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Oils have been hydrogenated for many decades, to prolong their shelf life and make the oils more stable. Hydrogenated oil is oil in which the essential fatty acids have been converted to a different form chemically, which has several effects. Hydrogenated oil is far more shelf stable, and will not go rancid as quickly as untreated oil. It also has a higher melting point, and is often used in frying and pastries for this reason. When hydrogenated, the chemical structure of the oil is changed, which scientists in the 1990s began to realize could result in negative health effects.

Hydrogenated oil is made by forcing hydrogen gas into oil at high pressure. Both animal and vegetable fats can be and are hydrogenated. In general, the more solid the oil is, the more hydrogenated it is. Two common examples of hydrogenated oil are Crisco and margarine. In the 1990s, it was realized that these products might have deleterious health effects, a tragic irony since they were originally produced and promoted as being healthier than conventional oils.

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