Dietary Inulin Improves Pork Quality and Systemic Health Via Gut Microbiome and Metabolome Modulation in Finishing Pigs

doi:10.21203/rs.3.rs-6562341/v1

Background Inulin is widely recognized for its ability to improve glucolipid metabolism and modulate the gut microbiome and metabolome. However, the potential to influence pork flavor development through gut environment changes in animal husbandry remains unexplored. This study investigated the relationships among systemic health, meat flavor, gut microbiome, and metabolome in pigs fed a diet supplemented with inulin. Thirty-six male Duroc × Landrace × Yorkshire pigs (75.0 ± 1.5 kg) were divided into 2 groups, and fed either a regular diet (CON group) or a diet containing 0.5% inulin (INU group) for 60 d.

Results Inulin supplementation did not adversely affect production or slaughter performance (P> 0.05) but enhanced systemic health by improving serum biochemistry indicators (P < 0.05). Additionally, it increased the level of C16:0, C18:0, C18:1, and glutamic acid in pork (P < 0.05), while modulating the gut microbiome to reduce alpha-diversity and increase specific microbes including Escherichia-Shigella, UCG-005, Streptococcus, Terrisporobacter, Lactobacillus, Lachnospiraceae_NK4A136_group, Romboutsia, Family_Xlll_AD3011_group, Roseburia, and Turicibacter. Furthermore, inulin supplementation significantly altered metabolic pathways, down-regulating arginine biosynthesis, linoleic acid metabolism, riboflavin metabolism, and alanine, aspartate, and glutamate metabolism. These microbial and metabolic changes strongly correlated with the observed improvements in pork quality and flavor.

Conclusion Dietary inulin supplementation is recommended to enhance pork quality and systemic health without compromising productive performance.

Inulin

Finishing pig

Pork quality

Flavor development

Microbiome

Metabolome

The increasing demand for healthier diets has heightened consumer interest in premium and flavorful meat. While rapid fattening techniques have successfully shortened pig growth cycles and boosted pork production, they have also led to declines in pork quality, including reduced sensory attributes, nutritional value, intramuscular fat content, and flavor [1, 2].

To address these challenges, animal husbandry has employed a diverse range of traditional medicinal herbs and naturally plant- derived products as nutrual solutions for enhancing the quality and flavor of animal products [3-9]. Inulin, a fructan linked by β-glycosidic bonds [10], is one such promising additive. It resists digestion in the stomach and small intestine but undergoes microbial fermentation in the gut [11, 12]. Extracted primarily from Jerusalem artichoke and chicory roots, inulin can be categorized by chain length into short, medium, and long chains, each with distinct health benefits [13]. The fractional precipitation method can be used to separate inulin of different chain lengths, which are divided into natural inulin (the degree of polymerization (DP) between 2 and 60), short-chain inulin (average DP ≤ 10), long-chain inulin (average DP ≥ 23), and those with a DP between 10 and 23 are typically referred to as medium-chain inulin [14].

Medium-chain inulin, for example, has demonstrated potential in mitigating high-fat diet-induced metabolic disorders, hepatic steatosis, and chronic inflammation in mice [15]. As a prebiotic, inulin enhances antioxidative capacity and improves pork juiciness without compromising other sensory attributes [2]. Based on our previous research, we identified a medium-chain inulin with a DP of 12 that effectively regulates glucolipid metabolism and modulates the gut microbiome (data not shown, under review). We hypothesized that the incorporating this inulin into the diets of finishing pigs could modify the intestinal microbiome and metabolic processes, thereby enhancing both pork quality and flavor.

In a previous statistical study, the median of inulin in pig farming was 0.1-2% [16]. Supplementing weaned piglets' diets with 2.5 and 5 g/kg of inulin is advantageous for intestinal development, reduces inflammation, and improves intestinal permeability [17]. Furthermore, 0.5% inulin diet can improve the growth performance and carcass traits of growing barrows [18]. However, higher doses may pose a potential risk of flatulence in the host [19]. Thus, the study aims to explore the effects of an appropriate amount of dietary inulin supplementation on the physical phenotypes, pork quality and flavor, gut microbiome, and metabolome in finishing pigs.

Production performance and slaughter traits

Dietary inulin supplementation did not apparently affect body weight (BW), average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency (G/F) over the 60-day experimental period (Table 1). However, significant differences were observed in slaughter traits between the two groups, with the INU group showing reduction in hoof, tail, liver indices, along with leather thickness, and an increased eye muscle area (EMA) index (P <0.05).

Table 1. Effects of inulin on production and slaughter performance of finishing pigs

Item¹	Treatments²		SEM	P-value
Item¹	CON	INU	SEM	P-value
Initial body weight, kg	74.93	75.00	0.263	0.906
Final body weight, kg	120.83	120.33	0.593	0.694
ADG, kg/d	0.77	0.76	0.008	0.600
ADFI, kg/d	2.75	2.73	0.019	0.687
G:F	0.28	0.28	0.002	0.690
Dressing percentage, %	71.18	70.91	0.483	0.797
Head index, %	5.22	4.68	0.113	0.008
Hooves index, %	2.23	2.02	0.039	0.002
Tail index, %	0.14	0.11	0.005	0.034
Liver index, %	1.40	1.12	0.049	<0.001
Leather thick, mm	3.39	2.92	0.118	0.038
Back fat thick, mm	39.63	42.09	0.810	0.133
EMA, mm²	38.50	50.63	2.299	0.002

¹ ADG, average daily gain; ADFI, average daily feed intake; G:F, gain-to-feed ratio; EMA, eye muscle area.

