Trans fats

The fats in food have cis and trans forms too. The trans form packs into solid fat, and it is hard on your heart.

A fat molecule is glycerol joined to three fatty acid chains. Oleic acid, the main fatty acid in olive oil, has one C=C double bond in the middle of its 18-⁠carbon chain, between carbons 9 and 10, and that double bond is cis. Both halves of the chain sit on the same side of it, so the chain has a kink. The oils plants make have cis double bonds like this, and the kinks keep the chains apart, so the oils stay liquid.

Elaidic acid has the same formula, with its double bond in the same place, in the trans form. The two halves of its chain sit on opposite sides of the double bond, so the chain runs on nearly straight, like a saturated chain. Straight chains pack tightly, and elaidic acid melts at 45 °C, more than 30 degrees above oleic acid. Its name comes from elaion, the Greek word for oil.

Where trans fats come from

In 1902 the German chemist Wilhelm Normann patented a way to add hydrogen to liquid oils with a nickel catalyst, the addition reaction that turns an oil into a solid spread. Stopping partway, called partial hydrogenation, gives a soft fat that spreads and bakes well and keeps for a long time. Margarine and shortening were made this way for most of the 20th century.

On the nickel, a hydrogen atom adds to one carbon of a double bond, and for a moment that bond is a single bond, free to turn. When the hydrogen atom leaves again, the double bond forms again, often in the trans form. So partly hydrogenated oils came out with many trans chains, such as elaidic acid.

Trans fats raise the "bad" LDL cholesterol in your blood and lower the "good" HDL cholesterol, which raises the risk of heart disease. Countries now limit the trans fats made in factories:

In 2018 the World Health Organization called on every country to remove factory-made trans fats from food, and many more countries have set limits since.