Oils that have been hydrogenated to be artificially "hard" at room temperature. Hydrogenated oils are found in just about any processed foods at the grocery store. They are found in all fast food chains, used to make french fries, doughnuts, and fried goods such as chicken. Hydrogenated oils are high in saturated fat (hydrogenated means artificially saturated) and are highly deficient in essential fatty acids like linolenic and linoleic acid, because these highly unsaturated fatty acids tend to deform into other fats and plastic-like trans isomers under high pressure and heat. Hydrogenated oils are associated with problems like insulin resistance, lipid peroxidation, increased LDL cholesterol with decreased HDL, and irritable bowel syndrome.

Hydrogenated oils are nearly impossible to avoid without preparing your food yourself. Just about all margarines contain partially hydrogenated oils, which contain molecular anomalies known as "trans fatty acids," which have no legitimate use in the human body. Unfortunately they can be shoved into cell membranes, and they literally have a hardening effect on your very cells.

In my own personal diet, I have noticed a correlation between trans fat consumption and irritable bowel syndrome / ulcerative colitis symptoms. In general, I eliminate them from my diet, but if I end up going to a restaurant (where they are almost completely unavoidable), I generally suffer from mild but quite noticeable <plug>rectal bleeding</plug> the next day.

Hydrogenated oils were first available in the United States in 1910 (I think it was first introduced in France but I'm not sure) and by 1911 Crisco was on the market. Margarine, which was originally made using lard and suet, began to contain more and more hydrogenated oils. Depending on the type of hydrogenation used, partial or complete, different products can be made. Complete hydrogenation yields a solid fat at room temperature while partial hydrogenation results in a softer fat which could be used for spreadable margarine for example.

Hydrogenation has many benefits (as far as food companies are concerned anyway): It gives the oils an increased melting point, making them more solid as well as increasing the oil's resistance to oxidation giving it a longer shelf life. Basically, during hydrogenation, fats are forced to bond with more hydrogen atoms than they would normally. The fats become saturated with hydrogen. Essential fatty acids and polyunsaturated fats are transformed into saturated fats. Any health benefits provided by the polyunsaturated fats are lost. The process also creates trans-fats which are very detrimental to one's health.

In order to acheive hydrogenation an oil is placed under high temperature and pressure and a metal catalyst is added. Hydrogen gas is then injected into the mix. The whole process takes about 8 hours.

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