylalanine by 28.3% (0.33 versus 0.46 g/bird/day). Table 9 shows the effects of dietary treatments on apparent proximal jejunal digestibility coefficients and disappearance rates of non-essential amino acids. In both instances significantly different observations were confined to aspartic acid, cysteine and glutamic acid.

Table 10 shows the effects of dietary treatments on apparent distal jejunal digestibility coefficients and disappearance rates of essential amino acids. Significant treatment differences were observed for all amino acids except methionine. A distinct pattern in responses is evident as diets 1A and 6F generated similar outcomes in digestibility coefficients that were statistically superior than diets 2 B to 5E inclusive for six amino acids; the exceptions being lysine, methionine and threonine. A similar pattern is apparent in amino acid disappearance rate responses although diet 5E generated significantly faster rates than all other diets in respect of isoleucine, leucine and lysine. The effects of dietary treatments on apparent distal jejunal digestibility coefficients and disappearance rates of non-essential amino acids are shown in Table 11. Again, the highest digestibility coefficients were associated with either diet
1A or 6F and the lowest digestibility coefficients were consistently associated with diet 4D. Diet 6F generated the most rapid disappearance rates which were significantly for six amino acids with the exception of glycine.

Table 7

Effect of dietary treatments on apparent digestibility coefficients of protein and protein disappearance rates (g/bird/day) in four small intestinal segments (PJ: proximal jejunum, DJ: distal jejunum, PI: proximal ileum, DI: distal ileum).

Table 8

Effect of dietary treatments on (top half) apparent digestibility coefficients and (bottom half) disappearance rates (g/bird/day) of essential amino acids in the proximal jejunum.

Table 9

Effect of dietary treatments on (top half) apparent digestibility coefficients and (bottom half) disappearance rates (g/bird/day) of non-essential amino acids in the proximal jejunum.

The significant effects of dietary treatments on apparent proximal ileal digestibility coefficients and disappearance rates of essential amino acids are shown in Table 12. The transition to low protein diets significantly compromised the digestibilities of arginine by an average of 8.21% (0.760 versus 0.828) and methionine by 3.82% (0.855 versus 0.889). For the balance of 7 amino acids the positive control diet supported the highest digestibility coefficients which were significantly higher than for the majority of the low protein diets with the exception of diet 6F which were numerically lower but not to significant extents. Moreover, diet 6F generated significantly faster disappearance rates than all other treatments for 6 essential amino acids. The exceptions included phenylalanine, threonine and, in the case of methionine, diet 6F supported the slowest disappearance rate. Table 13 shows the highly significant effects of dietary treatments on apparent proximal ileal digestibility coefficients of non-essential amino acids. In overall terms, diets
1A and 6F supported relatively superior digestibility coefficients. Diet 6F generated significantly faster disappearance rates than all other treatments for the non-essential amino acids.

Table 10

Effect of dietary treatments on (top half) apparent digestibility coefficients and (bottom half) disappearance rates (g/bird/day) of essential amino acids in the distal jejunum.

Table 11

Effect of dietary treatments on (top half) apparent digestibility coefficients and (bottom half) disappearance rates (g/bird/day) of non-essential amino acids in the distal jejunum.

The significant effects of dietary treatments on apparent distal ileal digestibility coefficients and disappearance rates of essential amino acids are shown in Table 14. The control diet 1A generated the highest digestibility coefficients which were statistically superior to the five low protein/high starch diets with the exception of Diet 6F. Diet 4D generated the lowest digestibility coefficients for the essential amino acids. The transition from the positive control to the five low protein/high starch diets collectively reduced average digestibility coefficients of nine amino acids by 6.68% (0.782 versus 0.838). The reductions were significant for each amino acid and the most pronounced reductions observed were 9.19% for leucine (0.751 versus 0.827; P< 0.001) and 9.23% for phenylalanine (0.757 versus 0.834; P< 0.001). Significant differences in the disappearance rates were observed for all essential amino acids with the exception of phenylalanine. Table 15 shows the significant effects of dietary treatments on apparent distal ileal digestibility coefficients and disappearance rates of non-essential amino acids. The same dietary transition collectively reduced average digestibility coefficients of seven amino acids by 7.26% (0.690 versus 0.744). The reductions were significant for each amino

Table 12

Effect of dietary treatments on (top half) apparent digestibility coefficients and (bottom half) disappearance rates (g/bird/day) of essential amino acids in the proximal ileum.

