Types of fats and definition of functional lipids

Fats are triglycerides (TAGs) composed of glycerol and fatty acids, serving as important components and energy reserves in living organisms. Based on the saturation of fatty acids, fats can be classified into saturated and unsaturated fats, with unsaturated fats further divided into monounsaturated and polyunsaturated fats. According to the carbon chain length of fatty acids, fats can also be categorized as short-chain fatty acid glycerides, medium-chain fatty acid glycerides, and long-chain fatty acid glycerides. TAGs containing long-chain fatty acids (LCTs, >12 carbon atoms) can be broken down by various enzymes to provide the energy required for life activities. Medium-chain TAGs (MCTs, containing 6–12 carbon atoms) serve as excellent solvents for lipophilic bioactive substances and can act as carriers for nutrients and bioactive compounds (Dima et al. Reference Dima, Assadpour and Dima2021). Compared to long-chain TAG, MCT can rapidly supply energy to the body and improve the digestibility and absorption of food, offering unique advantages.

Structurally simplifying from TAGs, monoglycerides (MAGs) represent glycerol molecules esterified with only a single fatty acid, which can range from short- to long chain. Depending on the position of the ester bond with glycerol, MAGs can be divided into two categories: 1-MAGs (α-MAGs) and 2-MAGs (β-MAGs). Common MAGs include glycerol monobutyrate (C4:0), glycerol monocaprylin (C8:0), glycerol monodecanoate (C10:0), glycerol monolaurate (C12:0), and others. α-MAG supplements serve as an alternative energy source, reducing the body’s reliance on other energy nutrients like proteins, thereby indirectly promoting protein synthesis. Additionally, MAGs can be directly utilized by intestinal cells, helping to maintain intestinal integrity in poultry and promoting nutrient absorption and utilization (Aar et al. Reference Aar, Molist and Klis2017). Notably, MAGs exhibit strong antibacterial, anticoccidial, and antiviral properties, and when used in combination with organic acids, essential oils, or probiotics, they can exert significant synergistic effects, positively impacting animal health, production performance, and feed digestibility (Baltić et al. Reference Baltić, Starčević and Đorđević2017).

Distinct from these glycerol-based esters, branched fatty acid esters of hydroxy fatty acids (FAHFAs) constitute a novel class of bioactive lipid molecules (Zhu et al. Reference Zhu, Hao and Yan2021). Most endogenous FAHFAs share a common chemical structure characterized by an ester bond connecting a hydroxy fatty acid backbone and a fatty acid. Based on the position of the ester bond, FAHFAs can be divided into two main superfamilies: (1) branched FAHFAs, which are involved in regulating metabolism and immune responses, and (2) linear (ω-hydroxylated) FAHFAs, primarily used as biosurfactants and skin barrier matrices (Riecan et al. Reference Riecan, Paluchova and Lopes2022). FAHFAs may become potential therapeutic targets for treating various metabolic disorders, such as type II diabetes, hepatic steatosis, cardiovascular diseases, and various cancers, generating significant interest in the field of human health (Benlebna et al. Reference Benlebna, Balas and Gailliet2021).

Collectively, these diverse lipids – including MCTs, LCTs, MAGs, and FAHFAs – exemplify functional lipids, defined as lipid components with specific physiological functions and health benefits. They typically exhibit physiological activities such as anti-inflammatory, antioxidant, and regulation of gut microbiota and the immune system, positively impacting health. Structured lipids, phospholipids, and fatty acids, including the critically important polyunsaturated fatty acids, also fall under this broad category. Their significant impact on intestinal health has been extensively reviewed elsewhere (Durkin et al. Reference Durkin, Childs and Calder2021; Lee et al. Reference Lee, Tang and Chan2022; Wei and Wang Reference Wei and Wang2024). Here, we will specifically focus on the impact of short- and medium-chain fatty acid glycerides on intestinal health in pigs and poultry, providing valuable references for their rational application in the livestock industry.