²CON, control group; INU, the same diet supplemented with 0.5% inulin.

Serum biochemistry

Inulin supplementation significantly improved serum biochemical indices as shown in Table 2. Levels of ALT, AST, UREA, GLU, TG, and TCHO were significantly lower in the INU group compared to the CON group (P < 0.05), with respective reduction proportions of 26.95%, 19.07%, 16.62%, 21.71%, 12.50%, and 31.36%. On the other hand, the HDL/LDL ratio increased in the INU group compared to the CON group (P < 0.01), primarily attributed to increased HDL concentrations (P < 0.01) and decreased LDL concentrations (P < 0.01). No significant differences were observed in total protein (TP) and albumin (ALB) levels.

Table 2. Effects of inulin on serum biochemistry of finishing pigs

Item¹	Treatments		SEM	P-value
Item¹	CON	INU	SEM	P-value
ALT, U/L	46.33	33.83	2.091	<0.001
AST, U/L	39.33	31.83	1.515	0.005
TP, mmol/L	74.32	74.23	0.873	0.965
ALB, mmol/L	30.55	29.85	0.863	0.705
UREA, mmol/L	7.70	6.42	0.220	<0.001
GLU, mmol/L	4.56	3.57	0.158	<0.001
TG, mmol/L	0.32	0.28	0.007	<0.001
TCHO, mmol/L	2.36	1.62	0.117	<0.001
HDL, mmol/L	0.62	0.77	0.025	<0.001
LDL, mmol/L	1.07	0.79	0.049	<0.001
HDL/LDL	0.59	0.96	0.063	<0.001

¹ ALT, alanine aminotransferase; AST, aspartatea aminotransferase; TP, total protein; ALB, albumin; GLU, glucose; TG, triglycerides; TCHO, total cholesterol; HDL, high-density lipoprotein; LDL, low-density lipoprotein.

Pork quality analysis

As shown in Table 3, inulin supplementation had no effect on pork moisture, dry matter, or ash content. Tenderness-related traits showed substantial improvements, including reductions in drip loss (29.66%, P < 0.001), cooking loss (6.05%, P < 0.05), and shear force (33.1%, P < 0.01). Flesh color was enhanced with decrease lightness (L*) and yellowness (b*) (P < 0.01) and increase redness (a*) (P < 0.05). Besides, dietary inulin promoted the contents of crude protein (P < 0.05) and ether extract (P < 0.05) in pork.

Table 3. Effects of inulin on pork quality traits of finishing pigs

Item¹		Treatments		SEM	P-value
Item¹		CON	INU	SEM	P-value
Initial water, %		71.98	71.75	0.214	0.620
DM, %		93.00	93.29	0.183	0.451
CP, %		81.96	82.48	0.122	0.024
EE, %		6.47	6.85	0.088	0.024
Ash, %		4.13	4.10	0.058	0.654
Drip loss, %		6.98	4.91	0.365	<0.001
Cooking loss, %		29.58	27.79	0.403	0.017
Shear force, N		116.39	77.82	6.187	<0.001
Longissimus pH 45min		6.01	6.02	0.006	0.402
Chromaticity	Lightness, L*	38.26	35.10	0.564	0.001
	Redness, a*	7.57	8.75	0.256	0.012
	Yellowness, b*	7.47	6.19	0.233	0.001

¹ DM, dry matter; CP, crude protein; EE, ether extract.

Fatty acid Profile of Pork

Fatty acids with concentrations greater than 0.1 μg/g were analyzed and the results are shown in Table 4. Pork from the INU group exhibited significantly higher concentrations of saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs), and polyunsaturated fatty acids (PUFAs) (P < 0.05). Briefly, the primary saturated fatty acids were palmitic acid (C16:0), stearic acid (C18:0), and myristic acid (C14:0). Monounsaturated fatty acids included oleic acid (C18:1), palmitoleic acid (C16:1), erucic acid (C22:1), and eicosenoic acid (C20:1). Polyunsaturated fatty acids included linoleic acid (C18:2), arachidonic acid (C20:4), eicosapentaenoic acid (C20:5n3), and docosahexaenoic acid (C22:6n3). However, the PUFA/SFA and n-3/n-6 ratios were lower compared to the CON group (P < 0.05).

