نوع مقاله : مقاله پژوهشی
عنوان مقاله English
نویسندگان English
Introduction: Sheep milk and its products, particularly cheese, possess high nutritional value due to their content of high-quality proteins, vitamins, and minerals. However, ruminant milk fat has long been criticized for its high proportion of saturated fatty acids, such as myristic acid and palmitic acid, whose excessive consumption is associated with increased cardiovascular disease risk. Conversely, ruminant milk naturally contains health-promoting fatty acids including conjugated linoleic acid (particularly rumenic acid), vaccenic acid (its endogenous precursor), and omega-3 polyunsaturated fatty acids like alpha-linolenic acid, which reduce inflammation and support brain health. The fatty acid profile of ruminant milk can be substantially manipulated through nutritional strategies. Including vegetable oils or oilseeds rich in unsaturated fatty acids in lactating ewe diets is particularly effective, as these undergo ruminal biohydrogenation, forming intermediates like vaccenic acid and conjugated linoleic acid that are incorporated into milk fat. While numerous studies have investigated various unsaturated fatty acid sources—linseed, sunflower seed, soybean oil, and olive by-products—results have often been conflicting due to differing experimental conditions. This systematic review and meta-analysis aimed to quantify the overall effects of plant-based unsaturated fatty acid sources on dry matter intake, milk yield, milk composition, and fatty acid profiles of milk and cheese in lactating ewes.
Materials and Methods: Following PRISMA guidelines, a systematic search was conducted in PubMed, Scopus, Web of Science, Google Scholar, and ScienceDirect for articles published between January 1990 and December 2022. The search strategy combined keywords related to population (ewe, sheep), intervention (unsaturated fatty acids, vegetable oil, oilseed), and outcomes (milk, cheese, fatty acid profile, production). After removing duplicates and screening titles and abstracts, full texts were assessed for eligibility. Inclusion criteria required studies with control groups, non-grazing systems, plant-based unsaturated fatty acid supplementation, and English publication. Thirty-three eligible studies comprising 52 independent comparisons were identified. Data were extracted using standardized forms. Effect sizes were calculated as standardized mean differences with 95% confidence intervals using Hedges' g. A random-effects model was employed, with heterogeneity assessed using I² statistics and Cochran's Q test. Publication bias was examined using funnel plots and Egger's test.
Results: The 33 included studies were conducted across Italy, Spain, Turkey, Iran, Canada, and Romania, using various breeds including Sarda, Manchega, Lacaune, Awassi, and Moghani. Investigated unsaturated fatty acid sources included linseed (whole, extruded, oil), sunflower seed or oil, soybean oil, and olive oil or cake.
Performance Parameters: Supplementation with plant-based unsaturated fatty acids had no significant effect on daily dry matter intake. However, milk yield increased significantly by approximately 0.15 kg per day. Milk fat percentage and lactose percentage were significantly higher in treatment groups, while milk protein percentage decreased significantly.
Milk Fatty Acid Profile: Unsaturated fatty acid supplementation significantly reduced most medium- and short-chain saturated fatty acids, particularly hypercholesterolemic myristic and palmitic acids, while stearic acid increased. Total saturated fatty acid content decreased substantially. Changes in unsaturated fatty acids were entirely positive: vaccenic acid, oleic acid, both conjugated linoleic acid isomers (c9,t11 rumenic acid and t10,c12), and alpha-linolenic acid all increased significantly. Total polyunsaturated and monounsaturated fatty acids also increased significantly.
Cheese Fatty Acid Profile: Changes observed in milk were effectively transferred to cheese. Saturated fatty acid content decreased while monounsaturated and polyunsaturated fatty acids increased. Beneficial fatty acids including vaccenic acid, oleic acid, conjugated linoleic acid, and alpha-linolenic acid were considerably higher in cheese from supplemented ewes.
Heterogeneity and Publication Bias: High statistical heterogeneity was observed for most variables, reflecting diversity in experimental conditions, supplement types and doses, breeds, and basal diets. Egger's test indicated potential publication bias for milk yield and milk protein percentage but not for other key outcomes.
Discussion: These findings demonstrate that plant-based unsaturated fatty acid supplementation in lactating ewe diets effectively improves both productive performance and nutritional product quality. The absence of negative effects on dry matter intake alleviates concerns about reduced feed consumption. Increased milk yield can be attributed to higher dietary energy density and glucose sparing. The increase in milk fat percentage contrasts with milk fat depression in dairy cows, suggesting lower sensitivity of ewes to inhibitory conjugated linoleic acid isomers. The slight reduction in milk protein percentage is a known phenomenon in fat-supplemented diets. The profound changes in milk fatty acid profile are consistent with ruminant lipid metabolism. Plant-based unsaturated fatty acid sources provide precursors like linoleic acid and alpha-linolenic acid, which undergo ruminal biohydrogenation producing intermediates such as vaccenic acid, subsequently converted to rumenic acid in the mammary gland. The reduction in harmful saturated fatty acids combined with increased beneficial fatty acids markedly improves the milk fat profile towards promoting cardiovascular health. The complete transfer of these improvements to cheese confirms that cheesemaking preserves beneficial fatty acids, enabling production of functional cheeses with enhanced health-promoting properties.
Conclusion: This comprehensive meta-analysis demonstrates that supplementing lactating ewe diets with plant-based unsaturated fatty acid sources is a practical strategy that simultaneously increases production efficiency and enhances product quality. It improves economic profitability through increased milk yield and fat percentage, while substantially increasing the nutritional value of milk and cheese by reducing harmful saturated fatty acids and increasing beneficial fatty acids. These findings provide strong scientific support for farmers, nutritionists, and the sheep dairy industry to implement targeted nutritional strategies towards producing healthier dairy products aligned with public health needs.
کلیدواژهها English
