Vegetable oils are purified fats of plant origin that are either liquid or solid at room temperature. Vegetable oils are used for cooking, frying, and baking; for salad dressings; and as dipping oils. Cooking oils typically are liquid at room temperature, including olive oil, canola oil, soybean oil, sunflower oil, peanut oil, walnut oil, and other nut oils. Solid vegetable oils, such as palm oil, palm kernel oil, and cocoa butter, are used in food preparation to provide desired textural qualities and flavor. Often, solid vegetable oils are used for manufacturing nonfood products such as cosmetics, soaps, and candles. They also can be hydrogenated or partially hydrogenated to increase resistance to oxidation or to raise the viscosity and melting point of the fat or oil. Vegetable oils are a dense source of energy, providing 9 cal/g; higher than that provided by carbohydrates (4 cal/g), protein (4 cal/g), and alcohol (7 cal/g). This entry summarizes various vegetable oils and describes why they are important for health.
Vegetable oils are composed of 1 glycerol molecule and 3 fatty acids (making a triglyceride [TG] molecule). These fatty acids are either saturated or unsaturated. The structures of saturated and unsaturated fats are shown in Figure 1. Saturated fatty acids (SFA) contain only single carbon-carbon bonds and therefore form straight chains. As a result, they pack tightly together. When a vegetable oil is proportionally high in SFA, the fat generally is solid at room temperature. Common SFA include palmitic and stearic acid, found in most fats and oils; lauric acid, found in coconut oil and palm kernel oil; and myristic acid, found in palm oil. All vegetable fats and oils contain saturated fat; however, those that are solid at room temperature have the highest concentrations of saturated fat.
Unsaturated fatty acids contain 1 (monosaturated) or 2 or more (polysaturated) double bonds and are named according to the position of the double bond closest to the methyl end of the molecule. The double bond produces a “kink” in the molecule, rendering these acids liquid at room temperature. Vegetable oils that are liquid at room temperature are a good source of monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs). The most common MUFA is oleic acid, an omega-9 fatty acid (i.e., its double bond is 9 carbons from the methyl end), found in olive oil. Common PUFAs include linoleic acid (an omega-6 fatty acid), which is found in sunflower oil, safflower oil, soybean oil, corn oil, cottonseed oil, and canola oil. Many oils also contain a small amount of α-linolenic acid (ALA), an omega-3 fatty acid. Vegetable oil sources of ALA are canola oil, flaxseed oil, soybean oil, and walnut oil. Other omega-3 fatty acids are eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are found in fish, and in high amounts in fatty fish. DHA is also found in algae. New “designer” oils that are high in EPA are being introduced into the market. Omega-3 and omega-6 fatty acids are considered “essential fatty acids” because they cannot be made in the body and therefore must be consumed in the diet.
Other components of vegetable oils include the antioxidant vitamin E oil (tocopherol), which helps protect red blood cells; sterols (sitosterol, stigmasterol, campesterol, and squalene), which help lower blood cholesterol; resveratrol, found in olive oil, which acts as an antioxidant; and the fat-soluble vitamin A, found in canola oil, which assists in growth, immune function, and vision.
Vegetable oils containing high amounts of SFA, such as coconut oil and palm oil, raise low-density lipoprotein cholesterol (LDL-C) more than vegetable oils containing lower amounts, such as canola oil, soybean oil, and sunflower oil. The cholesterolemic effects of individual SFAs also vary. Lauric acid has the highest potential to raise LDL-C and total cholesterol (TC) levels. However, because it increases high-density lipoprotein cholesterol (HDL-C) proportionally more than it does LDL-C, it lowers the TC:HDL-C ratio, an important risk factor for heart disease. Stearic acid has a neutral effect on LDL-C. Replacing coconut fat (18% of energy) with unsaturated fats (soybean oil and sesame oil, 24% of energy) decreases LDL-C by 14%, 16%, and 16% after 3, 6, and 12 months, respectively, with no significant change in HDL-C. Other studies have reported similar blood cholesterol-lowering effects when a high-lauric oil or palm oil is replaced with an oil high in oleic acid and when coconut oil is replaced with sunflower oil or rapeseed (canola) oil. Thus, replacing fats and oils high in SFAs with vegetable oils high in unsaturated fatty acids will decrease the risk of cardiovascular disease (CVD).
The concept of the Mediterranean diet originated from several observational studies in the 1950s, most notably the Seven Countries Study, led by Dr. Ancel Keys. Olive oil is the predominant fat consumed by these populations, accounting for as much as 85% of the total fat intake. The Mediterranean diet also typically includes fruits, vegetables, fish, whole grains, and nuts. In the Seven Countries Study, consumption of olive oil (high in oleic acid) was associated with lower all-cause and coronary heart disease (CHD) death rates. Subsequent epidemiologic studies also have reported that consumption of olive oil more than once daily was associated with a 25% lower breast cancer risk than consumption of olive oil only once daily. An olive oil intake of 77 to 84 g/d was associated with lower blood pressure, and including olive oil in a healthy dietary pattern was associated with a 38% reduced risk for CVD at 5 years.