Table 4. Effects of inulin on pork free fatty acids of finishing pigs

Item¹	Treatments		SEM	P-value
Item¹	CON	INU	SEM	P-value
C6:0, μg/g	0.05	0.05	0.000	0.585
C8:0, μg/g	0.06	0.06	0.000	0.604
C10:0, μg/g	0.06	0.06	0.002	0.098
C11:0, μg/g	0.06	0.06	0.001	0.047
C12:0, μg/g	0.06	0.07	0.002	0.066
C13:0, μg/g	0.06	0.07	0.002	0.082
C14:0, μg/g	0.08	0.13	0.012	0.005
C14:1, μg/g	0.07	0.07	0.002	0.123
C15:0, μg/g	0.05	0.06	0.002	0.130
C15:1, μg/g	0.06	0.07	0.002	0.068
C16:0, μg/g	1.06	1.85	0.186	0.003
C16:1, μg/g	0.12	0.31	0.042	<0.001
C17:0, μg/g	0.07	0.07	0.003	0.174
C17:1, μg/g	0.06	0.07	0.003	0.180
C18:0, μg/g	0.62	0.99	0.088	0.003
C18:1n9t, μg/g	0.06	0.07	0.003	0.044
C18:1n9c, μg/g	1.21	3.00	0.406	<0.001
C18:2n6t, μg/g	0.06	0.07	0.002	0.074
C18:2n6c, μg/g	0.48	0.67	0.045	0.003
C20:0, μg/g	0.06	0.71	0.004	0.060
C18:3n6, μg/g	0.07	0.07	0.002	0.081
C20:1, μg/g	0.07	0.10	0.008	0.039
C18:3n3, μg/g	0.06	0.08	0.004	0.066
C21:0, μg/g	0.05	0.06	0.002	0.080
C20:2, μg/g	0.07	0.09	0.005	0.029
C22:0, μg/g	0.06	0.06	0.002	0.049
C20:3n6, μg/g	0.07	0.08	0.003	0.036
C22:1n9, μg/g	0.19	0.24	0.011	0.034
C20:3n3, μg/g	0.06	0.07	0.003	0.031
C23:0, μg/g	0.05	0.06	0.002	0.071
C20:4n6, μg/g	0.16	0.22	0.013	0.001
C22:2, μg/g	0.06	0.07	0.002	0.045
C24:0, μg/g	0.06	0.07	0.002	0.049
C20:5n3, μg/g	0.06	0.07	0.002	0.048
C24:1, μg/g	0.07	0.08	0.002	0.036
C22:6n3, μg/g	0.10	0.11	0.002	0.026
SFA, μg/g	2.49	3.79	0.307	0.004
UFA, μg/g	3.09	5.50	0.542	<0.001
MUFA, μg/g	1.91	4.00	0.471	<0.001
PUFA, μg/g	1.17	1.50	0.075	0.001
PUFA/SFA	0.47	0.40	0.020	0.021
n-3/n-6	0.33	0.28	0.011	0.015

¹ SFA, saturated fatty acid; UFA, unsaturated fatty acid; MUFA, monounsaturated fatty acid; PUFA, polyunsaturated fatty acid; n-3, sum of the C18:3n3, C20:3n3, C20:5n3 and C22:6n3; n-6, sum of the C18:2n6t, C18:2n6c, C18:3n6, C20:3n6, C20:4n6.

The amino acids profile of the pork

Inulin supplementation led to a 10.20% increase in the total amino acid content, with a notable 45.44% rise in umami amino acids (Table 5). Glutamic acid levels increased significantly from 14.09 to 22.16 μg/g (P < 0.05). Essential amino acids like arginine and lysine also showed significant increase (P < 0.05).

Table 5. Effects of inulin on pork amino acids of finishing pigs

Item¹	Treatments		SEM	P-value
Item¹	CON	INU	SEM	P-value
Histidine, μg/g	12.1	14.03	0.784	0.258
4-Hydroxy-L-Proline, μg/g	5.47	5.13	0.280	0.605
Arginine, μg/g	17.13	21.46	1.082	0.016
Asparagine, μg/g	11.47	9.75	0.437	0.021
Glutamine, μg/g	1045.46	1190.83	35.935	0.013
Serine, μg/g	17.08	15.47	0.430	0.039
Glycine, μg/g	73.73	73.90	0.761	0.923
Aspartic acid, μg/g	3.66	3.67	0.114	0.985
Glutamic acid, μg/g	14.09	22.16	1.840	0.001
Threonine, μg/g	21.33	18.97	0.630	0.038
Alanine, μg/g	80.31	73.06	1.737	0.007
γ-Aminobutyric acid, μg/g	0.26	0.18	0.021	0.045
Proline, μg/g	24.30	22.00	0.575	0.016
(R)-2-Aminobutyric acid, μg/g	1.14	1.86	0.221	0.103
Lysine, μg/g	9.84	14.12	0.973	<0.001
Methionine, μg/g	8.71	7.10	0.376	0.002
Tyrosine, μg/g	19.41	16.81	0.628	0.008
Valine, μg/g	18.13	20.80	0.700	0.031
Isoleucine, μg/g	12.83	13.84	0.330	0.134
Leucine, μg/g	19.75	21.36	0.557	0.167
Phenylalanine, μg/g	18.66	13.02	1.306	0.002
Tryptophan, μg/g	7.09	8.72	0.504	0.104
TAAs, μg/g	1441.95	1589.09	36.719	0.016
EAAs, μg/g	116.34	117.92	0.720	0.322
UAAs & SAAs, μg/g	1291.43	1429.82	34.634	0.016

¹ TAAs, total amino acids; EAAs, essential amino acids; UAAs & SAAs, umami amino acids (asparagine, glutamine, aspartic acid and glutamic acid) and sweet amino acids (serine, glycine, threonine, alanine and proline).