Table 13

Effect of dietary treatments on (top half) apparent digestibility coefficients and (bottom half) disappearance rates (g/bird/day) of non-essential amino acids in the proximal ileum.

acid except cysteine and the most pronounced significant reduction observed was 9.35% for aspartic acid (0.611 versus 0.674;
P< 0.04) Significant differences in the disappearance rates were observed for all non-essential amino acids.

Pearson correlations between digestibility coefficients of starch and 16 amino acids in the four corresponding small intestinal segments are shown in Table 16. In the proximal jejunum there were significant positive correlations for methionine, threonine, and cysteine and negative correlations with arginine and isoleucine. In the distal jejunum, starch digestibility coefficients were negatively correlated to nine amino acids to significant extents. Proximal ileal starch digestibility coefficients were negatively correlated with twelve amino acids to significant extents; similarly, there were significant negative correlations with eleven amino acids in the distal ileum. In summary there were significant negative correlations between starch digestibility and the digestibilities of eight amino acids (arginine, histidine, isoleucine, methionine, phenylalanine, threonine, valine, glutamic acid) in the three posterior segments if the small intestine.

Table 14

Effect of dietary treatments on (top half) apparent digestibility coefficients and (bottom half) disappearance rates (g/bird/day) of essential amino acids in the distal ileum.

Table 15

Effect of dietary treatments on (top half) apparent digestibility coefficients and (bottom half) disappearance rates (g/bird/day) of non-essential amino acids in the distal ileum.

The effects of three dietary treatments on free amino acid concentrations in plasma taken from the anterior mesenteric vein are shown in Table 17 where significant differences were observed in five amino acids. For example, diet 3C supported greater concentrations than diet 1A on the basis of pair-wise comparisons in lysine by 94.4% (76.8 versus 39.5 mg/mL; P< 0.001), methionine by 68.8% (21.6 versus 12.8 mg/mL; P=0.001), threonine by 60.8% (82.0 versus 51.0 mg/mL; P=0.006) and valine by 36.6% (41.8 versus 30.6 mg/mL; P=0.012). Conversely, relative to 1A, diet 5E reduced concentrations of histidine by 41.6% (10.1 versus 17.3 mg/mL; P=0.007). Also dietary treatments had were negative linear effects on plasma concentrations of histidine (r=−0.545; P< 0.01), arginine (r=−0.468; P< 0.025) and leucine (r=−0.452; P< 0.03).

Table 16

Pearson correlations between apparent digestibility coefficients of starch and apparent digestibility coefficients of 16 amino acids in four segments of the small intestine.

Table 17

Effect of three dietary treatments on free plasma amino acid concentrations (mg/mL) in the portal (anterior mesenteric vein) circulation.

4. Discussion

Growth performance of male birds offered the conventional, positive control diet from 7 to 28 days post-hatch compared favourably with Ross 308 performance objectives in weight gain (1433 versus 1387 g/bird), feed intake (1972 versus 2052 g/bird) and FCR (1.379 versus 1.479). However, the transition from the conventional to five test diets with lower protein contents, higher inclusions of supplemental amino acids and higher starch contents elevated average feed intakes by 6.39% (2098 versus 1972 g/bird) and compromised FCR by 6.02% (1.462 versus 1.379). The diets were formulated to be iso-energetic but this necessitated a reduction in canola oil from 68.0 g/kg in the positive control diet to an average of 16.1 g/kg in the five test diets. This reduction in dietary lipids probably contributed to the elevated feed intakes as lipid may trigger the ‘ileal brake’ and depress feed intakes in poultry (Martinez et al., 1995). In addition, the transition from the positive control to low protein diets increased gizzard pH and decreased relative pancreas weights. Relative gizzard weights were correlated to relative pancreas weights (r=0.438; P< 0.001) in Moss et al. (2017). These outcomes are indicative of depressed gizzard functionality and reflect the reduced fibre content (5.8 versus 8.8 g/kg) in the low protein diets.

