Generally, non-starch polysaccharides (NSPs) in feed are considered as anti-nutritional factors for pigs. These NSPs not only have a great negative impact on digestion and absorption of nutrients in pigs but also are associated with the intestinal microbial fermentations, which is the energy source of microbial fermentation in gut. Additionally, NSP content in feed can affect the digestibility of dietary protein and amino acid. It was reported that the apparent ileal digestibility of crude protein and most of amino acids decreased by 3–5 % in growing pigs when β-glucan in diet increased from 29.0 to 31.8 g/kg (Yin et al. 2000b, c). It was also reported that the apparent ileal digestibility of crude protein and amino acid is reduced by 12 % and 6 %, respectively, when total NSP in diet increased from 83 to 193 g/kg (Yin et al. 2000a).

NSP not only can affect secretion of endogenous nitrogen but also affect deposition and biological value of feed nitrogen. It was reported that cereal fiber content can significantly increase the excretion of endogenous nitrogen and amino acids in pigs (Low and Rainbird 1984; Yin et al. 2000a). Pectin can reduce the amount of nitrogen deposition and nitrogen microbiological value (Xu et al. 2005). It has been suggested that pectin and other gel of polysaccharides (such as methyl cellulose) can affect the secretion of endogenous protein and amino acids, and increase endogenous excretion of nitrogen, which increased the amount of nitrogen coming from the small intestine to the large intestine, and thus led to the decrease of body nitrogen retention and nitrogen biological value.

The main factor responsible for the intestinal secretion of endogenous nitrogen in roughage feed is not the insoluble part of non-starch polysaccharides (INSP) but the soluble part of non-starch polysaccharides (SNSP) (Yin et al. 2000a, b, c, 2004). Although SNSP do not affect the digestive process of feed nitrogen and amino acids (that is the true digestibility), there is a positive linear relationship between SNSP content and intestinal microbial fermentation or endogenous nitrogen (amino acid) excretion in pigs that thereby reduces the amino acid digestibility. However, there is no positive linear relationship between INSP content and intestinal microbial fermentation. INSP does not affect the endogenous excretion of nitrogen and total nitrogen. When soluble NSPs in growing pig diets are more than 0.8 %, the protein digestibility and growth performance are significantly affected. There is a linear relationship between diet SNSP (X) and ileal excretion of nitrogen (y) as follows: y = 1.7832X − 8.2074 (R ² = 0.99). Also, there is a linear relationship between the INSP and ileal excretion of nitrogen as follows: y = 0.026X + 5.0431 (R ² = 0.43), which has however no significant correlation. Interestingly, the fecal nitrogen excretion (y) has the similar linear relationship with two types of NSPs (X) as follows: y = 0.2563X + 0.5773 (R ² = 0.89, SNSP) and y = 0.3165X + 1.546 (R ² = 0.35, INSP). This indicates that the two types of NSPs have similar influence on fecal nitrogen excretion (Yin et al. 2004).

Lastly, the dietary NSPs accelerated the basal metabolism of PDV tissues, thus affecting the utilization of amino acids in feedstuffs.

Phytic acid in plant feedstuff not only form phytate combined to phosphorus but also form insoluble complexes combined with protein and amino acids, thus reducing the utilization of these nutrients. Studies have showed that some other anti-nutritional factors, including insulin-inhibitory factor, tannin, and so on, can increase the endogenous ileal amino acid and nitrogen content (Huang et al. 2001). Tannin can reduce the true digestibility of amino acids by combination with endogenous nitrogen in digestive tract (Jansman et al. 1995).

