Lipids are a class of biomolecules defined by solubility--by definition, lipids don't dissolve in water. There is a wide variety of lipid compounds in the body, including cholesterol, many different hormones, and phospholipids, which make up cell membranes. Nutritional lipids, meaning the lipids that we eat, include unsaturated fats, saturated fats, and trans fats.
Chemistry
Chemically speaking, lipids are important to the human body. Not only do a wide variety of lipids perform biochemical roles in the body, but we also eat certain lipids as a source of energy and nutrition. The nutritional lipids include saturated, unsaturated and trans fats, all three of which consist of molecules called triglycerides. A triglyceride consists of three molecules called fatty acids, chemically bonded to a molecule called glycerol.
Types of Fats
The difference between nutritional lipids lies in the chemical composition of the fatty acids in triglycerides, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." Saturated fats contain fatty acids that have no carbon-carbon double bonds. This makes the fats solids at room temperature. Unsaturated fats contain fatty acids that have kinked carbon-carbon double bonds. As a result of these double bonds, unsaturated fatty acids are liquids at room temperature. Saturated and unsaturated fats are the only naturally occurring nutritional triglycerides.
Trans Fats
Trans fats don't occur to any significant extent in nature. Instead, they're made in a lab by chemically modifying unsaturated fatty acids to make them solid at room temperature. This chemical process, called "partial hydrogenation," results in formation of solid fats that are convenient for cooking and baking purposes--margarines and shortenings typically contain trans fat. Trans fats, because they don't occur in nature, resist bacterial degradation and have long shelf lives, explain Drs. Garrett and Grisham.
Considerations
With regard to health, different nutritional lipids have different effects. Saturated fats typically increase LDL, or bad cholesterol. Unsaturated fats don't affect LDL or HDL, which is good cholesterol, and they further play important roles in physiological function. Humans need healthy, unsaturated fats to maintain the immune system and to maintain fat stores. Trans fats not only increase LDL, but decrease HDL, making them the least heart-healthy of the fats.
Expert Insight
Because of the negative effect of trans fats and saturated fats on heart health, the American Heart Association recommends limiting consumption of saturated fat and significantly limiting consumption of trans fat. Dietary sources of saturated fat include most animal fats and some tropical plant fats like coconut oil. Dietary sources of trans fat include processed foods and many fast foods. Oils that are liquid at room temperature, like olive oil, contain healthy unsaturated nutritional lipids.
References
- "Biochemistry"; Reginald Garrett, Ph.D. and Charles Grisham, Ph.D.; 2007
- American Heart Association: Fats
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