A trans-18:1 fatty acid found at notable levels in milk fat and other foods derived from ruminants. Vaccenic acid — the predominant natural trans fatty acid — is formed by bacterial biohydrogenation of polyunsaturated fatty acids in the rumen. Unlike industrial trans fats linked to cardiovascular risk, ruminant trans fats occur at low dietary levels, and vaccenic acid is partly converted in human tissues to conjugated linoleic acid (CLA), studied for potential health benefits. Its presence is a marker of ruminant origin in dairy authenticity testing, and pasture-based feeding increases its concentration in milk fat.