Both slowly (amylose) and rapidly (waxy rice) digestible starch sources supported superior FCR than diets containing maize in this study. Starch digestion rates differ across feedstuffs [3, 12] and some studies have indicated that slowly digestible starch may advantage boiler growth performance [13,14,15]. Indeed, a high 90.4% of starch digestion along the small intestine took place in proximal jejunum (not shown) indicating that rapidly digestible starch enhanced FCR is ostensibly a curious outcome. However, Sydenham et al.  demonstrated that proximal jejunal starch/protein disappearance rate ratios in broilers quadratically influenced FCR and weight gain so it may not be prudent to consider starch digestion rates in isolation.
mRNA expression for glucose transporters
The majority of glucose is co-absorbed with sodium from the gut lumen via SGLT-1 transporters; whereas, GLUT-2 provides the basolateral exit of glucose from enterocytes into the portal circulation . SGLT-1 is located in the apical membrane of enterocytes and SGLT-1 mRNA expression occurs in response to high dietary carbohydrate levels . In the present study, waxy rice in low protein diets generated the highest jejunal SGLT-1 expression, which is not surprising as it is a rapidly digested starch source. However, this was not the case in high protein diets as there was a highly significant treatment interaction. That ileal SGLT-1 expression was higher with waxy rice and maize diets than amylose may have been a compensatory mechanism due to the relative lack of starch and glucose in the ileum in comparison to amylose. GLUT-2, which is not a sodium dependent transporter, is located in the baso-lateral membrane of enterocytes and starch source was very influential in respect of mRNA expression. However, the mRNA response patterns in the two intestinal segments were almost exact opposites; waxy rice was highest in the jejunum and amylose was highest in the ileum. This difference may reflect the relative amounts of starch and glucose along the small intestine dependent of starch source and their digestion rates. As considered later, GLUT-2 may be recruited into the apical membrane of enterocytes to amplify glucose absorption from the gut in certain physiological contexts .
mRNA expressions for PepT-1 and amino acid transporters
Several proton-coupled oligopeptide transporters have been identified in poultry including peptide transporters 1 and 2 (PepT1 and PepT2) and the peptide/histidine transporter 1 (PHT1) as reported by Zwarycz and Wong . Relatively large quantities of amino acids as di- and tri-peptides can be absorbed rapidly via PepT1 and Krehbiel and Matthews  argued that the majority of amino acids is absorbed into enterocytes via PepT1. Therefore, it is noteworthy that PepT1 mRNA expressions in the ileum (r = − 0.323; P = 0.077) tended to be associated with improvements in FCR of birds from 6 to 35 d post-hatch.
Relative mRNA expression for amino acid and oligopeptide transporters is a complex subject and dietary protein level, protein quality, feed restriction and bird age have all been shown to influence mRNA expression to variable extents [22,23,24]. In the present study, waxy rice starch generated the highest mRNA expressions. In contrast, the low dietary protein level increased ileal mRNA expressions of only two transporters, EAAT3 and LAT1. According to Gilbert et al. , B0AT1 is broad scope, Na+-dependent transporters of neutral amino acids. The y+LAT1 transporter mediates Na+-dependent neutral amino acids and Na+-independent cationic amino acids; whereas, LAT1 mediates the Na+-independent transport of branched-chain and aromatic amino acids. Moss et al.  proposed that glucose and amino acids compete for intestinal uptakes via their respective Na+-dependent transporters. If so, this competition would take place mainly in the anterior small intestine with a rapid starch source such as waxy rice which may have resulted in a relative surplus of amino acids becoming available for absorption in the ileum; this may be reflected in the higher mRNA expressions for ATB0,+, EAAT3, B0AT1 and y+LAT1 with diets containing waxy rice. In the case of B0AT1 and y+LAT1, waxy rice generated significantly higher mRNA expressions than maize or amylose. That dietary starch can influence mRNA expressions for amino acid transporters is intriguing and emphasises the interactions that may occur between starch and glucose with protein and amino acids.
ATP, Na+/K+-ATPase and citrate synthase activities
The ‘sodium pump’, or Na+/K+-ATPase, is located in the baso-lateral membrane of enterocytes and drives the co-absorption of sodium with glucose or amino acids via Na+-dependent transporters. Instructively, diets containing amylose as a starch source had the highest sodium pump activities in the ileum. This suggests that amylose, a slowly digested starch, generated additional glucose to be absorbed in the ileum via SGLT-1. Interestingly, ileal sodium pump activity was positively correlated (r = 0.406; P < 0.025) with weight gains and tended to be negatively correlated (r = − 0.324; P = 0.062) with FCR from 21 to 35 d post-hatch. That sodium pump activity in the ileum was associated with enhanced broiler performance illustrates the importance of intestinal uptakes of nutrients for the performance of broiler chickens . Starch source and protein level had tangible, significant impacts on citrate synthase activity in both small intestinal segments; in contrast; they did not influence ATP contents. This is surprising given that citrate synthase is a pivotal enzyme in the tricarboxylic acid cycle which generates ATP . When glucose is a limiting factor, many anaplerotic amino acids can supply OAA and acetyl-CoA, providing support for the TCA cycle. The higher activity levels of CS in the intestinal mucosa of amylose-fed animals suggests that nutrient-limited conditions may require cells to engage alternative metabolic pathways, including degradation of proteins, to supply carbon to the TCA cycle .
