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:  A family of compounds that includes triglycerides, phospholipids, and sterols.  Glycerol:  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:  A nitrogen-containing compound found in foods as part of lecithin and other phospholipids.  Lecithin:  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 fat- digesting 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):  The type of lipoprotein made primarily by liver cells to transport lipids to various tissues in the body.  Low-Density-Lipoprotein (LDL):  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:  An essential fatty acid with 18 carbons and two double bonds.  Linolenic Acid:  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 triglyceride- storing 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.