Trans fats are a type of unsaturated fats which are uncommon in nature but can be created artificially. Fats contain long hydrocarbon chains, which can either be unsaturated, i.e. have double bonds, or saturated, i.e. have no double bonds. In nature, unsaturated fatty acids generally have cis (as opposed to trans) configurations. In food production, liquid cis-unsaturated fats such as vegetable oils are hydrogenated to produce saturated fats, which have more desirable physical properties, e.g. they melt at a desirable temperature (30–40 °C). The process of hydrogenation upon the unsaturated fat converts some of the “cis” double bonds into “trans” double bonds, which yields a trans fat. Trans fats are a contaminant introduced by a side reaction with the catalyst in partial hydrogenation. Although trans fats are edible, consumption of trans fats has shown to increase the risk of coronary heart disease in part by raising levels of the lipoprotein LDL (so-called “bad cholesterol”), lowering levels of the lipoprotein HDL (“good cholesterol”), increasing triglycerides in the bloodstream and promoting systemic inflammation. Trans fats also occur naturally in a limited number of cases. Vaccenyl and conjugated linoleyl (CLA) containing trans fats occur naturally in trace amounts in meat and dairy products from ruminants. Most artificial trans fats are chemically different from natural trans fats, but there is no scientific consensus about differences in their health effects. Two Canadian studies have shown that the natural trans fat vaccenic acid, found in beef and dairy products, could actually be beneficial compared to hydrogenated vegetable shortening, or a mixture of pork lard and soy fat, by lowering total and LDL and triglyceride levels. A study by the US Department of Agriculture showed that vaccenic acid raises both HDL and LDL cholesterol, whereas industrial trans fats only raise LDL without any beneficial effect on HDL. In lack of recognized evidence and scientific agreement, nutritional authorities consider all trans fats as equally harmful for health and recommend that consumption of trans fats be reduced to trace amounts. In 2013 the United States FDA issued a preliminary determination that partially hydrogenated oils (which contain trans fats) are not “generally recognized as safe”, which is expected to lead to a ban on industrially produced trans fats from the American diet. In other countries, there are legal limits to trans fat content. Trans fats levels can be reduced or eliminated using saturated fats such as lard, palm oil or fully hydrogenated fats, or by using interesterified fat. Other alternative formulations can also allow unsaturated fats to be used to replace saturated or partially hydrogenated fats. Hydrogenated oil is not a synonym for trans fat: complete hydrogenation removes all unsaturated fats, both cis and trans forms.