The importance of intestinal health in pigs and poultry

A healthy gut is crucial for the overall metabolism, physiology, disease defense, and growth performance of pigs and poultry. However, there is currently a lack of precise and unified standards for defining “gut health.” Based on the research by Kogut and Arsenault, a healthy gut is defined as “the ability of an animal to perform its physiological functions normally, resisting both exogenous and endogenous stressors through the prevention or avoidance of disease” (Kogut and Arsenault Reference Kogut and Arsenault2016). Celi et al. emphasized the importance of effective digestion and absorption of feed, maintaining the effective structure and function of the intestinal barrier, good interaction between the host and gut microbiota, and an effective immune status in gut health (Celi et al. Reference Celi, Cowieson and Fru-Nij2017). Pluske et al. proposed that gut health should be viewed as a state of homeostasis within the gastrointestinal tract. They suggested that general criteria for assessing gut health in weaned pigs encompass efficient digestion and absorption of nutrients, effective waste excretion, functional and protective intestinal barriers, stable and balanced microbial communities, functional and protective intestinal immune systems, minimal stress/neuro-pathway activation, and a disease-free state (Pluske et al. Reference Pluske, Turpin and Kim2018). In summary, a healthy gut should enhance the host’s ability to cope with and adapt to infections/stress, helping animals achieve or maintain optimal growth, production, or performance levels.

In practical pig and poultry farming, gut health issues frequently arise, particularly in neonatal and weaned piglet populations. These animals have not fully developed their gut functions due to their age and physiological state, making them more susceptible to external factors (Tang et al. Reference Tang, Xiong and Fang2022). For high-yield poultry breeds, the high feed intake undoubtedly places a heavy burden on their digestive systems. This burden may lead to maldigestion and inadequate absorption of nutrients, further disrupting the balance of gut microbiota, thus triggering a series of gut health issues (Ducatelle et al. Reference Ducatelle, Goossens and Eeckhaut2023). In recent years, the interactions between the digestive system and other organ systems have garnered increasing attention. For example, when the intestinal barrier is compromised, potentially harmful compounds and microorganisms can leak from the gut lumen into the portal circulation, directly entering the liver, which may disrupt liver cell metabolism and even lead to liver diseases. The gut–brain axis also indicates that gut health can influence neurotransmitter release and nerve signal transmission, subsequently affecting the animal’s behavior and physiological state. Scientists have found that even minor gut health issues can significantly affect overall animal health and production performance (Bindari and Gerber Reference Bindari and Gerber2022). Therefore, the gut health of pigs and poultry is closely linked to the overall health of the animals, making it an indispensable key factor in ensuring their production performance.

Metabolism of functional lipids in the intestine

After oral intake, functional lipids that orally intake are broken down into free fatty acids and other compounds under the action of lipase. These breakdown products then mix with phospholipids, cholesterol, and bile acids to form micelles. When lipid micelles enter the brush border membrane of intestinal epithelial cells, fatty acid-binding proteins (FABPs) bind to free fatty acids and transport them into the cytoplasm. FABP1 in the intestine tends to guide fatty acids toward oxidation to provide energy for cells, while FABP2 is more inclined to direct fatty acids toward re-synthesis of TAGs with MAGs under the action of diacylglycerol acyltransferase (DGAT) in the endoplasmic reticulum (Ko et al. Reference Ko, Qu and Black2020). MCTs and short-chain TAGs bypass the MAG pathway and are directly absorbed into the portal circulation, then transported to the liver for rapid oxidation or re-routing to form very low-density lipoprotein with the help of albumin. Newly synthesized TAGs primarily take two fates: one forms chylomicrons, while the other forms cytoplasmic lipid droplets (CLDs). TAGs produced by DGAT2 mainly contribute to the formation of chylomicrons and CLDs, while TAGs produced by DGAT1 primarily form endoplasmic reticulum lipid droplets and may help increase the size of chylomicron precursors, although the growth of CLDs in intestinal epithelial cells may be limited (Hung et al. Reference Hung, Carreiro and Buhman2017). Chylomicrons and CLDs share similar structural characteristics: both contain neutral lipids such as TAGs and cholesterol esters at their core, surrounded by a complex of phospholipid monolayers, free cholesterol, and various proteins (Ko et al. Reference Ko, Qu and Black2020). Chylomicrons are synthesized in the form of chylomicron precursors, a process primarily completed with the help of microsomal triglyceride transfer protein and apolipoprotein B48 (apoB48). The synthesized chylomicron precursors are transported from the endoplasmic reticulum to the Golgi apparatus through pre-CM transport vesicles, where they are further processed into mature chylomicrons. Chylomicrons are the main carriers of lipid transport, working in conjunction with lipoproteins to transport lipids to the basolateral membrane of intestinal epithelial cells and release them into the lymphatic system, effectively delivering lipids into the systemic circulation. As neutral lipid droplets continue to accumulate, newly formed lipid droplets separate from the endoplasmic reticulum membrane in a “budding” manner, subsequently becoming mature CLDs. Lipid droplets can either re-synthesize chylomicrons or serve as temporary storage sites for lipids. When lipid content is excessive, droplets can also be reconverted into fatty acids under the action of adipose triglyceride lipase or lysosomal acid lipase. CLDs are known to play important roles within cells, including energy storage, regulation of lipid metabolism, assembly and maintenance of cell membranes, signaling regulation, and modulation of inflammatory responses. Abnormal metabolism and regulatory disturbances of chylomicrons and CLDs may lead to gut damage through mechanisms such as lipid deposition, oxidative stress, inflammatory responses, impaired intestinal barrier function, and dysregulation of hydroelectrolytic balance (Demignot et al. Reference Demignot, Beilstein and Morel2014).

