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Trans fatty acids (commonly termed trans fats) are a type of unsaturated fat (and may be monounsaturated or polyunsaturated).

Trans fats occur, in small quantities, in meat and dairy products from ruminants. Most trans fats consumed today, however, are industrially created as a side effect of partial hydrogenation of plant oils — a process developed in the early 1900s and first commercialized as Crisco in 1911. Partial hydrogenation changes a fat's molecular structure (raising its melting point and reducing rancidity), but this process also results in a portion of the changed fat becoming trans fat.

Unlike other fats, trans fats are neither required nor beneficial for health.^[1] Eating trans fat increases the risk of coronary heart disease.^[2] For these reasons, health authorities worldwide recommend that consumption of trans fat be reduced to trace amounts. Trans fats from partially hydrogenated oils are generally considered to be more of a health risk than those occurring naturally.^[3]

Trans fats are tightly regulated in a few countries, must be disclosed on product labels in many others, and are the central issue in several ongoing lawsuits (particularly against fast food outlets). Many companies are voluntarily removing trans fats from their products, or establishing trans-free product lines.

Chemically, trans fats are made of the same building blocks as non-trans fats, but have a completely different shape. In trans fat molecules, the hydrogens on the doubly bonded carbon atoms (characteristic of all unsaturated fats) are in the trans rather than the cis configuration, resulting in a straighter, rather than kinked, shape. As a result, trans fats are less fluid and have a higher melting point than the corresponding cis fats.