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safflower oil—illustrates these differences: lard is the most saturated and the hardest; chicken fat is less saturated and somewhat soft; and safflower oil, which is the most unsaturated, is a liquid at room temperature. Stability Saturation also influences stability. Fats can become rancid when exposed to oxygen. Polyunsaturated fatty acids spoil most readily because their double bonds are unstable. The oxidation of unsaturated fats produces a variety of compounds that smell and taste rancid; saturated fats are more resistant to oxidation and thus less likely to become rancid. Other types of spoilage can occur due to microbial growth, however. Manufacturers can protect fat-containing products against rancidity in three ways—none of them perfect. First, products may be sealed airtight and refrigerated— an expensive and inconvenient storage system. Second, manufacturers may add antioxidants to compete for the oxygen and thus protect the oil (examples are the additives BHA and BHT and vitamins C and E).* Third, manufacturers may saturate some or all of the points of unsaturation by adding hydrogen atoms—a process known as hydrogenation. Hydrogenation Hydrogenation offers two advantages: it protects against oxidation (thereby prolonging shelf life) and also alters the texture of foods by increasing the solidity of fats. When partially hydrogenated, vegetable oils become spreadable margarine. Hydrogenated fats make piecrusts flaky and puddings creamy. A disadvantage is that hydrogenation makes polyunsaturated fats more saturated. Consequently, any health advantages of using polyunsaturated fats instead of saturated fats are lost with hydrogenation. Trans-Fatty Acids Another disadvantage of hydrogenation is that some of the molecules that remain unsaturated after processing change shape from cis to trans. In nature, most unsaturated fatty acids are cis-fatty acids—meaning that the hydrogen atoms next to the double bonds are on the same side of the carbon chain. Only a few fatty acids in nature (notably a small percentage of those found in milk and meat products) are trans-fatty acids—meaning that the hydrogen atoms next to the double bonds are on opposite sides of the carbon chain (see Figure 3-4). These arrangements result in different configurations for the fatty acids, and this difference affects function: in the body, trans-fatty acids behave more like saturated fats, increasing blood cholesterol and the risk of heart disease (as a later section describes).2 Researchers are trying to determine whether the health effects of naturally occurring trans fats differ from those of commercially created trans fats.3 In any case, the important distinction is that intake of naturally occurring trans-fatty acids is typically low. At current levels of consumption, naturally occurring trans fats are unlikely to have adverse effects on blood lipids. The naturally occurring trans-fatty acid conjugated linoleic acid may even have health benefits. Saturated Fats and Blood Cholesterol The main dietary factors associated with elevated blood LDL cholesterol are high saturated fat and high trans fat intakes.*14 High LDL cholesterol levels increase the risk of heart disease because high LDL concentrations promote the uptake of cholesterol in the blood vessel walls. Nutrition in Practice 3 examines various types of fats and their roles in supporting or harming heart health. Solid fats, introduced in Chapter 1, are foods or ingredients in foods (such as shortening in cakes or pies) that provide abundant saturated fat, trans fat, and/or cholesterol and many kcalories. The current American diet delivers excessive amounts of solid fats—representing an average of almost one-fifth of the day’s total kcalories.15 The easiest way to lower saturated fat, then, is to limit solid fats in the diet. Grain-based desserts, pizza, cheese, and processed and fatty meats are major providers of solid fats. Solid fats from animal sources contribute a great deal of the saturated fat in most people’s diets. Some vegetable fats (coconut oil, palm kernel oil, and palm oil) and hydrogenated fats such as shortening or stick margarine provide smaller amounts of saturated fats. It is important to note that replacing dietary saturated fats with added sugars and refined starches is often counterproductive.16 The best diet for health not only replaces saturated fats with polyunsaturated and monounsaturated oils (as discussed in a later section), but also is adequate, balanced, kcalorie controlled, and based on mostly nutrient-dense whole foods. Trans-Fatty Acids and Blood Cholesterol Consuming commercially derived trans fat poses a risk to the health of the heart and arteries by raising LDL and lowering HDL cholesterol, and by producing inflammation.17 Commercially derived trans fats are found in the partially hydrogenated oils used in some margarines, snack foods, and prepared desserts. The risk to heart health from trans fats is similar to or slightly greater than that from saturated fat, so the Dietary Guidelines for Americans suggest that people keep trans fat intake as low as possible.18 Limiting the intake of trans fats can improve blood cholesterol and lower the risk of heart disease. To that end, many restaurants and food manufacturers have taken steps to eliminate or greatly reduce trans fats in foods.19 For example, margarine makers have reformulated their products to contain much less trans fat. Soft or liquid varieties are made from unhydrogenated oils, which are mostly unsaturated and so are less likely to elevate blood cholesterol than the saturated fats of butter. Some margarines contain olive oil, omega-3 fatty acids, or plant sterols (mentioned earlier), making these products preferable to butter and other margarines for the heart.† *** The words hydrogenated vegetable oil or shortening in an ingredients list indicate trans-fatty acids in the product. In the past, most commercially fried foods, from doughnuts to chicken, delivered a sizeable amount of trans fats to consumers. Today, newly formulated commercial oils and fats perform the same tasks as the previously used hydrogenated fats but with fewer trans-fatty acids. Some manufacturers, however, merely substitute saturated fats—which pose well-established risks to heart health—for trans fats. When reformulating their products, food companies must consider not only the fat composition, but also the taste, texture, cost, and availability of materials. No health benefits can be expected when saturated fats replace trans fats in the diet. Dietary Cholesterol and Blood Cholesterol Although its effect is not as strong as that of saturated fat or trans fat, dietary cholesterol may contribute to elevated blood cholesterol in some people. Less clear is its role in heart disease.20 The Dietary Guidelines recommend limiting dietary cholesterol to less than 300 milligrams per day for healthy people (less than 200 milligrams for some people with or at high risk of heart disease). On average, women take in about 240 milligrams a day and men take in Are you pleased with your body weight? If you answered yes, you are a rare individual. Nearly all people in our society think they should weigh more or less (mostly less) than they do. Usually, their primary reason is appearance, but they often perceive, correctly, that their weight is also related to physical health. Chapter 6 addressed the health risks of being overweight or underweight. Overweight and underweight both result from energy imbalance. The simple picture is as follows. Overweight people have consumed more food energy than they have expended and have banked the surplus in their body fat. To reduce body fat, overweight people need to expend more energy than they take in from food. In contrast, underweight people have consumed too little food energy to support their activities and so have depleted their bodies’ fat stores and possibly some of their lean tissues as well. To gain weight, they need to take in more food energy than they expend. This chapter’s missions are to present strategies for solving the problems of excessive and deficient body fatness and to point out how appropriate body composition, once achieved, can be maintained. The chapter emphasizes overweight and obesity, partly because they have been more intensively studied and partly because they represent a major health problem in the United States and a growing concern worldwide. Henceforth, this chapter will use the term obesity to refer to excess body fat. Excess body fat accumulates when people take in more food energy than they expend. Why do they do this? Is it genetic? Metabolic? Psychological? Behavioral? All of these? Most likely, obesity has many interrelated causes. This section reviews the two major contributing and interacting factors—genetics and the environment.1