In recent years, the impact of dietary carbohydrates on health has become one of the main focuses in the field of public health. As one of the three essential nutrients, carbohydrates provide about 60% of energy in the Asian diet (Cui and Dibley, 2012). Therefore, it plays an important role in the nutrition supply of human or monogastric animals during lactation and nursery stages. During lactation, lactase is the main source of energy; after weaning, the starch in food is the main source of energy (Corring et al., 1978). However, due to an immature gastrointestinal tract, young children may experience temporary malabsorption when consuming starchy foods, which may lead to disease in serious cases (Lin, 2018; Shulman, 2018). In contrast to the slow weaning process of human infants, piglets in commercial animal production experienced a huge shift from high-fat, low-carbohydrate breast milk to high-carbohydrate, and low-fat solid feed in 21–28 days of life. This change will lead to diarrhea and intestinal damage in piglets and then affect growth performance (Xiong et al., 2019). Therefore, this may bring an economic burden on the swine industry (Dou et al., 2017). Over the last decade, animal nutritionists have improved the overall health of weaned piglets by continuously optimizing feed formulations to meet the needs of weaned piglets and exploring different nutritional factors or management (Xiong et al., 2019). Studies have found that the gut of mammals is home to trillions of microbes that play a crucial role in nutrient absorption and metabolism (Hu et al., 2016). Intestinal microbial composition and ecological succession are determined by some complex internal and external factors, such as age, weaning, and diet (Kim et al., 2012; Yatsunenko et al., 2012). Because microorganisms reproduce in the host and change with age, it is very important to study the community structure of intestinal microorganisms (Green et al., 2006; Kim et al., 2012; Krajmalnik-Brown et al., 2012). In addition, the role of microbiota in health has attracted more and more attention (Ramayo-Caldas et al., 2016). Miniature piglets are similar to humans in physiological, anatomical, and endocrine systems, especially in infancy. Therefore, it is of great significance to study the changes in intestinal digestive enzymes and microbiota in miniature weaned piglets (Hu et al., 2016). Few studies have examined changes in intestinal digestive enzymes and microbes in piglets at different time periods, and there are currently no longitudinal studies that follow the correlation between digestive enzymes and microbes in piglets from birth to nursery.

In this study, we followed the changes in the digestive function, microbial communities, and the correlation between intestinal microorganisms and digestive enzymes and microbial metabolites in piglets from birth through weaning, up to 6 weeks of age. More importantly, this experiment covered the effects of diet, weaning, and age on the health of piglets. It is hoped that the results of this experiment will contribute to a deeper understanding of the changes in the digestive physiology of piglets and pave the way for the formulation of better nutritional strategies to promote the overall health of infancy and piglets.

The first goal of this study was to better understand the changes in the digestive function of piglets from birth to post-weaning. So far, under the background that early weaning models are widely used, there are few studies on the changes in the intestinal digestive function with age in piglets (Corring et al., 1978; Owsley et al., 1986; Hedemann et al., 2003). Therefore, there are differences between the early research and the current situation of piglets, and new research is needed to provide a theoretical basis to guide production.

The intestine is the main place for digestion and absorption of nutrients, and the early postnatal period is a key time for intestinal development. The tissue mass and absorption surface area of the small intestine of newborn piglets increased significantly, for example, the number of mucosal cells increased by 50% on the first day after birth and doubled on the third day after birth (Widdowson and Crabb, 1976; Xu et al., 1992). These studies show that the intestine develops most rapidly in the early stages of piglets. In this study, our measurement of intestinal tissue characteristics is basically consistent with the data in the literature, which confirms that the animals used in this study are healthy and normal (Skrzypek et al., 2018; Verdile et al., 2019). Research showed that the morphological structure of the duodenum will change with the growth of piglets during lactation, such as the increase in villus height, crypt depth, and intestinal wall thickness (Wiyaporn et al., 2013). For instance, the villus height of the small intestine of piglets increases by 33% in the first week after birth and reaches its maximum in the second week after birth (Skrzypek et al., 2005). Consistent with previous studies, duodenal villus height increased continuously from 0 to 14 d, peaked at 14 d, decreased at weaning (21–28 d), and then increased at 35–42 d. However, the changing trend of jejunum and ileum is inconsistent with that of the duodenum, and the maximum value appeared at 7 and 0 d, respectively. One plausible explanation for the inconsistent peak time of villus height may be that the development of the small intestine structure is basically completed and the villus height is high at birth. Therefore, through this experiment, we once again demonstrated that the structure and digestive function of small intestine of piglets began from prenatal.