Effects of INU treatment on cecum microbiome

Gut microbiome composition

Based on 97% sequence similarity, a total of 3,846 Operational Taxonomic Unit (OTUs) were identified in the INU group, which were then assigned to 39 phyla, 92 classes, 185 orders, 267 families, and 466 genera. In the CON group, 4,118 OTUs were obtained and were clustered into 39 phyla, 98 classes, 181 orders, 271 families, and 481 genera. There were 2,847 OTUs that were common across the two experimental groups (Fig. 1A). Inulin supplementation significantly reduced alpha-diversity in the gut microbiome, as indicated by lower Shannon, Simpson, Chao1, and observed species indices in the INU group compared to the CON group (P < 0.01, Table 6).

Table 6. Alpha-diversity of the cecum microbiota of pigs feeding inulin

Item		Treatments		SEM	P-value
Item		CON	INU	SEM	P-value
Coverage percentage, %		>99	>99
Richness Estimators	Observed species	1075.83	935.83	27.264	0.003
	Chao1	1174.87	1068.72	19.109	0.001
	ACE	1185.72	1073.22	22.576	0005
Alpha-diversity Indexes	PD_whole_tree	89.56	90.86	5.043	0.905
	Shannon	7.57	6.79	0.136	<0.001
	Simpson	0.99	0.97	0.003	<0.001

Beta-diversity analysis, using PCoA, revealed distinct microbiome community structures between the two groups (Fig 1B). The INU group exhibited an increased relative abundance of Firmicutes and Proteobacteria and a reduced abundance of Bacteroidetes, resulting in a significantly higher Firmicutes/Bacteroidetes (F/B) ratio (P < 0.05, Fig 1C). The relative abundance of microbes exceeding 0.5% were displayed at the phylum, family, and genus levels (Fig. 1D-F). Concretely, within the Firmicutes phylum, the dominant families included in both groups were Lachnospiraceae, Oscillospiraceae, Clostridiaceae, and Peptostreptococcaceae, while in phyla of Bacteroidetes were Prevotellaceae, Rikenellaceae, Muribaculaceae, and p-251-o5.

Gut microbiome biomarker

At the genus level, inulin supplementation enriched beneficial microbes, including Lactobacillus, Lachnospiraceae_NK4A136_group, Roseburia, Turicibacter, Streptococcus, Terrisporobacter, Romboutsia, and UCG-005, while decreasing potentially harmful genera such as Treponema, Rikenellaceae_RC9_gut_group, and Methanobrevibacter (Fig. 2A). Linear discriminant analysis effect size (LefSe, with LDA > 4.0) further identified significant biomarkers differentiating the two dietary groups (Fig 2B). Additionally, the cladograms illustrated the phylogenetic distribution of discrepant bacteria, as shown in Fig. 2C. By integrating these two logical analyses, we identified and selected 15 microbial genera for further investigation.

Gut metabolome

Composition and biomarker

Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis of gut metabolome identified 854 metabolites (582 positive and 272 negative ions) in the treatment groups. To compare the distribution of cecum metabolites, orthogonal partial least squares discriminant analysis (OPLS-DA) was conducted. This revealed a distinct clustering pattern between the CON and INU groups, suggesting that inulin supplementation significantly altered gut metabolites. As shown in Fig. 3A, the OPLS-DA model (R²X = 0.735, R²Y = 1, and Q² = 0.996) score plot revealed that the first principal component explained 70.2% of the features between the groups, while the second principal component explained 3.29%.

The variable importance in projection (VIP) value of the OPLS-DA model was used to measure the contribution of metabolites to distinguishing characteristics between groups. Univariate statistical analysis was combined to calculate the P-value and fold change (FC) between the INU and CON groups, leading to the identification of 379 differentiated metabolites with 71 upregulated and 308 downregulated in the INU group (Fig. 3C, VIP > 1.0, FC > 2.0 and FC < 0.5, P < 0.05). In addition, the heatmap shows that the samples from the CON and INU groups cluster well, indicating a strong correlation with the additive treatment (Fig. 3C).

The KEGG (Kyoto Encyclopedia of Genes and Genomes) pathway enrichment analysis of differential metabolites revealed that the highlighted metabolic pathways primarily involved amino and fatty acid metabolism (Fig. 3D). Based on their impact, the top five differential metabolic pathways were identified as linoleic acid metabolism, arginine biosynthesis, riboflavin metabolism, alanine, aspartate, and glutamate metabolism, and taurine and hypotaurine metabolism (the profile of metabolic pathways modulated by inulin supplementation can be referred to in Table 7). Among them, the taurine and hypotaurine metabolism pathway was upregulated in the inulin dietary supplementation, while the other four pathways were downregulated. Classification analysis of significantly modulated metabolites within the top five pathways revealed that those regulated by inulin primarily included lipids and lipid-like molecules, organic acids and derivatives, as well as organoheterocyclic compounds. Thirteen primary differential metabolites and their respective categories are detailed in Fig. 3E.