The low protein dietary treatments 2 B to 6F were supplemented with varying arrays of supplemental amino acids but the performance of birds offered these diets did not differ to tangible extents. For example, diet 5E supported a numerically more efficient FCR by 2.86% (1.429 versus 1.471) in comparison to the balance of four low protein diets. Diet 5E was supplemented with twelve supplemental amino acids and this could suggest that non-essential amino acids should not be overlooked. Nevertheless, the overall pattern of results observed in the present study effectively precludes any consideration of which array of supplementary supplemental amino acids should be adopted in the formulation of low protein diets.

The maize starch component of the five low protein diets was associated with unequivocal increases in starch digestibility coefficients in four small intestinal segments. This outcome supports the Wiseman et al. (2000) contention that ‘starch extrinsic’ factors influence starch digestibility more so than the inherent properties of starch per se where starch-protein interactions in the endosperm of feed grains are believed to hold importance (Rooney and Pflugfelder, 1986). The precise identification of these starchprotein interactions remains lacking (Truong et al., 2016) but such interactions may not be confined to feed grain endosperms but extend to more general interactions between dietary starch and proteins during feed processing or in the avian gut. Certainly purifiedmaize starch enhanced digestibility and modified the starch digestive dynamics. In the control diet 84.2% of starch digested along the small intestine was digested in the proximal jejunum (‘rapid’ starch) leaving a balance of 15.8% (‘slow’ starch). In contrast, 93.2% of starch was ‘rapid’ leaving a balance of 6.2% ‘slow’ starch in the five low protein diets with higher maize starch inclusions. Given that ‘slow starch’ in broiler diets has been shown to be beneficial (Weurding et al., 2003; Enting et al., 2005; Truong et al., 2016), the ‘rapid starch’ in the low protein diets was probably disadvantageous.

Nevertheless, the maize starch component of the five low protein diets was associated with average improvements of 0.67 MJ in AME, 14.2% in ME:GE ratios, 5.0 percentage units in N retention and 0.64 MJ in AMEn in comparison to the positive control diet. The maize starch component of the five low protein diets was also associated with significant declines in protein digestibility coefficients in the three posterior small intestinal segments with corresponding reductions in protein disappearance rates.

The decline in protein digestibility coefficients is reflected in the significant negative correlations between starch digestibility coefficients with twelve amino acids in proximal ileum and eleven amino acids in distal ileum (Table 16). There were significant negative correlations with nine amino acids in the distal jejunum but there was also one positive correlation (cysteine). In the proximal jejunum there were two significant negative correlations but four positive correlations including methionine and threonine which were both negatively correlated with starch digestibility coefficients in the three posterior segments to significant extents. The likelihood is that both the supplemental methionine and threonine components of the diets would have been absorbed in the proximal jejunum. These outcomes suggest that glucose and amino acids are competing for absorption from the small intestine following the digestion of starch and protein and this probably applies more to protein-bound than crystalline amino acids. The capacity of the small intestine to absorb nutrients, including glucose and amino acids, may be rate-limiting on poultry performance (Croom et al., 1999) and Vinardell (1990) concluded that intestinal uptakes of glucose and amino acids are subject to mutual inhibition. As reviewed by Stevens et al. (1984), intestinal uptakes of glucose and amino acids are complex and interactive and there are documented indications that glucose and amino acids may compete for absorption along the small intestine (Alvarado and Robinson, 1975; Murer et al., 1975). This may apply particularly to competition between glucose and amino acids for co-absorption with sodium (Na) via their respective Na+-dependent transport systems, which are driven by the activity of the ‘sodium pump’ (Na+,/K+-ATPase) in the baso-lateral membrane of enterocytes.