Impact of functional lipids on intestinal health in pigs and poultry

Impact of functional lipids on the gut microbiota

Increasing evidence suggests that functional lipids are closely related to the regulation of gut microbiota (Fig. 1). For instance, a study found that tributyrin increased the relative abundance of several bacterial genera, such as Oscillospira, Oscillibacter, Mucispirillum, and Butyrivibrio, which positively correlated with average daily gain and/or body weight in weaned piglets. Conversely, the abundance of Mogibacterium, Peptococcus, Atopobium, and Collinsella significantly decreased, correlating negatively with average daily gain and body weight. Furthermore, the reduction of Collinsella may be associated with improved intestinal barrier function and decreased gut permeability (Miragoli et al. Reference Miragoli, Patrone and Prandini2021) (Table 1). Feeding a mixture of tributyrin and fennel to weaned piglets increased beneficial bacteria like Lactobacillus reuteri, Lactobacillus amylovorus, and Clostridium butyricum, while reducing pathogenic bacteria Prevotella copri (Dang et al. Reference Dang, Lee and Lee2023). However, some studies found that tributyrin did not affect the numbers of Escherichia coli and Enterobacteriaceae in piglet feces, while Lactobacillus and Bifidobacterium counts significantly decreased (Sotira et al. Reference Sotira, Dell’Anno and Caprarulo2020). This discrepancy may arise because the microbial results were based on specific strains detected via polymerase chain reaction, thus not representing the entire populations of these bacteria. Moreover, it was noted that Butyrivibrio could only be detected in pigs with higher feed efficiency, suggesting its potential role in enhancing feed utilization (Wu et al. Reference Wu, Li and Yi2018, Reference Wu, Li and Lyu2020).

Figure 1. Mechanisms by which functional lipids improve intestinal health in pigs and poultry. Note: LCFAG, long-chain fatty acid glycerides; LCFA, long-chain fatty acids; MCFAG, medium-chain fatty acid glycerides; MCFA, medium-chain fatty acids; SCFAG, short-chain fatty acid glycerides; SCFA, short-chain fatty acids; Tregs, regulatory T cells; FABP, fatty acid-binding protein; TAG, triglycerides; LDs, lipid droplets; CM, chylomicrons; apoB48, apolipoprotein B48.