Table 7. Profile of metabolic pathways modulated by inulin supplementation

Names of pathway	Total	Hits	Raw P	− log 10 (P)	Impact
Linoleic acid metabolism	5	2	0.02	1.65	1.00
Arginine biosynthesis	14	8	<0.01	7.19	0.52
Riboflavin metabolism	4	1	0.19	0.73	0.50
Alanine, aspartate and glutamate metabolism	28	4	0.05	1.32	0.49
Taurine and hypotaurine metabolism	8	2	0.06	1.25	0.43

Pathways correlation

The interconnection of metabolic pathways suggested inulin supplementation reshaped the gut metabolome, promoting beneficial changes linked to pork flavor and host health (Fig. 4).

Correlation among the phenotypes, differentiated microbes and metabolites

Spearman correlation analysis revealed strong association among health parameters, gut microbiota, metabolites, and pork quality traits. Beneficial genera enriched in the INU group, such as Escherichia-Shigella, Treponema, UCG-005, Streptococcus, Rikenellaceae_RC9_gut_group, Terrisporobacter, Lactobacillus, Romboutsia, Lachnospiraceae_NK4A136_group, Ruminococcus, Methanobrevibacter, NK4A214_group, Roseburia, Family_XIII_AD3011_group, Turicibacter, were positively correlated with improved phenotypes, including higher levels of HDL, crude protein (CP), ether extract (EE) and redness (a*) (P < 0.05). These genera were also associated with reduced levels of ALT, AST, glucose, triglycerides (TG), total cholesterol (TCHO), and shear force, indicating enhanced systemic health and meat tenderness (Fig. 5B).

In addition to their correlations with systemic health and pork quality, these beneficial microbes positively influenced the levels of taurine, taurocholic acid in the gut, and fatty acids like SFAs and MUFAs in meat, which serve as flavor precursors in pork. In contrast, genera more abundant in the CON group, such as NK4A214_group, Methanobrevibacter, Treponema, Rikenellaceae_RC9_gut_group, and Ruminococcus, were associated with less favourable phenotypes including higher levels of drip loss, LDL and yellowness (b*) (Fig. 5B).

Correlation analysis also identified 13 key metabolites significantly linked to both host phenotypes and pork flavor traits. Inulin-modulated metabolic pathways, particularly those involving amino acids like alanine, aspartate, and glutamate, as well as fatty acids, were strongly connected to these observed improvements (Fig. 5C&D). For example, higher levels of umami-related amino acids such as glutamic acid, and beneficial fatty acids in pork from the INU group were closely linked to the reshaped gut microbiota and metabolome. The integration of microbiome, metabolome, and phenotype data highlights the pivotal role of inulin in optimizing systemic health and pork quality through gut microbiome modulation.

This study demonstrated that dietary inulin supplementation improved systemic health, pork quality, and flavor while having no adverse effects on production performance in finishing pigs. These findings align with previous studies showing that polysaccharides in pig diets do not directly enhance production performance but can significantly improve the quality of animal by-products [7, 20]. The observed reduction in by-products, such as liver and tail indices, also suggest potential economic benefits from inulin supplementation.

Inulin’s role as a non-digestible carbohydrate likely contributed to the observed improvements in systemic health by balancing carbohydrate and protein metabolism. Reduced fasting blood glucose levels and improved serum indicators, such as lower ALT, AST, and TG levels, reflect enhanced hepatic function and metabolic stability [21, 22]. The increase in HDL/LDL ratio further supports the hypothesis that inulin promotes systemic health by regulating lipid metabolism, as seen in previous studies [23].. Our findings indicated that a diet supplemented with inulin balanced the metabolism of carbohydrates and protein, maintaining energy metabolism and homeostasis, thus contributing to enhanced systemic health in the host.

The enhancements in pork quality, including reduced drip loss, cooking loss, and shear force, suggest an improvement in water-holding capacity and tenderness. Increased crude protein and fatty acid content also highlight the nutritional benefits of inulin supplementation. These improvements can be attributed to the modulation of oxidative stress and increase levels of essential nutrients in the meat, consistent with earlier findings [2, 24]. The degradation of SFAs, MUFAs, and PUFAs, including oleic acid, linoleic acid, stearic acid, and palmitic acid, produces volatile flavor substances such as aldehydes, acids, ketones, hydrocarbons, esters, alcohols, and heterocyclic compounds [25, 26]. The consumption of inulin results in an elevation of the levels of various fatty acids deposited in pork, thereby enhancing the flavor during the cooking process. However, while the higher levels of UFAs enhance pork flavor, the reduced PUFA/SFA and n-3/n-6 ratios may slightly diminish its health benefits [27, 28]. It is worthwhile to mention that higher levels of UFA may shorten the shelf life of meat product [29]. Therefore, future studies should aim to balance flavor enhancement with nutritional quality.

The elevated levels of umami-related amino acids, such as glutamic acid, contributed to the superior flavor profile of pork from the INU group. These amino acids are critical for enhancing the umami taste, sweetness, and overall flavor of meat [30, 31]. Serine and alanine primarily influence the meat's sweetness [32], while arginine generates pyrazine compounds during ripening [33]. Valine produces caramel via the Maillard reaction when combined with glucose [34], and phenylalanine degradation leads to benzaldehyde, imparting a nutty flavor [35]. Dietary interventions, such as inulin supplementation, can modify amino acid profiles to enhance distinctive pork flavor. Additionally, inulin’s ability to increase the abundance of beneficial microbes, such as Lactobacillus and Lachnospiraceae_NK4A136_group, likely supported the production of flavor precursors. These microbes are known for their roles in gut health, nutrient metabolism, and reducing inflammation [36, 37].