At birth, the small intestine is almost complete and digestion can begin after the first intake of colostrum. The integrity of intestinal development will affect the utilization efficiency of carbohydrates because it will affect the disaccharidase activity of piglets (Tsukahara et al., 2016). Since the pancreas is an important organ involved in nutrient digestion, in order to further understand the changes in the digestive ability of piglets with the increase in age, we measured the digestive enzymes secreted by the pancreas. Our results showed that the activities of sucrase and maltase in the intestine and amylase in the pancreas increased with age during lactation, decreased during weaning, and then increased during nursery, while the activities of lactase continued to decrease with age. The activities of lipase, trypsin, and chymotrypsin decreased with age during lactation and weaning but increased during the nursery. These results are consistent with previous studies (Bellinge et al., 2005; Ito et al., 2019). In addition, sucrase and maltase activities are important markers for evaluating intestinal development, and the activity intensity of pancreatic enzymes is an indicator to measure digestive ability (Huygelen et al., 2015; Pieper et al., 2016; Yuan et al., 2017). Thus, increased sucrase, maltase, and pancreatic enzyme activities imply rapid maturation of the small intestine and digestive capacity, respectively. Furthermore, higher lactase activity during lactation and higher sucrase and maltase activities during nursery were helpful to degrade polysaccharides in sow milk and feed into monosaccharides, respectively. This facilitates the absorption and utilization of carbohydrates by the body, thereby promoting intestinal maturation and host growth. Overall, these results suggest that the digestive capacity of the intestine and pancreas increases with age, and the increased carbohydrate degradation rate in the diet promotes the digestion and absorption of nutrients by increasing the activity of amylase and disaccharidase.

This study also measured short-chain fatty acids in the hindgut, which are key metabolites interacting with the intestinal microbiota and can affect intestinal health and systemic metabolism (Yao et al., 2020; Zhou et al., 2020). Our results showed that the concentration of SCFAs increased with age but decreased at weaning. Studies have confirmed that higher concentrations of SCFAs in the intestine contribute to better growth performance (Le Gall et al., 2009). In addition, butyrate has been shown to have positive effects on pathogen control and intestinal barrier function, particularly energy utilization in the colon, gastrointestinal cell proliferation, and pH stabilization in the intestinal lumen (Guilloteau et al., 2010; Kelly et al., 2015).

The mammalian gastrointestinal tract contains a wide variety and active microbial community that serves as an important barrier against pathogens and plays an integral role in promoting the development of the intestinal immune system and maintaining normal intestinal function (Buffie and Pamer, 2013; Kamada et al., 2013). The present study shows a significantly increased α-diversity in the intestinal bacterial community with the age of piglets but decreased at the weaning stage. Moreover, recent studies showed that the α-diversity of intestinal bacteria increased significantly with the time interval of ~1 month after weaning (Niu et al., 2015; Zhao et al., 2015). One of the most striking observations in this study was that with the exception of Firmicutes and Proteobacteria, which were the two most dominant phyla in the piglet intestinal microbiota at 0 and 21 d, the predominant phylum in the other groups was Firmicutes and Bacteroidetes. Our results are consistent with the finding of Zhao et al. which indicated the maximum abundance of Proteobacteria at birth and then it begins to decline (Zhao et al., 2015). In this study, Escherichia_Shigella was the most abundant bacteria in the intestinal of newborn piglets and subsequently, its abundance decreased during lactation and increased again during weaning. Piglet diarrhea can be attributed to the presence of certain bacteria in the microbiota, such as Escherichia_Shigella (Schokker et al., 2015; Hu et al., 2016). This result suggests that pathogenic species are often present in the gastrointestinal tract of infants or early piglets, waiting for a potential opportunity to become pathogens. The results of this study also showed that Lactobacillus belonging to the phylum Firmicutes was the most abundant genera in intestinal bacterial communities during lactation and nursery periods. It is well known that Lactobacillus will produce acetate, and the increase in Lactobacillus abundance will increase the concentration of acetate in the hindgut, which is consistent with our previous results on SCFAs. Furthermore, Lactobacillus and Escherichia_Shigella are the core microbiota in pre-weaning piglets, suggesting that they play a key role in the establishment and maintenance of the postnatal intestinal microbiota in piglets. In addition, the abundance of Prevotella increased with age during lactation, which is in line with the results of the study based on the intestinal microbiota of infants (from newborn to 12 months) (Bäckhed et al., 2015). Prevotella is a key microbe member of the animal gastrointestinal tract, and it is not only important for the degradation of starch and plant polysaccharides but also has strong catabolism of mucin (Kovatcheva-Datchary et al., 2015; Fang et al., 2017).