Table 1. Key findings on the use of functional lipids as feed additives

In broilers, adding tributyrin can increase the counts of Bacillus and Lactobacillus in the ileum and cecum while reducing pathogenic E. coli (Hu et al. Reference Hu, Yin and Li2021). Additionally, Gong et al. observed that after adding tributyrin, the increase of Eisenbergiella in the cecum might negatively affect feed conversion rates (Gong et al. Reference Gong, Xiao and Zheng2021). Yang et al. found that butyric acid glycerides (a mix of 30% mono-butyrin, 50% di-butyrin, and 20% tri-butyrin) supplementation effectively reduced harmful bacteria such as Mollicutes and Holdemania in the cecum while significantly enhancing the diversity and abundance of Bifidobacterium (Yang et al. Reference Yang, Yin and Yang2018). Furthermore, serum metabolites associated with Bifidobacterium, such as choline, dimethylamine, and succinate, were also significantly elevated, indicating that butyric acid glycerides can influence lipid and energy metabolism by modulating gut microbial metabolites. Numerous studies have investigated the effects of MAGs on intestinal microbiota in poultry. Research has demonstrated that dietary supplementation with a mixture of monocaprin and monooctanoin can reduce the abundance of Proteobacteria in the cecum of laying hens by approximately 5.86%. Notably, conditional pathogens within Proteobacteria, such as Escherichia and Shigella, have been shown to exert significant negative impacts on intestinal health (Liu et al. Reference Liu, Tang and Feng2020). When broilers were fed with a combination of monocaprin and monolaurin, their intestinal microbiota exhibited age-dependent changes. At 3 and 7 weeks of feeding, the populations of Firmicutes, Rikenellaceae RC9 group, Barnesiellaceae, and Collinsella were significantly reduced, while the abundance of Bacteroides and Lachnospiraceae NK4A136 group was increased. The microbial community transitioned to a stable phase between weeks 7 and 10, and these microbial changes were potentially correlated with growth performance and feed conversion ratio in chickens (Liu et al. Reference Liu, Ruan and Mo2023). Overall, functional lipids contribute to optimizing the composition of gut microbiota, improving intestinal health in pigs and poultry, and promoting animal growth.

Research progress on functional lipid products: applications and industrialization of short- and medium-chain fatty acid glycerides

Given the numerous advantages of functional lipids, particularly short- and medium-chain fatty acid glycerides, they are increasingly recognized as key factors in promoting animal growth and improving production efficiency. Researchers have conducted extensive studies on their practical applications. For instance, Yen et al. found that adding 3% MCT to the diet of weaned piglets improved their weight gain-to-feed ratio by 18.5% from days 3 to 7, and by 8.3% after 14 days (Yen et al. Reference Yen, Lai and Lin2015). Papadopoulos et al. conducted trials in two pig farms with the same genetic background but different management practices (weaning time, antibiotic use). They found that adding mixtures of tributyrin and monolaurin significantly enhanced piglet growth performance without significant differences compared to high zinc oxide groups while notably reducing feed conversion rates (Papadopoulos et al. Reference Papadopoulos, Poutahidis and Chalvatzi2022). Another study by Sotira et al. showed that feeding 0.2% tributyrin to 120 weaned piglets for 40 days significantly increased average daily weight gain and reduced feed conversion rates (Sotira et al. Reference Sotira, Dell’Anno and Caprarulo2020). Dang et al. discovered that a mixture of tributyrin and fennel could dose-dependently improve weight gain, feed intake, and feed conversion ratio in weaned piglets, positively impacting the digestibility of dry matter, crude protein, and energy, and linearly reducing ammonia emissions in feces (Dang et al. Reference Dang, Lee and Lee2023). These studies indicate that adding short- and medium-chain fatty acid glycerides positively affects pig growth. However, the growth-promoting effects of tributyrin in broilers remain controversial, with varying results across different studies (Gong et al. Reference Gong, Xiao and Zheng2021; Li et al. Reference Li, Hou and Yi2015). Nevertheless, some researchers suggest that adding butyrate products to low-protein and/or metabolizable energy diets can improve weight gain and feed conversion in broilers (Hu et al. Reference Hu, Yin and Li2021). In terms of layer hens, Liu et al. reported that feeding 300 mg/kg of medium-chain alpha-MAGs (containing monolaurin and mono-caprylic acid) to 40-week-old hens maintained egg production rates above 90% at 56 weeks, compared to a decline to 84.42% in the control group. Additionally, eggshell hardness and thickness significantly increased (Liu et al. Reference Liu, Tang and Feng2020). In summary, functional lipid products significantly benefit animal health and growth, holding substantial application value in formulating cost-saving and high-efficiency feed for livestock and poultry.