Early weaning is a prevalent practice in the contemporary swine industry. It exposes piglets to a range of environmental stressors known as weaning stress, increasing their susceptibility to diarrhea. The occurrence of post-weaning diarrhea raises concerns regarding intestinal health and growth retardation in piglets, ultimately leading to economic losses [1]. The impairment of intestinal barrier function has been implicated as the key determinant of post-weaning diarrhea in piglets [2]. Hence, it is critical to understand the mechanisms underlying intestinal barrier dysfunction and investigate effective interventions that can avert or alleviate its consequences.

According to current knowledge, the integrity of the intestinal barrier is intricately linked to multifaceted biological processes, including oxidative stress, inflammation, gut microbiota composition, and metabolic regulation [3], [4]. Increased permeability and subsequent dysfunction of the intestinal tract may result from pro-inflammatory responses and oxidative stress that compromise barrier function [5], [6]. Immune cells mount a response to the passing pathogen and subsequently undergo differentiation, leading to the activation of inflammatory responses [7]. It has been known that the overproduction of reactive oxygen species (ROS) plays a role in initiating the activation of NF-κB, which in turn contributes to the progression of intestinal inflammation [8]. In addition, barrier dysfunction can be exacerbated by an imbalance in the gut microbiota, which is crucial for the maintenance of intestinal homeostasis. A dysbiosis in the gut microbiome may be accompanied by an increase in pathogenic microbes and a stimulation of the immune system, leading to inflammation and oxidative stress [9]. The gut microbiota of weaned piglets with diarrhea exhibited dysbiosis, further disturbing the host metabolism and potentially resulting in adverse changes [10]. Additionally, metabolites derived from gut bacteria are considered a major component of host-microbiota crosstalk and participate in various physiological functions [11]. Taken together, targeting the preservation of intestinal barrier function by enhancing antioxidant and anti-inflammatory capacities, as well as improving metabolic profile and gut microbiome composition, may offer promising strategies for strengthening intestinal barrier integrity and alleviating post-weaning diarrhea in piglets.

Hydroxyproline (Hyp), a naturally imino acid abundant in sow’s milk [12], [13], has been reported to exert antioxidant and anti-inflammatory activities [14], [15], [16]. From a metabolic perspective, Hyp is not only recognized as a substrate for glycine (a precursor of glutathione) biosynthesis [17], [18], but also capable of being utilized by gut microbes through the enzyme hydroxyproline dehydratase (HypD) [19]. The activity of metabolic enzymes related to Hyp in the gut of suckling piglets increases with age, followed by an increase in glycine levels, emphasizing the crucial role of Hyp in supporting growth and health [20]. In addition, dietary supplementation of glycine in weaned piglets enhanced mucosal immunity in the gut and ameliorated diarrhea [21]. The capacity of Hyp to mitigate oxidative and inflammatory insults, as well as modulate the gut microbiota and host metabolism, presents a promising approach for addressing intestinal barrier dysfunction. After weaning, piglets obtain much less Hyp from their feed compared to the amount acquired from sow's milk before weaning. Therefore, Hyp supplementation may confer benefits on intestinal health and growth in weaned piglets. The overarching goal of this study was to investigate the effects of dietary Hyp on the integrity and function of the intestinal barrier in early-weaned piglets. Additionally, it aimed to dissect the intricate interaction with the gut microbiota and metabolites in response to Hyp supplementation. Through meticulous study, this research aspires to furnish a comprehensive theoretical framework conducive to enhancing intestinal health, optimizing growth performance, and ameliorating post-weaning diarrhea in piglets via Hyp interventions.