In the ongoing PREDIMED (Prevención con Dieta Mediterránea) Study, a large, multicenter, randomized controlled, 5-year clinical trial of participants at high risk for CVD, consumption of a Mediterranean diet with olive oil (equivalent to 143 mL/d) for 3 months reduced important CVD risk factors, including blood pressure, plasma glucose concentrations, and C-reactive protein, and also decreased LDL oxidation. Other trials that included 25 mL/d of olive oil in place of other fats in the diet decreased TC and TG levels over a 3-week period. In addition, consumption of a diet rich in olive oil (22%-23% MUFAs) resulted in a significant decrease in the blood-clotting factor, plasminogen activator inhibitor type 1 activity, compared with a high-carbohydrate diet. Four weeks’ consumption of an olive oil-rich diet reduced coagulation factors VIIc, XIIc, XIIa, and Xc, compared with sunflower oil (9% MUFAs). These responses would be expected to decrease risk of thrombosis.
The Lyon Diet Heart Study was a large secondary prevention study using canola oil as the main source of ALA in the diet. Patients who had survived a myocardial infarction were randomized to either a standard blood cholesterol-lowering diet or a Mediterranean-type diet rich in ALA, derived mainly from a canola oil-based margarine. After a follow-up period of 27 months, there was a 73% reduction in the risk of new major cardiac events and a 70% reduced risk of overall mortality. In the Multiple Risk Factor Intervention Trial, a primary prevention study in men at high risk for CHD, death from CHD and all causes was lowest in subjects with the highest intakes of ALA. In the prospective Nurses’ Health Study, at 10-year follow-up, higher ALA intakes were associated with a 45% reduced risk of fatal ischemic heart disease; and higher intakes of oil and salad dressing (5 or more times per week), which provided high amounts of ALA, also were associated with reduced risk of fatal ischemic heart disease compared with low consumption of these high-ALA foods (relative risk, 0.46; 95% CI, 0.27-0.76; P for trend = .001).
Some, but not all, controlled intervention studies have shown that canola oil (15-22 g/d) reduces serum TC and/or LDL-C when it replaces foods higher in saturated fat in the diet.
In healthy individuals, consumption of unhydrogenated soy oil, partially hydrogenated soy oil, or hydrogenated soy oil for 19 to 26 days was found to lower TC and LDL-C and to decrease TG and apolipoprotein B (apoB), thus reducing CVD risk.
In a meta-analysis of 60 controlled trials, rapeseed (i.e., canola), soybean, sunflower, and olive oils decreased the TC:HDL-C ratio when they replaced saturated fat or carbohydrate (10% of energy). These data indicate that replacing saturated fat or carbohydrate with unsaturated fatty acids, such as unhydrogenated vegetable oils, is most effective for improving blood lipid profiles.
Olive oil has an antithrombotic effect by virtue of its promoting antioxidant and anti-inflammatory effects and enhancing endothelial function. Oleic acid, the main unsaturated fat in olive oil, reduces platelet sensitivity to collagen and other coagulatory factors and inhibits thromboxane and prostaglandin synthesis, which promotes the formation of blood clots. The polyphenols (tocopherol, hydroxytyrosol, and oleuropein) in olive oil also may exert antioxidant effects. Compared with regular virgin olive oil, extra-virgin olive oil has the highest antioxidant phenolic content (approximately 89% higher). Olive oil also contains olecanthal, which is a nonselective inhibitor of cyclooxygenase similar to classical NSAID (nonsteroidal anti-inflammatory) drugs such as ibuprofen, and thus acts as an anti-inflammatory compound.
Canola oil contains approximately 11% ALA. α-Linolenic acid is the precursor for the formation of the marine long-chain omega-3 fatty acids EPA and DHA, which can displace arachidonic acid and reduce the production of pro-inflammatory eicosanoids, prostaglandins, and leukotrienes. The beneficial effects of ALA include cardioprotective effects, anti-inflammatory effects, antithrombotic effects, and improved endothelial function.
The 2010 U.S. Dietary Guidelines recommend inclusion of 27 g vegetable oils based on a 2,000-cal diet, an amount that has been supported by the American Dietetic Association and Dietitians of Canada. Table 1 represents the U.S. Department of Agriculture Food Guide recommendations for inclusion of oils at 12 different calorie levels.
The inclusion of vegetable oils in a healthy diet that meets nutrient- and food-based dietary recommendations is advocated by the following:
The National Cholesterol Education Program
• Most PUFAs should be derived from liquid vegetable oils, semiliquid margarines, and other margarines low in trans fatty acids.
• Higher dietary intakes of n − 3 fatty acids in the form of fatty fish or vegetable oils are an option for reducing risk of CHD.
The American Heart Association
• Use liquid vegetable oils in place of solid fats.
Food and Drug Administration—Qualified Health Claims
• Limited and not conclusive scientific evidence suggests that intake of about 2 tablespoons (23 g) of olive oil daily may reduce the risk of CHD due to the monounsaturated fat in olive oil. To achieve this possible benefit, olive oil is to replace a similar amount of saturated fat and not increase the total number of calories one consumes in a day.
• Canola oil (19 g—about 1½ tablespoons per day) may reduce the risk of CHD due to its unsaturated fat content, according to supportive but not conclusive research. Canola oil should replace a similar amount of saturated fat in the diet, without increasing the calories consumed.
Vegetable oils are prevalent in the diet because they are present in a wide variety of foods. Liquid vegetable oils are a good source of unsaturated fats and confer numerous health benefits. Including vegetable oils in the diet reduces TC and LDL-C levels. Olive oil decreases LDL oxidation and blood pressure and improves hemostatic markers, thereby reducing the risk of CHD. Canola oil also reduces blood cholesterol levels, thereby reducing the risk of CHD. Furthermore, replacing saturated fat or carbohydrates with vegetable oils containing unsaturated fats has the most marked effect in improving the lipid profile. Thus, including liquid vegetable oils in a healthy diet, consistent with current, food-based guidance, is recommended to promote health and well-being.
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