Summary for Chapter 5 – The Lipids: Triglycerides, …
Summary for Chapter 5 – The Lipids: Triglycerides, Phospholipids, and Sterols
The predominant lipids both in foods and in the body are triglycerides: glycerol backbones with three fatty acids attached. Fatty acids vary in the length of their carbon chains, their degrees of unsaturation, and the location of their double bond(s). Those that are fully loaded with hydrogens are saturated; those that are missing hydrogens and therefore have double bonds are unsaturated (monounsaturated or polyunsaturated). The vast majority of triglycerides contain more than one type of fatty acid. Fatty acid saturation affects fats’ physical characteristics and storage properties. 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.
Phospholipids, including lecithin, have a unique chemical structure that allows them to be soluble in both water and fat. In the body, phospholipids are part of cell membranes; the food industry uses phospholipids as emulsifiers to mix fats with water.
Sterols have a multiple-ring structure that differs from the structure of other lipids. In the body, sterols include cholesterol, bile, vitamin D, and some hormones. Animal-derived foods contain cholesterol. To summarize, the members of the lipid family include:
• Triglycerides (fats and oils), which are made of:
• Glycerol (1 per triglyceride) and
• Fatty acids (3 per triglyceride); depending on the number of double bonds, fatty acids may be:
• Saturated (no double bonds)
• Monounsaturated (one double bond)
• Polyunsaturated (more than one double bond); depending on the location of the double bonds, polyunsaturated fatty acids may be:
• Omega-3 (first double bond 3 carbons away from methyl end)
• Omega-6 (first double bond 6 carbons away from methyl end)
• Phospholipids (such as lecithin)
• Sterols (such as cholesterol)
The body makes special arrangements to digest and absorb lipids. It provides the 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, for transport so that cells all over the body may select needed lipids from them.
The liver assembles lipids and proteins into lipoproteins for transport around the body. All four types of lipoproteins carry all classes of lipids (triglycerides, phospholipids, and cholesterol), but the chylomicrons are the largest and the highest in triglycerides; VLDL are smaller and are about half triglycerides; LDL are smaller still and are high in cholesterol; and HDL are the smallest and are rich in protein.
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 (18 carbons, omega-6) and linolenic acid (18 carbons, omega-3) are essential nutrients. They serve as structural parts of cell membranes and as precursors to the longer fatty acids that can make eicosanoids—powerful compounds that participate in blood pressure regulation, blood clot formation, and the immune response to injury and infection, among other functions. Because essential fatty acids are common in the diet and stored in the body, deficiencies are unlikely.
The body can easily store unlimited amounts of fat if given excesses, and this body fat is used for energy when needed. (Remember that 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.
High blood 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.
In foods, triglycerides:
• Deliver fat-soluble vitamins, energy, and essential fatty acids
• Contribute to the sensory appeal of foods and stimulate appetite
Although some fat in the diet is necessary, health authorities recommend a diet moderate in total fat and low in saturated fat, trans fat, and cholesterol. They also recommend replacing saturated fats with monounsaturated and polyunsaturated fats, particularly omega-3 fatty acids from 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.
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