Unsaturated Fats can be trans fat or cis fat. Trans fats are rarely found in naturally growing food other than small amounts in cattle and sheep. So in the past, animal-based fats were the only trans fats consumed. But today the largest amount of trans fat consumed is created by the processed food industry as a side effect of partially hydrogenating unsaturated plant fats (generally vegetable oils). In November 2013, the U.S. FDA announced that they will require the food industry to phase out the use of trans fats.
edit Health Consequences
The National Academy of Sciences has concluded that trans fatty acids, whether of plant or animal origin, are not essential and provide no benefit to human health. What's more, trans fats decrease the levels of HDL (good cholesterol) in the body, thereby increasing the risk of coronary heart disease. So the consumption of trans fat should be as low as possible.
Cis fats do not pose the same level of risk. While consuming cis fats in unnaturally large quantities poses a health risk, unsaturated (cis) fats generally increase the levels of good cholesterol (HDL) in the body.
edit Video slides
The slides in this video explain some of the health implications of trans fatty acids.
edit Chemical Structure
In unsaturated fatty acids, the carbon atoms that are missing a hydrogen atom are joined by double bonds rather than single bonds so that each carbon atom participates in four bonds. If the hydrogen atoms are on the same side of the double bonds of the carbon chain then it is said to be in cis configuration. If the hydrogen atoms are on the opposite side of the double bonds of the carbon chain then it is said to be in trans configuration.
edit Video explaining the structure
The change in the arrangement of molecules changes the chemical and physical properties of the fat. For example, the trans fatty acid elaidic acid and naturally occurring oleic acid have the same chemical formula (C9H17C9H17O2) but they have different chemical and physical properties:
- Oleic acid has lower melting point 13.4 °C.
- Oleic acid is liquid at room temperature because cis molecules are loosely packed.
- Elaidic acid has a much higher melting point, 45 °C.
- Elaidic acid is solid at room temperature because trans molecules are tightly packed.
This also explains why trans fats grew in the processed food industry: they make the food last longer and decrease refrigeration requirements.