Using RDA, amylase, SCFAs, and disaccharidase were significantly associated with colonic microbiota. Furthermore, Spearman's correlation analysis of the top 50 microbiota genera and hindgut environmental factors showed that the microbiota bacteria with positive effects on SCFAs were Lactobacillus, unclassified_Porphyromonadaceae, and Faecalibacterium, while the microbiota negatively correlated with SCFAs were Escherichia_Shigella, Acinetobacter, and Bifidobacterium. Previous studies have found that Lactobacillus bacteria promote early piglet development and improve the intestinal health of newborn piglets by modulating intestinal microbiota (Dowarah et al., 2017; Yang et al., 2018; Zhang et al., 2019). The abundance of Bifidobacteria decreased with increasing age. As Bifidobacteria have been proved to inhibit the inflammatory response of intestinal epithelial cells, the decrease in its relative abundance with age may be related to the increase in chronic inflammation in the elderly. In addition, Faecalibacterium is a butyrate-producing bacterium, which increases with age. Unclassified_Bacteroidetes were positively correlated with amylase, sucrase, and maltase. Bacteroidetes are involved in nutrient metabolism, including carbohydrate fermentation, polysaccharides, and steroid metabolism, and are essential for maintaining the normal physiological function of the intestine (Yang et al., 2019).

Overall, our results suggested that the most significant difference between weaning, lactation, and the nursery was the decrease in the abundance of bacteria producing SCFAs and bacteria involved in carbohydrate metabolism. Many of the bacterial taxa that showed reduced numbers in piglets with diarrhea were related to the carbohydrate production of SCFAs, which was also observed in humans with Crohn's disease, suggesting that abnormal carbohydrate metabolism may be an important feature of diarrheal disease in piglets (Erickson et al., 2012). Therefore, we should further study the carbohydrate metabolism of piglets in future research.

In this experiment, we explored the digestive function and microbial changes in piglets from birth to nursery and the correlation between intestinal microorganisms and digestive enzymes and metabolites. Based on the results, we infer that age and growth environment, including diet composition and weaning, are the key factors for the formation of piglet intestinal microbiota. Diarrhea caused by carbohydrate digestion deserves the attention of nutritionists. Supplementing bacteria conducive to carbohydrate digestion at an appropriate time will be beneficial to the health of piglets and infants. Our results broaden the understanding of the digestive function and microbial changes in piglets at different stages. These results will provide help for the experimental design of host–microbial interaction.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: NCBI BioProject—PRJNA816983.

The animal study was reviewed and approved by Institutional Animal Care Advisory Committee for Sichuan Agricultural University.

JH and DC designed the whole experiment. XG performed the experiment, including chemical analysis, statistical analysis, and manuscript writing. BY, ZH, and JY verified the validity of the experiment and checked the results. PZ, JL, and YL participated in the experimental design and provided valuable advice. All authors have read and approved the final version of the manuscript.

This research was supported by grants from the National Natural Science Foundation of China (Project No. 31730091).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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