The type and source of starch have significant impact on the digestibility of protein and amino acid absorption, particularly on ileal digestibility (Li et al. 2007; Laplace et al. 2001). It was reported that the ileal digestibility of the protein and some amino acids (cysteine, serine, threonine, phenylalanine, and tyrosine) were significantly reduced when pig ingested the diet including slow digestion of pea starch (Everts et al. 1996). It was also reported that diets including high level of rapidly digestible amylopectin (glutinous rice) and high level of slowly digestible amylose (resistant starch) not only reduce the apparent and true digestibility of dietary protein but also can reduce the apparent and true ileal digestibility of aspartic acid, glutamic acid, serine, histidine, threonine, arginine, tyrosine, methionine, phenylalanine, leucine, isoleucine, and lysine. Furthermore, when the ratio of amylose to amylopectin was equal to 0.23 in the corn-based diet, the ileal amino acid digestibility was significantly better (Li et al. 2008).

It is believed that the different structures of starch can affect differently the digestion of amino acids. Slowly digestible starch can increase the endogenous nitrogen and amino acid secretion and the microbial nitrogen and amino acid content in digesta because slowly digestible starch has relatively slow digestion property. That is the reason why slowly digestible starch reduces amino acid digestibility. Rapidly digestible starch can accelerate glucose absorption, which competitively inhibits the absorption of amino acids, thereby reducing the digestion and absorption of dietary amino acids (Yin et al. 2001a, b, 2011).

Starch sources, composition, and structures can affect microbial biomass nitrogen and the flow of nitrogen in feces and urine. It was reported that urinary nitrogen excretion has a downward trend with the increased content of resistant starch diet and increased excretion of fecal nitrogen (Li et al. 2008). There is a quantitative relationship between fecal nitrogen excretion (y) and resistant starch content of the diet (X) as follows: y = 0.785X + 9.1739 (R ² = 0.997). It was reported that pigs fed resistant starch diet have higher fecal nitrogen content. It is supposed that there are two reasons for such a result: (1) resistant starch may reduce the intestinal absorption of nitrogen and (2) the microbial fermentation in the large intestine increased microbial protein synthesis (Heijnen and Beynen 1997).

Studies have shown that starch sources, composition, and structures can affect the net absorption and amino acid composition in the portal vein. The content of essential amino acids (valine, isoleucine, phenylalanine, tryptophan, arginine, serine, cystine, tyrosine, lysine, histidine) in portal vein of pigs fed pea starch diet was higher than that in pigs fed the corn starch diet (van der Meulen et al. 1997).

The diet, including high percentage of rapidly digestible amylopectin (glutinous rice) and slowly digestible amylose (resistant starch), not only reduced the total absorption of amino acids in the portal venous blood after feed intake but also changed the composition pattern of absorbed portal amino acids, as well as reduced the proportion of EAA in the total pool of absorbed amino acids. However, the total absorption of amino acids in the portal venous blood and the composition pattern of amino acids absorbed in the portal blood were significantly improved when the ratio of amylose to amylopectin was 0.23 in the cornstarch diet (Deng et al. 2010). Since the different starch in feed have different digestibility, the different starch composition will not only affect digestion and absorption of dietary amino acids but also affect amino acid metabolism in intestinal mucosa, thus affecting the net portal absorption and composition pattern of amino acids (Huang et al. 2006).

Starch source, composition, and structure can affect the amino acid metabolism in visceral tissues. It has been reported that pigs fed corn-based diet (the ratio of amylose to amylopectin is 0.23) have the highest protein synthesis rate (FSR) in PDV tissues and liver. The order of FSR in other diets is as follows: brown rice-based diet > starch-based diet (including high percentage of slowly digestible amylose) > glutinous rice-based diet (including high percentage of rapidly digestible amylopectin). The FSR (%/day) of spleen, pancreas, duodenum, jejunum, ileum, and colon in pigs fed glutinous rice-based diet were significantly lower than those in the corn-based group (P < 0.05) and brown rice-based group (P < 0.05). The FSR of spleen, pancreas, duodenum, jejunum, ileum, and colon in pigs fed glutinous rice-based diet were lower than corn-based group by 84.76 %, 30.34 %, 46.20 %, 32.19 %, 27.16 %, and 36.02 %, respectively. The FSR were lower than brown rice-based group by 81.97 %, 21.72 %, 46.15 %, 30.48 %, 25.14 %, and 32.78 %, respectively. The FSR also were lower than resistant starch group by 60.49 %, 17.79 %, 34.10 %, 23.23 %, 13 %, and 28.9 %, respectively. Additionally, the liver FSR in pigs fed glutinous rice-based diet were lower than corn, brown rice, and resistant starch groups by 14.99 %, 14.38 %, and 9.27 %, respectively (Deng et al. 2010).

