Hydrogenation is the addition of hydrogen to another substance at a high temperature and in the presence of a metallic catalyst to increase or decrease the number of chemical bonds within the substance's molecules. In terms of food processing, hydrogenation is the addition of hydrogen to a liquid oil to transform it into a denser oil or even a solid fat.
Hydrogenation transforms unsaturated fats into saturated fats. The term "saturation" refers to the chemical bonds within the fat molecule. Unsaturated fats have bonds between some carbon atoms. Saturated fats have bonds between carbon and hydrogen, but not between carbon and carbon. Carbon-to-carbon bonds are relatively unstable and can be broken by oxygen, whereas carbon-hydrogen bonds are very stable and resist oxygenation. Oxygenation of fats leads to food spoilage.
Hydrogenation replaces carbon-to-carbon bonds with carbon-to-hydrogen bonds. A fat can be partially or fully hydrogenated. Partial hydrogenation transforms a liquid oil into a denser, but not completely solid oil. It leaves some carbon-to-carbon bonds, but rearranges their structure so that the two carbons are on opposite sides of the molecule. This is called a "trans" bond. In non-hydrogenated, unsaturated oils both carbons in a carbon-to-carbon bond are on the same side of the molecule, which is called a "cis" bond. Fats with trans bonds are called "trans fats." Trans fats are very stable -- sometimes too stable, because your body can't easily break them down into a utilizable product.
Once inside your body, trans or partially hydrogenated fats raise LDL cholesterol; the type that gets stuck in your arteries. They also lower HDL cholesterol; the type that removes LDL cholesterol. Trans fats increase blood triglyceride levels, which may increase the risk of atherosclerosis and stroke, and increase blood levels of Lp(a) lipoprotein, which forms arterial plaques.
Full Is Fine ... Kind Of
Oddly enough, fully hydrogenated oils aren't as problematic as partially hydrogenated ones. Full hydrogenation replaces all carbon-to-carbon bonds with carbon-to-hydrogen bonds and transforms liquid oils into completely solid fats. These fats don't contain trans bonds and are no more harmful than any other saturated fat -- in fact, they may be less harmful than some naturally occurring saturated fats because your body converts most of them into oleic acid, an unsaturated fat that does not raise LDL cholesterol levels.
Trans fats naturally occur in only a few foods, primarily beef and sheep body fat and milk fat. The amount you'd eat compared to your overall fat intake is relatively small -- but trans fats really add up when it comes to partially hydrogenated vegetable oils. These oils replaced most natural solid fats in baked goods, snack foods and fried foods between the 1950s and the 2000s because they are cheaper than naturally saturated fats, such as butter, lard and palm oil. As of 2013, the FDA revoked the designation "generally regarded as safe" from trans fats -- the preliminary step to ending their recognition as an acceptable food product.
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