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The Lipids: Triglycerides, Phospholipids, and Sterols Unit #5 Learning Targets The Chemist’s View of Fatty Acids Fatty Acid: An organic compound composed of a carbon chain with hydrogens attached and an acid group (COOH) at one end and a methyl group (CH3) on the other end. Fatty acids vary in: The length of their carbon chains Their degrees of unsaturation: Fatty acid saturation affects fats’ physical characteristics and storage properties: Those that are fully loaded with hydrogens are saturated. Those that are missing hydrogens and therefore have double bonds are unsaturated (monounsaturated and polyunsaturated). Hydrogenation, which makes polyunsaturated fats more saturated, gives rise to trans-fatty acids, altered fatty acids that may have health effects similar to those of saturated fatty acids. Hydrogenation: A chemical process by which hydrogens are added to monounsaturated or polyunsaturated fatty acids to reduce the number of double bonds, making the fats more saturated and more resistant to oxidation. Location of their double bonds. A Chemist’s View of Triglycerides Triglycerides: The chief form of fat in the diet and the major storage form of fat in the body; composed of a molecule of glycerol with three fatty acids attached. The predominant lipids both in foods and in the body are triglycerides: glycerol backbones with three fatty acids attached. Lipids: Glycerol: A family of compounds that includes triglycerides, phospholipids, and sterols. An alcohol composed of a three-carbon chain, which can serve as the backbone for a triglyceride. The vast majority of triglycerides contain more than one type of fatty acid. Phospholipids Phospholipids, including lecithin, have a unique chemical structure that allows them to be soluble in both water and fat. Phospholipids: A compound similar to a triglyceride but having a phosphate group and choline in place of one of the fatty acids. Choline: Lecithin: A nitrogen-containing compound found in foods as part of lecithin and other phospholipids. One of the phospholipids. In the body, phospholipids are part of cell membranes; the food industry uses phospholipids as emulsifiers to mix fats with water. Sterols Sterols have a multiple-ring structure that differs from the structure of other lipids. Sterols: Compounds containing a four-carbon ring structure with any variety of side chains attached. In the body, sterols include: Cholesterol: One of the sterols containing a four-carbon ring structure with a carbon side chain. Bile Vitamin D Some Hormones Only animal-derived foods contain cholesterol. Digestion and Absorption of Lipids The body makes special arrangements to digest and absorb lipids. It provides an emulsifier bile to make them accessible to the fatdigesting lipases that dismantle triglycerides, mostly to monoglycerides and fatty acids, for absorption by the intestinal cells. The intestinal cells assemble freshly absorbed lipids into chylomicrons, lipid packages with protein escorts, from transport so that cells all over the body may select needed lipids from them. Chylomicrons: The class of lipoproteins that transport lipids from the intestinal cells to the rest of the body. Lipoproteins: Clusters of lipids associated with proteins that serve as transport vehicles for lipids in the lymph and blood. Lipid Transport The liver packages lipids with proteins into lipoproteins for transport around the body. All four types of lipoproteins carry all classes of lipids (triglycerides, phospholipids, and sterol), but the chylomicrons are the largest and the highest in triglycerides; VLDL are smaller and are about half triglycerides: LDL are smaller still and hare high in cholesterol; and HDL are the smallest and are the smallest and are rich in protein. Very-Low-Density-Lipoprotein (VLDL): Low-Density-Lipoprotein (LDL): The type of lipoprotein made primarily by liver cells to transport lipids to various tissues in the body. The type of lipoprotein derived from very-low-density-lipoproteins (VLDL) as VLDL triglycerides are removes and broken down. High-Density-Lipoprotein (HDL): The type of lipoprotein that transports cholesterol back to the liver from the cells. Lipids in the Body In the body, triglycerides: Provide an energy reserve when stored in the body’s fat tissue. Insulate against temperature extremes. Protect against shock. Help the body use carbohydrate and protein efficiently. Linoleic acid and linolenic acid are essential nutrients. Linoleic Acid: Linolenic Acid: An essential fatty acid with 18 carbons and two double bonds. An essential fatty acid with 18 carbons and three double bonds. They serve as structural parts of cell membranes and as precursors to the longer fatty acids that can make eicosanoids. Eicosanoids: Derivatives of 20-carbon fatty acids; biologically active compounds that help regulate blood pressure, blood clotting, and other functions. Because essential fatty acids are common in the diet and stored in the body, deficiencies are unlikely. A Preview of Lipid Metabolism The body can easily store unlimited amounts of fat if given excesses, and this body fat is stored for energy when needed. Adipose Tissue: The body’s fat tissue; consists of masses of triglyceridestoring cells. The liver can also convert excess carbohydrate and protein into fat. Fat breakdown requires simultaneous carbohydrate breakdown for maximum efficiency; without carbohydrate, fats break down to ketone bodies. Health Effects and Recommended Intakes of Lipids High LDL cholesterol poses a risk of heart disease, and high intakes of saturated and trans fats, specifically, contribute most to high LDL. Cholesterol in foods presents less of a risk. Omega-3 fatty acids appear to be protective. Omega-3 Fatty Acids: A polyunsaturated fatty acid in which the first double bond is three carbons away from the methyl (CH3) end of the carbon chain. Recommended Intakes of Fat In food, triglycerides: Deliver fat-soluble vitamins, energy, and essential fatty acids. Contribute to the sensory appeal of foods and stimulate appetite. While some fat in the diet is necessary, health authorities recommend a diet moderate in total fat and how saturated fat, trans fat, and cholesterol. They also recommend replacing saturated fats with monounsaturated and polyunsaturated fats, particularly omega-3 fatty acids and foods such as fish, not from supplements. Many selection and preparation strategies can help bring these goals within reach, and food labels help to identify foods consistent with these guidelines. Bibliography Rolfes, S.R. & Whitney, E. (2005). “Understanding nutrition.” Thomson Wadsworth; Belmont, California.