The gut microbiome of pigs in the INU group showed reduced alpha-diversity and increased abundance of beneficial genera, such as Turicibacter and Romboutsia. These microbial changes were associated with improved systemic health, as well as increased deposition of fatty acids and amino acids, which enhance pork flavor [38]. The observed increase in the F/B ratio aligns with findings from a recent study by [39], further supporting the role of inulin supplementation in modulating gut microbiota (i.e., Lactobacillus and Bifidobacterium) to enhance nutrient absorption and energy metabolism. Additionally, Lu et al. reported increased intramuscular fat content and enhanced redness (a*) in pork, signifying improved meat color and quality. In contrast, harmful genera such as Treponema and Rikenellaceae_RC9_gut_group were more prevalent in the CON group, correlating with unfavorable phenotypes and metabolic outcomes [40-43]. These results support the hypothesis that inulin supplementation reduces the prevalence of potentially harmful microbes, thereby conferring protection against pathogen-associated issues. Furthermore, the increase abundance of UCG-005, a member of the Oscillospiraceae family is essential for host fatness [38], in the INU group suggests the inulin may have the potential to promote production performance, consistent with the rise in the F/B ratio.

As a non-protein amino acid, taurine reduces oxidative stress and supports lipid metabolism by forming taurocholate with bile acids, promoting fat digestion and lowering serum triglycerides and cholesterol levels [44-46]. Riboflavin (vitamin B₂) also plays a key role in protein and lipid metabolism [47]. In our study, inulin supplementation reduced protein and lipid degradation by modulating related metabolic pathways.

The alanine, aspartate and glutamate metabolism pathway, central to amino acid metabolism and pork flavor [48], is linked to arginine synthesis initiated by glutamate [49]. Additionally, linoleic acid, precursor for conjugated linoleic acid, enhanced pork flavor by increasing fatty acid content [50, 51]. Collectively, our findings demonstrate that inulin can effectively inhibit the breakdown of flavor-enhancing amino acids and preserve umami compounds, as evidenced by the high glutamate content and increased fatty acid levels in pork. These results confirm the down-regulation of fatty acid catabolism pathways observed in the omics analysis.

In conclusion, pigs receiving inulin supplementation displayed a gut microbiome characterized by decreased alpha-diversity and increased the abundance of beneficial microbes, accompanying by the downregulation of flavor-degrading metabolic pathways. Furthermore, inulin supplementation resulted in superior phenotypic traits, improved pork flavor, and maintained average growth and fattening performance compared to the control group. Therefore, feeding 0.5% inulin to finishing pigs was feasible for superior meat quality in intensive rearing conditions.

Experimental design, Animals, and feeding management

Thirty-six healthy Duroc × Landrace × Yorkshire (DLY) pigs with an average body weight of 75.0 ± 1.5 kg were randomly allocated into six pens (4 m × 8 m), with six pigs per pen. The pens were equally divided between two dietary treatment groups: a control group (CON) fed a standard diet and an inulin-supplemented group (INU) fed a diet containing 5 g/kg inulin. Each treatment was replicated three times. Dietary formulation (Table 8) was based on the China national standard outlined in GB/T 39235-2020 [52] to satisfy the nutrient requirements of fatting pigs, and the crude protein (the method of 954.01), crude fat (920.39), ash (942.05), Ca (927.02) and TP (965.17) content in diet were all analysed according to the AOAC [53]. The dietary digestible energy (DE) and metabolizable energy (ME) was calculated according to the following formula: DE (ME) = corn × DE1 (ME₁) + wheat middlings × DE2 (ME₂₎ + wheat bran × DE3 (ME₃₎ + … + minerals × DE14 (ME₁₄₎. All values from DE₁ (ME₁) to DE₁₄ (ME₁₄) were based on nutritional value listed on the GB/T 39235-2020 [52]. The inulin was provided by Gansu Lircon Biological Co., Ltd. The molar mass of inulin is 1,800 g/mol, and mean degree of polymerization (DP) is 12. Inulin was first combined with a premix that was subsequently mixed with other ingredients and then stored in covered containers. All diets were prepared in a single batch and stored in a cool warehouse. The pigs were provided ad libitum access to fresh water and fed three times daily (6:00 a.m., 12:00 p.m., and 6:00 p.m.). Feed intake was calculated based on the quantity provided and leftovers. After a 7-day acclimation period, the pigs were fed their respective diets for 60 days.

