Role of milk carbohydrates in intestinal health of nursery pigs: a review

Abstract

Intestinal health is essential for the resistance to enteric diseases and for nutrient digestion and absorption to support growth. The intestine of nursery pigs are immature and vulnerable to external challenges, which cause negative impacts on the structure and function of the intestine. Among nutritional interventions, the benefits of milk are significant for the intestinal health of pigs. Milk coproducts have traditionally been used in starter feeds to improve the growth of nursery pigs, but their use is somewhat limited due to the high costs and potential risks of excessive lactose on the intestine. Thus, understanding a proper feeding level of milk carbohydrates is an important start of the feeding strategy. For nursery pigs, lactose is considered a highly digestible energy source compared with plant-based starch, whereas milk oligosaccharides are considered bioactive compounds modulating intestinal immunity and microbiota. Therefore, milk carbohydrates, mainly composed of lactose and oligosaccharides, have essential roles in the intestinal development and functions of nursery pigs. The proper feeding levels of lactose in starter feeds could be variable by weaning age, body weight, or genetic lines. Effects of lactose and milk oligosaccharides have been broadly studied in human health and animal production. Therefore, this review focuses on the mechanisms of lactose and milk oligosaccharides affecting intestinal maturation and functions through modulation of enterocyte proliferation, intestinal immunity, and intestinal microbiota of nursery pigs.

Keywords: Intestinal health; Lactose; Milk carbohydrates; Milk oligosaccharides; Nursery pigs.

Fig. 1

Changes in G:F of nursery pigs fed diets by inclusion level of lactose in diets during 5 to 7 kg (A) and 7 to 11 kg (B) BW using a broken-line analysis. The meta-analysis is conducted by Proc NLMIXED in SAS 9.4 to determine break-points on the regression of feed efficiency of nursery pigs calculated based on the data from 10 published studies (Phase 1 for 5 to 7 kg BW: 14 experiments) and 14 published studies (Phase 2 for 7 to 11 kg BW: 23 experiments). The breakpoints (one-slope broken-line model) were inclusion levels of lactose at 20% during 5 to 7 kg BW and 13% during 7 to 11 kg BW when G:F were 0.80 and 0.77, respectively. One-slope broken-line models; The equation for G:F during 5 to 7 kg BW was G:F = 0.80–0.39 × 10^− 2 × z1, R² = 0.90. The equation for G:F during 7 to 11 kg BW was G:F = 0.77–0.24 × 10^− 2 × z1, R² = 0.76 was; if lactose supplementation is ≥ breakpoint, then z1 = 0; if lactose supplementation is < breakpoint, then z1 = lactose supplementation - breakpoint

Fig. 2

Changes in G:F of nursery pigs fed diets by average daily lactose intake during 5 to 7 kg (A) and 7 to 11 kg (B) BW using a broken-line analysis. The meta-analysis is conducted by Proc NLMIXED in SAS 9.4 to determine break-points on the regression of feed efficiency of nursery pigs calculated based on the data from 10 published studies (Phase 1 for 5 to 7 kg BW: 14 experiments) and 14 published studies (Phase 2 for 7 to 11 kg BW: 23 experiments). The breakpoints (one-slope broken-line model) were average daily lactose intake at 45.61 g/d during 5 to 7 kg BW and 57.13 g/d during 7 to 11 kg BW when G:F were 0.79 and 0.77, respectively (P < 0.05). One-slope broken-line models; The equation for G:F during 5 to 7 kg BW was G:F = 0.79–0.12 × 10^− 3 × z1, R² = 0.89. The equation for G:F during 7 to 11 kg BW was G:F = 0.77–0.83 × 10^− 4 × z1, R² = 0.76 was; if average daily lactose intake is ≥ breakpoint, then z1 = 0; if average daily lactose intake is < breakpoint, then z1 = average daily lactose intake - breakpoint

Fig. 3

Overview of the possible functions of milk carbohydrates on the intestine of nursery pigs. Lactose and various type of milk oligosaccharides could improve the intestinal maturation and health of nursery pigs through positively modulating enterocyte proliferation, intestinal immune response, and microbiota. (1) Lactose would be hydrolyzed by lactase producing galactose and lactose, which are utilized to produce energy for the growth and maintenance of nursery pigs. (2) lactose at optimal level could induce a favorable condition against pathogenic bacteria by microbial fermentation of saccharolytic bacterial groups, including Lactobacillus and Bifidobacterium, leading to preventing pathogen colonization in the intestine of nursery pigs. In addition, (3) milk oligosaccharides could modulate the growth of intestinal epithelium through binding to toll-like receptors (TLRs) and epidermal growth factor receptors (EGFR) on the enterocytes. (4) Milk oligosaccharides can increase gene expression of mucus secretion and prevent the pathogen from binding to intestinal immune-related receptors including TLRs and various types of glycan receptors, leading to enhanced mucosal immune homeostasis and tolerance. (5) Milk oligosaccharides can prevent pathogen colonization by increased abundance of Bifidobacterium and production of lactic acid and volatile fatty acids (VFA)