Trans fats occur naturally, 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. Partial hydrogenation changes a fat's molecular structure (raising its melting point and reducing rancidity) but this process also results in a proportion of the changed fat becoming trans fat.

Unlike other fats, trans fats are neither required nor beneficial for health. Eating trans fat increases the risk of coronary heart disease. 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.