The measurement of true ileal digestible amino acids can eliminate the impacts of measurement conditions. The values of true ileal digestible amino acids can be additive. The data of true ileal digestible amino acids can make diet formulations more accurate, which can reduce the safety margin and dietary crude protein level without reducing animal performance. Some synthetic amino acids can be supplemented in the diets in order to meet the requirements of some limiting amino acids, thereby improving the overall efficiency of amino acids.

The feasibility of reducing nitrogen excretion by amino acid-balanced low-protein diet was investigated in growing pigs, which was formulated based on the true digestible amino acids and synthetic amino acids (Deng et al. 2007a, b, 2009). Experiment designed 5 crude protein levels, namely, 18.2, 16.5, 15.5, 14.5, and 13.6 %. The results showed that fecal nitrogen, urinary nitrogen, and total nitrogen excretion (g/day) decreased with the reduced dietary crude protein levels. Compared with those of crude protein group of 18.2 % (control group), fecal nitrogen were reduced by 7.45, 13.04, 13.82, and 17.39 % in crude protein groups of 16.5 %, 15.5 %, 14.5 %, and 13.6 %, respectively. Urine nitrogen decreased by 19.25, 26.86, 37.43, and 44.64 %. Total nitrogen decreased by 15.13 %, 22.00 %, 29.18 %, and 35.14 %, respectively. In addition, the ratio of urinary nitrogen to fecal nitrogen levels decreased with reduced dietary crude protein (P < 0.0001). The nitrogen retention also decreased with decreased dietary crude protein (P < 0.001).

However, the percentage of nitrogen deposition to nitrogen intake increased with the reduced dietary crude protein levels (P < 0.01). Dietary crude protein levels did not affect nitrogen digestibility (P > 0.05). Nitrogen digestibility in each group fluctuated between 83.86 and 82.19 %. Moreover, the apparent digestibility of nitrogen in groups supplemented with synthetic amino acids was lower than that of control group. But the apparent biological value of nitrogen increased with the decrease of dietary crude protein (P < 0.001). From the results of growth performance in different crude protein levels, it was indicated that the production performance of growing pigs was not affected by the decrease of dietary crude protein levels from 18.2 to 16.5 % in the whole trial period. On the contrary, the ADG, average daily feed intake, and the feed conversion rate were enhanced to a certain extent, while the production performance decreased with further reduction of protein levels. Carcass measurement indicated that the back fat thickness of pigs increased with the decrease of dietary protein levels (P < 0.03). In summary, the production performance, carcass quality, and nitrogen utilization as well as economic and ecological efficiency can be achieved when dietary crude protein levels reduced from 18.2 to 16.5 % (thus at less than 2 %).

During several decades, it has been thought that the role of carbohydrates is mainly related to their structure for energy storage material. But in recent years, it has been found that glycosylation in vivo is involved in the synthesis of numerous proteins as well as numerous life processes. In addition, many natural polysaccharides and oligosaccharides can affect various physiological functions and activities of animals and humans. Particularly oligosaccharides can regulate animal growth and metabolism, enhance immune functions, and maintain intestinal health (Huang et al. 2005, 2007; Deng et al. 2007c, d; Kong et al. 2007; Yin et al. 2008). These functions are more and more recognized by scientists. Animal growth, immunity, and gut microflora changes are closely associated with digestion and metabolism of amino acids. Therefore, these functional carbohydrates in feed have an important regulatory role in digestion and metabolism of amino acids.