Table 8. Basic diet composition and nutritional level

Items	Content
Ingredients
Corn, %	60.50
Wheat middlings, %	10.00
Wheat bran, %	10.00
Soybean meal, %	9.00
Defatted rice bran, %	3.00
Peanut meal, %	4.00
CaHPO₄·2 H₂O, %	0.34
NaCl, %	0.45
Soybean oil, %	1.00
Sulfate lysine (70%), %	0.30
DL-methionine (99%), %	0.01
Limestone, %	1.00
Vitamins¹, %	0.20
Minerals², %	0.20
Total, %	100.00

Nutrients content³^{, 4}
DE, MJ/kg	14.08
ME, MJ/kg	13.46
CP, %	14.43
EE, %	3.71
Lys, %	0.84
Lys/ME, g/MJ	0.62
Ca, %	0.57
TP, %	0.47
Met, %	0.25
Thr, %	0.51
Cys, %	0.29

¹The Vitamins provided the following per kg DM of diets: Vitamin A 8000 IU, Vitamin D₃ 3500 IU, Vitamin E 30 IU, Vitamin K₃ 1.0 mg, Vitamin B₁ 1.5 mg, Vitamin B₂ 5.5 mg, Vitamin B₆ 2.0 mg, Vitamin B₁₂ 0.02mg, Pantothenic acid 7.50 mg, Niacin 26.5 mg, Biotin 0.07mg, Folic acid 0.6 mg;

²The Minerals provided the following per kg DM of diets: Mn (as manganese sulfate) 40 mg, Fe (as ferrous sulfate) 70 mg, Zn (as zinc sulfate) 80 mg, Cu (as copper sulfate) 25 mg, I (as potassium iodide) 0.2 mg, Se (as sodium selenite) 0.4 mg;

³The content of CP, EE, Ca, and TP were measured values (n = 6), methionine (Met), lysine (Lys), Threonine (Thr), and Cystine (Cys) were the actual addition. The value of dietary DE and ME was calculated based on the DE and ME values of each ingredient in the formula.

⁴DE, digestible energy; ME, Metabolizable energy; CP, Crude protein; EE, Ether extract; Ca, Calcium; TP, Total phosphorus.

Sample collection

At the end of trial, 12 pigs (two randomly selected per pen, six per treatment group) were chosen for sample collection. Blood samples were obtained from fasted pigs on the day before slaughter. The pigs were euthanized using electrical stunning following guidelines from the Animal Ethics Committee of Shandong Agricultural University. Longissimus dorsi muscle samples were collected to access meat quality and nutrient composition. Gut content was sampled from the junction of the cecum and colon using sterile surgical procedures. Samples were immediately flash-frozen in liquid nitrogen and stored for further microbiome and metabolome analysis.

Meat quality assessment

Muscle samples were evaluated for physical traits, including the nutrient content, drip loss, cooking loss and shear force. The nutritional values of the pork also determined by the methods mentioned in the AOAC [53], which the moisture according to the method of 950.46, while the 928.08 for analysing the crude protein, 960.39 for the ether extract, and the ash refer to the 920.153. Drip loss was determined as a percentage of weight loss for a sample (30 ± 3 g) after hanging at 4°C for 24 hours. Similarly, cooking loss was calculated as the weight difference of muscle (50 ± 5 g) before and after being water-bathed in a sealed bag at 80°C for 60 mins. The cooked samples were then trimmed into rectangular chunks (30 mm × 10 mm × 10 mm) parallel to the muscle fibers. The chunks were subsequently sliced perpendicular to the myofiber orientation using a tenderness meter (C-LM38, NEAU, China) to determine the pork's shear force.

Briefly, one gram of the freeze-dried samples was weighed onto filter paper and placed in a Soxhlet extractor with petroleum ether for 5 hours to determine the crude fat content. Concurrently, 0.4 grams of the samples were subjected to sulfuric acid digestion, followed by the determination of the crude protein content using a protein analyzer (K9860, Hanon, China). pH values were measured at 45 mins post-mortem (PHBJ-260, INESA, China), and color parameters (L* (lightness), a* (redness), and b* (yellowness)) were accessed using a colorimeter (CR 300, Minolta, Japan) [54].

Fatty acids and amino acids analysis

The profiles of free fatty acids and amino acids in pork samples were analyzed by Shanghai Sanshu Biotechnology Co., Ltd. Fatty acids were identified using gas chromatography-mass spectrometry (GC-MS, Agilent Technologies Inc. CA, USA), and amino acids were determined using ultra-high-performance liquid chromatography (UPLC, (Vanquish, Thermo, USA)) coupled with high-resolution mass spectrometry (Q Exactive, Thermo, USA).

Briefly, the GC system utilized Agilent 6890 (Agilent Technologies, USA), with an INU-Sil 88(100m×0.25mm×0.25 µm) column and an injection volume of 1 μL at a split ratio of 10:1 and nitrogen flow rate of 1.0 mL/min. The initial temperature in the column chamber was set at 100 ℃ for a duration of 5 mins, followed by a heating ramp to reach 240 ℃ at a rate of 4 ℃/min. For MS detection, Agilent 5977 (Agilent Technologies, USA) equipped with electron impact ion source (EI) source and MassHunter workstations were used. The MS conditions were as follows: the injection port temperature was maintained at 260 ℃ while the Quadrupole temperature remained constant at150 ℃; full SCAN mode was selected for detection with a Mass (m/z) scan range from30-550.

The UPLC system was equipped with a Waters BEH C18 column (50×2.1 mm, 1.7 μm). Ultra-pure water containing 0.1% formic acid was used as mobile phase A, while acetonitrile containing 0.1% formic acid served as mobile phase B. The flow rate was 0.5 mL/min, the temperature was set at 55°C, and the injection volume was 1 μL. The MS employed electrospray ionization (ESI) with the following settings: sheath gas, 40 arb; assisted gas, 10 arb; ion spray voltage, +3000V; temperature, 350°C; capillary temperature, 320°C. The scanning mode was set to FullScan using the positive ion method.

