Lipids Structure, Basic Functions, and Food Forms
Lipids are a macronutrient providing energy and other essential nutrients to the body. Lipids are composed of carbon (C), hydrogen (H), and oxygen (O) similar to carbohydrate. Lipids are the most energy dense of the macronutrients and provide 9 kcal/kg, versus the four calories in 1 g of carbohydrate and protein. Lipids are also anhydrous, meaning they do not store water with them, whereas for every gram of glycogen that is stored 3 g of water are stored with it. We would weigh a lot more if we stored all of our energy as glycogen. These characteristics make lipids a very efficient storage form of energy.
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Triglycerides are the main form of lipid consumed in the diet. Triglycerides are composed of three fatty acids, or chains of carbon and hydrogen atoms with a carboxyl group at one end and a methyl group at the other, and a “backbone” glycerol molecule that is attached via an ester bond (see Figure 4.1).
Dietary triglycerides (or dietary fats) are classified as either saturated or unsaturated (mono or poly). In reality, individual fatty acid chains may be fully saturated with hydrogen atoms (saturated fats), may have one double bond between carbon atoms (monounsaturated fatty acid), or may have two or more carbon-to-carbon double bonds (polyunsaturated fatty acids). As a consequence, the dietary triglyceride (or fat) is categorized based on the chemical structure of the majority of fatty acids. For example, olive oil is considered to be a monounsaturated fat, but it contains 8 percent polyunsaturated fatty acids and 13 percent saturated fatty acids. In general terms, saturated fats are typically solid at room temperature; butter and the white marbling in meats are considered to be primarily made up of saturated fatty acids. Saturated fats are also found in tropical oils such as coconut, palm, and palm kern oil but these tend to be liquid at room temperature. Unsaturated fats are usually liquid at room temperature. Monounsaturated fats are found in foods such as almonds, avocado, and olive oil. Food sources of polyunsaturated fats include vegetable oils such as soybean and corn oil, and fatty fish such as salmon and tuna fish. Trans fats are unsaturated fats that are naturally occurring in very small amounts, but primarily are consumed from products containing artificially derived trans fats. These fats have been chemically modified to be more shelf stable by making the cis configuration of the double bond a trans configuration. Trans fats are found in commercial baked goods, some fast food products, coffee creamers, and snacks foods and should be avoided.
While a primary function of fat is to provide energy at rest and during low- to moderate-intensity exercise, fat has many roles in the body. Body fat acts as storage vessel of energy during times of famine and as an
3 Fatty acid chains insulator to help maintain body temperature, as well as surround and protect vital organs. Dietary fat is also required for absorption of fat-soluble vitamins A, D, E, and K. Fat lines the CNS and plays a role in signal transduction between cells and throughout the body. Fat is also a precursor to other compounds, including cholesterol, which is important for cell membranes, vitamin D synthesis, and the synthesis of important hormones.