**Abstract**

Adding essential oils to the diet of ruminants is a novel strategy that improves milk and meat quality by enhancing production and fatty acid content. Including essential oils has various effects, such as modifying the biohydrogenation of unsaturated fatty acids. As a result, the fatty acid profile leaving the rumen can be affected, which in turn can affect the levels of important fatty acids in the milk and meat produced by ruminants. In the rumen, microorganisms convert unsaturated fatty acids to mostly saturated fatty acids and some unsaturated fatty acids through biohydrogenation. Added essential oils can shift the rumen microbiota, followed by changes in the fatty acid profile. The impact of essential oils on the biohydrogenation of fatty acids depends on various factors such as the type of essential oil used, its chemical composition, interactions with nutrients present in the feed, the ability of ruminal microbes to adapt to essential oils, and type of animal. Studies have indicated that various essential oils can influence ruminal fermentation and biohydrogenation of dietary fatty acids, and thus, affect the presence of polyunsaturated fatty acids in milk and meat, which is associated with positive effects on human health.

**Keywords:** essential oils, milk, meat, rumen microbiota, fatty acids