Microbiome and metabolome analysis

Gut microbiome composition was analyzed through 16S rRNA sequencing, and metabolome profiles were determined using liquid chromatography-tandem mass spectrometry (LC-MS/MS), as previously described [55]. Microbial alpha- and beta-diversity were evaluated, and key metabolic pathways were identified using KEGG pathway enrichment analysis.

Statistical analysis

Individual pigs were used for phenotypes, pork traits data collection, microbiome analysis, and metabolome analysis (n = 6). A single pen was used as the unit for free amino acid and medium-long chain fatty acid determination (n = 3). Statistical differences were assessed using IBM SPSS 26.0 (SPSS Inc., Chicago, IL, USA) to perform the independent two-sample t-tests, the statistical analysis model used as follows:

where Y_i is the outcome variable, X_i is a binary indicator (0 = Group A, 1 = Group B), β₀ represents the mean of Group A, β₁ is the mean difference between Group B and Group A, ϵ_i are independent normally distributed errors. The null hypothesis H₀ : β₁= 0 was tested against the alternative H₁ : β₁≠ 0. The P-value <0.05 was considered to have statistical significance. Gut microbiome data were analysed and plotted using Prism 8.1 (GraphPad, LaJolla, CA, USA) and http://www.ehbio.com/Cloud_Platform/. Metabolome data were analysed using MetaboAnalyst 6.0, and heatmaps were generated using https://cloud.metware.cn/.

DLY	Duroc × Landrace × Yorkshire pigs
CON	Control group
INU	Inulin group
DP	the degree of polymerization
BW	Body weight
ADG	Average daily gain
ADFI	Average daily feed intake
G:F	Gain-to-feed ratio
EMA	Eye muscle area
ALT	Alanine aminotransferase
AST	Aspartatea aminotransferase
TP	Total protein
ALB	Albumin
GLU	Glucose
TG	Triglycerides
TCHO	Total cholesterol
HDL	High-density lipoprotein
LDL	Low-density lipoprotein
DM	Dry matter
CP	Crude protein
EE	Ether extract
SFA	Saturated fatty acid
UFA	Unsaturated fatty acid
MUFA	Monounsaturated fatty acid
PUFA	Polyunsaturated fatty acid
TAAs	Total amino acids
EAAs	Essential amino acids
UAAs	Umami amino acids
SAAs	Sweet amino acids
OTUs	Operational taxonomic units
PCoA	Principal coordinate analysis
LefSe	Linear discriminant analysis effect size
LDA	Linear discriminant analysis
LC-MS/MS	Liquid chromatography-tandem mass spectrometry
OPLS-DA	Orthogonal partial least squares discriminant analysis
VIP	Variable importance in projection
FC	Fold change
KEGG	Kyoto encyclopedia of genes and genomes
AOAC	Association of official analytical chemists
DE	Digestible energy
ME	Metabolizable energy
GC-MS	Gas chromatography-mass spectrometry
UPLC	Ultra-high-performance liquid chromatography
EI	Ion source

Author contributions

Conceptualization, Yunkyoung Lee and Guiguo Zhang; Methodology, Yunpeng Wang and Kayeon Ko; Investigation, Yunpeng Wang and Minhyeok Kang; Formal Analysis and Writing - Original Draft, Yunpeng Wang; Writing - Review & Editing, Yunkyoung Lee, Miroslava Kačániová, and Guiguo Zhang; Funding Acquisition and Supervision, Yunkyoung Lee and Guiguo Zhang; All authors read and approved the final manuscript.

Funding

This work was supported by the National Key R&D Program of China-Korea cooperative project (2019YFE0107700, NRF-2019K1A3A1A20081146), the National Research Foundation Grant of Korea (NRF-2020R1A2C2004144, RS-2024-00334577), the key project for foreign experts of Shandong Province (WRS2023075), Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (RS-2024-00410255), the Forage Industrial Innovation Team Project (SDAIT-23-05), and the Key R&D Program of Shandong Province (2022TZXD0018).

Data availability

The Raw 16S rRNA sequences datasets described in this study have been deposited in the NCBI database (https://www.ncbi.nlm.nih.gov: Accession number PRJNA971324). The metabolome data reported in this paper have been deposited in the OMIX, China National Center for Bioinformation / Beijing Institute of Genomics, Chinese Academy of Sciences at https://ngdc.cncb.ac.cn/omix: Accession number OMIX 010210. All the other original data are not publicly available due to privacy or ethical restrictions, but will be made available upon request from [email protected].

Ethic approval

The current experiment adhered to animal welfare guidelines and experimental protocols established by the Research Ethics Committee of Shandong Agricultural University. Approval was granted by the Laboratory Animal Management Committee of Shandong Agricultural University (Protocol No. S20240136).

Competing interest statement

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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No competing interests reported.

Dietary Inulin Improves Pork Quality and Systemic Health Via Gut Microbiome and Metabolome Modulation in Finishing Pigs

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Abstract

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Introduction

Results

Discussion

Conclusions

Methods

Abbreviations

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References

Additional Declarations

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