Impact of Pasture-Based Diets on the Untargeted Metabolomics Profile of Sarda Sheep Milk

In this work, untargeted metabolomics was used to shed light on the impact of different pasture-based diets on the chemical profile of Sarda sheep milk. The study considered 11 dairy sheep farms located in Sardinia, and milk samples were collected in 4 different periods, namely January, March, May, and July 2019, when all sheep had 58, 98, 138, and 178 days in milk, respectively. The animal diet composition was based on the intake of grazed herbage in natural pasture, hay, and concentrate. Overall, the combination of two comprehensive databases on food, namely the Milk Composition Database and Phenol-Explorer, allowed the putative identification of 406 metabolites, with a significant (p < 0.01) enrichment of several metabolite classes, namely amino acids and peptides, monosaccharides, fatty acids, phenylacetic acids, benzoic acids, cinnamic acids, and flavonoids. The multivariate statistical approach based on supervised orthogonal projections to latent structures (OPLS-DA) allowed us to predict the chemical profile of sheep milk samples as a function of the high vs no fresh herbage intake, while the prediction model was not significant when considering both hay and concentrate intake. Among the discriminant markers of the herbage intake, we found five phenolic metabolites (such as hippuric and coumaric acids), together with lutein and cresol (belonging to carotenoids and their metabolites). Additionally, a high discriminant power was outlined for lipid derivatives followed by sugars, amino acids, and peptides. Finally, a pathway analysis revealed that the herbage intake affected mainly five biochemical pathways in milk, namely galactose metabolism, phenylalanine metabolism, alpha-linolenic acid metabolism, linoleic acid metabolism, and aromatic amino acids involved in protein synthesis (namely tyrosine, phenylalanine, and tryptophan).

Automated summary

Untargeted metabolomics was used to investigate the effect of different pasture-based diets on the chemical composition of Sarda sheep milk. A total of 11 dairy sheep farms located in Sardinia were included in the study, and milk samples were collected in four different periods. The analysis identified 406 metabolites, including amino acids, monosaccharides, fatty acids, phenolic acids, and flavonoids. The results showed that the composition of sheep milk varied based on the intake of fresh herbage, while the intake of hay and concentrate did not significantly affect the milk composition. Discriminant markers of herbage intake included phenolic metabolites, carotenoids, and their metabolites, as well as lipid derivatives, sugars, amino acids, and peptides. Pathway analysis revealed the impact of herbage intake on several biochemical pathways related to metabolism and protein synthesis.