[Medical Q&A] Why are fatty acids divided into saturated and unsaturated?

Planner: Chinese Medical Association

Reviewer: Gong Weiyan, Associate Researcher, Institute of Nutrition and Health, Chinese Center for Disease Control and Prevention

I believe you often hear the terms "saturated fatty acids" and "unsaturated fatty acids" in your daily life, and you also know that unsaturated fatty acids are beneficial to the human body. The reason why fatty acids are divided into "saturated" and "unsaturated" is because their molecular structures are different, and their functions are also different.

Lipids are important nutrients for the human body, and fatty acids are the basic substances that make up lipids. They are a class of compounds composed of three elements: carbon (C), hydrogen (H), and oxygen (O). There are many ways to classify them. The carbon chain in fatty acids (a chain structure formed by adjacent carbon atoms connected by covalent bonds) has different forms, such as single bonds (CC), double bonds (C=C), etc. If a fatty acid has one or more carbon-carbon double bonds (C=C) in its carbon chain, it is an unsaturated fatty acid; if a fatty acid does not contain a carbon-carbon double bond (C=C) in its carbon chain, it is a saturated fatty acid. The more carbon-carbon double bonds there are, the more curved the carbon chain will be, which makes its molecular structure more flexible. Such molecules are arranged more loosely and are usually liquid at room temperature, such as most vegetable oils; on the contrary, molecules without carbon-carbon double bonds are arranged more tightly and are usually solid at room temperature, such as most animal fats. This difference in chemical structure makes the physical properties and effects on health different between the two: saturated fatty acids are solid, and excessive intake may increase the risk of cardiovascular disease; while unsaturated fatty acids are liquid, and moderate intake is beneficial to health and can reduce the risk of cardiovascular disease.