Goldberg and Guggenheim (1962) compared the digestion of amino acids and their appearance in portal circulation in rats offered either casein or soy flour as protein sources. Both the small intestinal digestion and the entry into the portal circulation of lysine from casein were more rapid than lysine from soy flour. Moreover, the data suggests that a greater proportion of lysine was absorbed and entered the portal circulation from casein, the more rapidly digested protein source. Supplemental amino acids do not undergo digestion and are directly available for absorption in the upper small intestine (Wu, 2009); therefore, supplemental amino acids axiomatically constitute ‘rapid protein’. Both starch and protein require digestion before glucose and amino acids can be absorbed in the small intestine but the metabolic fates of glucose and amino acids in enterocytes is the final determinant their post-enteral availability. Rather than transferring into the portal circulation, amino acids may be synthesised into proteins to maintain gut integrity or serve as precursors for digestive enzymes, mucin, nucleotides, polyamines and amino acids (Wu, 1998). However, both glucose and amino acids are catabolised to provide energy to the gut and are approximately equally important energy substrates for small intestinal mucosal cells in the rat with the indication that energy is more efficiently derived from glucose (Fleming et al. (1997). A critical issue is whether or not the ‘catabolic ratio’ of glucose to amino acids can be manipulated by dietary strategies but there is evidence in pigs that slowly digestible starch will increase the net portal flux of amino acids in pigs (Van der Meulen et al., 1997)

In the present study diet 3C contained higher inclusions of six supplemental amino acids (arg, ile, lys, met, thr, val) than diet 1A (11.48 versus 3.71 g/kg). Significantly greater concentrations of lysine (94.4%), methionine (68.8%), threonine (60.8%) and valine (36.6%) in plasma in the anterior mesenteric vein than diet 1A were observed. Moreover, this was associated with a collective increase of 33.7% (333 versus 249 mg/mL) in concentrations of the six relevant amino acids in the portal circulation. In contrast, for amino acids present only as protein-bound forms, the corresponding increase was a more modest 5.13% (738 versus 702 mg/mL). Diet 5E contained higher inclusions of twelve supplemental amino acids than diet 1A (31.20 versus 3.71 g/kg) and there was a more modest collective increase of 9.43% (882 versus 806 mg/mL) in their concentrations in the portal circulation.

Higher concentrations of lysine, methionine, threonine and valine were observed in plasma from the anterior mesenteric vein in diet 3C than diet 1A. However, ileal digestibility coefficients were lower in diet 3C, in the case of lysine by 7.43% (0.822 versus 0.888), methionine by 1.74% (0.905 versus 0.921), threonine by 5.86% (0.707 versus 0.751) and valine by (0.768 versus 05.77%), than diet 1A. These reductions in amino acid digestibilities effectively amplify the increases observed in concentrations of free amino acid in the anterior mesenteric vein.

Interestingly, the above comparison between diets 1A and 3C, in particular, indicate that the transfer of amino acids into the portal circulation can be influenced by dietary manipulations. Wu (2009) indicated that supplemental amino acids appear in the portal circulation more rapidly than protein-bound amino acids and the present study suggests that higher concentrations of amino acids in the portal circulation are generated by supplemental rather than protein-bound amino acids. One interpretation of this outcome is that supplemental amino acids escape catabolism in the gut mucosa to some extent because they are absorbed in the upper small intestine where more glucose is available as an alternative energy substrate. However, it should be noted that birds offered diet 3C had significantly higher feed intakes than their 1A counterparts which may have influenced this comparison.

5. Conclusion

The inclusion of maize starch in low protein diets enhanced starch digestibility coefficients, starch disappearance rates and parameters of nutrient utilisation (AME, ME:GE ratios, N retention, AMEn). However, the same maize starch inclusion depressed protein digestibility coefficients and protein disappearance rates. Moreover, amino acid digestibility coefficients were depressed to varying extents in the three posterior segments of the small intestine. Instructively, proximal ileal starch digestibility coefficients were negatively correlated with digestibility coefficients of twelve amino acids to significant extents in the same segment. This outcome suggests that glucose and amino acids were competing for intestinal uptakes. Significant differences in concentrations of free amino acids in plasma from the anterior mesenteric vein were observed for histidine, lysine methionine threonine and valine in birds offered diets 1A, 3C and 5E. This indicates that the metabolic fate of amino acids in enterocytes is subject to dietary manipulation and there is the suggestion that crystalline amino acids are less prone to undergo catabolism in the gut mucosa. In short, this study demonstrates that starch and protein digestive dynamics are relevant to the performance of broiler chickens.

Dietary starch influences growth performance, nutrient utilisation and digestive dynamics of protein and amino acids in broiler chickens offered low-protein diets

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