Proliferation, apoptosis and mRNA expression for CASP-3 and mTOR
Dietary carbohydrate sources might influence rates of cell proliferation and apoptosis or programmed cell death in the gut mucosa and the measurement of BrdU positive cells is a means to detect proliferating cells along the small intestine. Diets containing waxy rice as a starch source generated the highest numbers of BrdU positive cells in the jejunum, which suggests that a rapidly digestible starch source promotes development of the anterior small intestine. Somewhat reciprocally, waxy rice diets generated the highest mRNA expression for CASP-3 in the ileum, which is indicative of apoptosis. Apoptosis-induced proliferation maintains tissue homeostasis as dying cells induce proliferation of the surviving cells to compensate for the tissue loss and restore organ size . Also, suppressed ileal mRNA expression of CASP-3 by amylose suggests less apoptosis of enterocytes in the ileum which is consistent with an increased load of slowly digestible starch reaching the posterior small intestine. Diets containing amylose as a starch source generated the highest mRNA expression for mTOR in the ileum which was more pronounced in low protein diets. One interpretation of this outcome is that a slowly digestible starch source was associated with increased protein synthesis and cell proliferation in the ileal gut mucosa [30,31,32].
Intestinal GOT activities and free amino acid concentrations in portal circulation
Glutamic acid is almost certainly the most catabolised amino acid in the gut mucosa for energy provision and GOT activity is indicative of the extent of that catabolism . Ileal GOT activity tended to decline (P = 0.057) when either maize or amylose were the starch sources in comparison to waxy rice which is consistent with the proposition that rapidly digestible provides little glucose as an energy substrate to the posterior small intestine resulting in more catabolism of amino acids, especially glutamic acid. The significant increase in GOT with the transition from high to low protein diets indicates greater catabolism of glutamic acid which may have been a compensatory response to mitigate the catabolism of essential amino acids.
The pivotal question, whether or not amino acid catabolism in the gut mucosa is subject to nutritional regulation, was posed by Reeds et al. . The outcomes of this study indicate that free amino acid concentrations in the portal circulation can be modified by dietary strategies. As a main effect, starch source influenced glutamine concentrations where waxy rice supported higher levels than maize and amylose. This was the only significant impact of starch source observed and may related to the fact that glutamine entry into enterocytes from the arterial circulation exceeds that of other amino acids . As main effects, protein levels significantly influenced concentrations of six amino acids and significant treatment interactions were observed for 10 amino acids from the total of 18 assessed. Thus dietary treatments, significantly influenced the concentrations of free amino acids in plasma taken from the anterior mesenteric vein for 17 of the 18 amino acids assessed, the exception was tryptophan. However, the outcomes are inconsistent and interpretations are not straightforward.
Nevertheless, Li et al.  concluded that dietary sources of starch profoundly affect the net appearance of amino acids and glucose in the portal vein of growing pigs and that starch source has important implications for the efficiency of nutrient utilisation. Concentrations of non-essential amino acids in the portal circulation are more dominant. Therefore, it is noteworthy that waxy rice as a starch source generated 13.6% numerically higher concentrations than maize and 22.4% significantly higher concentrations of non-essential amino acids than amylose in this study. This outcome conflicts with the findings of van der Meulen et al.  who reported that slowly digestible starch (peas) increased the net portal flux of amino acids more than relatively rapidly digestible starch (maize) in pigs. However, it is in agreement with the report by Yin et al.  who found that rapidly digestible starch increased post-prandial concentrations of free amino acid in the systemic circulation of pigs. These researchers conceded that the precise mechanisms responsible have yet to be clarified but suggested that “rapidly digestible starch ameliorates the digestive and absorptive function” which promotes higher systemic circulating concentrations of most amino acids within four hours post-prandially.
As mentioned earlier, GLUT-2 may be recruited into the apical membrane of enterocytes to amplify glucose absorption from the gut . There is the possibility that rapidly digestible waxy rice starch triggered this recruitment in the anterior small intestine to absorb glucose which would reduce the quantity of glucose absorbed via the Na+-dependent transporter SGLT-1. This in turn would allow greater co-absorption of amino acids and sodium and ameliorate any competition for intestinal uptakes between glucose and amino acids. Thus, the apical recruitment of GLUT-2 may be the genesis of the responses observed by Yin et al.  in pigs and the present study in poultry. While speculative, it may have been that neither pea nor maize starch prompted the apical recruitment of GLUT-2 in the van der Meulen et al.  study resulting in the dichotomy of outcomes.