Hydrogenation is a class of chemical reactions in which the net result is an addition of hydrogen (H2)…. The classical example of a hydrogenation is the addition of hydrogen on unsaturated bonds between carbon atoms (Wikipedia, "Hydrogenation").

In a natural fatty acid, the hydrogen atoms are usually on the same side of the double bonds of the carbon chain. However, partial hydrogenation reconfigures most of the double bonds that do not become chemically saturated, twisting them so that the hydrogen atoms end up on different sides of the chain. This type of configuration is called trans, which means "across" in Latin. (Note that it is not just a swapping of the position of the single hydrogen atom, rather it is a twisting of the whole molecule from that point on that creates the trans alignment (Wikipedia. "Trans fat").

After closely scrutinizing data from scientific studies and reviews, many European countries have either banned hydrogenated and partially hydrogenated oils altogether or have instituted future dates for elimination of their use in foods. These government actions concerning the trans fatty acids (hydrogenated and partially hydrogenated oils) is directly related to studies that link trans fatty acid (hydrogenated and partially hydrogenated oil) consumption from processed foods to the development of diabetes, cancer and cardiovascular disease (Recovery).

Moreover:

Most of the trans fats in the American diet are found in commercially prepared baked goods, margarines, snack foods, and processed foods. Commercially prepared fried foods, like French fries and onion rings, also contain a good deal of trans fat (Harvard).

An intake of trans-fatty acids of 5 grams per day is associated with an increase of 25% in the risk of ischemic heart disease (Stender).

Partial hydrogenation of oil results in fats containing unusual isomeric fatty acids characterized by cis and trans configurations. Hydrogenated fats containing trans fatty acids increase plasma total cholesterol (TC) and LDL-cholesterol while depressing HDL-cholesterol levels….

Subjects exhibited a higher total cholesterol and LDL-cholesterol level when consuming the diet containing trans fatty acids while also depressing the HDL-cholesterol level. Consuming the partially hydrogenated fat diet treatment increased the fractional synthesis rate of free cholesterol. Consumption of hydrogenated fats containing trans fatty acids in comparison to a mixture of palmitic and oleic acids increase plasma cholesterol levels apparently by increasing endogenous synthesis of cholesterol (Sundram).

Trans fats are even worse for cholesterol levels than saturated fats because they raise bad LDL and lower good HDL. They also fire inflammation, an overactivity of the immune system that has been implicated in heart disease, stroke, diabetes, and other chronic conditions. While you should limit your intake of saturated fats, it is important to eliminate trans fats from partially hydrogenated oils from your diet (Harvard).

Trans fatty acid intake has been associated with a higher risk of cardiovascular disease. The relation is explained only partially by the adverse effect of these fatty acids on the lipid profile. We examined whether trans fatty acid intake could also affect biomarkers of inflammation and endothelial dysfunction … CRP [C-Reactive Protein] levels were 73% higher among those in the highest quintile of trans fat intake, compared with the lowest quintile. … In conclusion, this study suggests that higher intake of trans fatty acids could adversely affect endothelial function, which might partially explain why the positive relation between trans fat and cardiovascular risk is greater than one would predict based solely on its adverse effects on lipids (Lopez-Garcia).

Trans fats can wreak havoc on humans, lowering good cholesterol, raising bad cholesterol and reducing the quality of everything from testosterone to breast milk (MedIndia).

Diets rich in trans fat cause a redistribution of fat tissue into the abdomen and lead to a higher body weight even when the total dietary calories are controlled," said Lawrence L. Rudel, Ph.D., professor of pathology and biochemistry and head of the Lipid Sciences Research Program (ScienceDaily).

The relationship between trans fat and cancer risk is not clear. Some research has suggested that trans fats might increase the risk of breast cancer. Other equally good studies have not confirmed this observation (Discovery).

Using hydrogenated oils in foods is a strategy used by manufacturing companies to enhance the taste of their foods, add calories to their foods and extend the shelf life and shelf stability of those foods. This all adds up to higher profits for food companies. The real costs of using this ingredient, however, are shifted to consumers (a process called externalizing costs). Those include the starkly increased risk of a number of degenerative chronic diseases including heart disease, diabetes, birth defects, cancer, malabsorption of healthy oils and the universal breakdown of cell wall structures throughout all organs and tissues of the body, including nervous system tissues (eyes, brain, spinal cord, etc.). (NewTarget).