For more than 65 years, doctors have told their patients that they could lower blood cholesterol levels and prevent heart attacks and premature death by substituting polyunsaturated fats in plants for saturated fats, primarily those found in animal products. A recent review of research done more than 40 years ago showed that this lowers blood cholesterol levels, but is associated with increased risk for heart attacks and premature death (BMJ, Feb 14, 2016;352:i919). Since the majority of the scientific literature show that polyunsaturated fats in vegetables are healthful and help to prevent heart attacks, we have to find an explanation of why adding large amounts of polyunsaturated fats extracted from plants could increase the rate of heart attacks.
The old studies that substituted polyunsaturated fats for saturated fats were done by giving vegetable oils primarily in the form of solid margarines that were full of trans fats, toxic aldehydes and other toxic oxidation products. Vegetable oils extracted from their plant sources may also be harmful because they are separated from the protective fiber, protein, and micronutrients that are naturally present in vegetables and seeds.
Problems of Extracted Oils Fats are classified by their chemical structures into saturated, polyunsaturated and monounsaturated types. The chemical stability of a fat is determined by its structure. All fats are made of carbon atoms held together by electrical bonds. These bonds can be single bonds that are stable and double bonds that are far less stable. The stability of a fat or oil depends on the number of double bonds between the carbon atoms. The more double bonds, the less stable the fat.
Saturated fats have only single bonds, so they are very stable. Polyunsaturated fats have two or more double bonds so they are far less stable than saturated fats, particularly when you heat them. Most polyunsaturated oils are heated after they are extracted from vegetables and therefore form all sorts of broken molecules. These molecules can turn on your immunity, causing the inflammation that punches holes in arteries to start forming plaques that can lead to heart attacks. Societies with the highest blood levels of polyunsaturated fats have the highest heart attack death rates (Ann N Y Acad Sci. Dec, 2005;1055:179-92), and heated polyunsaturated fats are associated in humans and animals with increased risk for cancer (BMC Medicine, May 21, 2012;10:50; Free Radic Biol Med, Oct 15, 2007;43(8):1109-20).
Which Vegetable Oils are Most Healthful? The most healthful vegetable oils are those that are still in plants, so they have not had to be heated and processed, and are still paired with fiber, minerals, vitamins and other nutrients. You get these most healthful oils by eating nuts, avocados, beans, sunflower seeds and other plant parts, particularly those that are high in fats.
Processing oils to remove them from their plant sources and to stabilize them can make them less healthful. We have already learned that the process called partial hydrogenation, which forms trans fats, is harmful and these oil products have largely been removed from our food supply. However, many people are unaware that other vegetable oils that they purchase in bottles -- corn oil, safflower oil, peanut oil, olive oil and others -- have been purified and stabilized through processing with heat. Scientists have found that these heated polyunsaturated oils are full of toxic oxidized aldehydes (Foodservice Research International, June 2006;13(1):41 - 55). Processed coconut oil, which is high in saturated fats, produces the lowest levels of aldehydes, while heating corn oil and sunflower oil produced three times more aldehydes than were found in butter.
Low levels of aldehydes were found in olive, coconut, avocado, peanut and rapeseed (canola) oils, butter, lard and goose fat. High levels of aldehydes were found in palm, corn, soy, sunflower, safflower, cottonseed, rice bran and grapeseed oils. Using any of these oils for frying at high temperatures considerably increases their levels of toxic aldehydes.
• Eat lots of nuts, seeds, vegetables and fruits that are good sources of healthful unprocessed fats -- polyunsaturated, monounsaturated and saturated.
• Restrict your consumption of bottled vegetable oils to reasonable amounts and use them uncooked, such as for dressing salads, or for low-temperature cooking such as stir-frying or sauteing. Olive oil should not be used for high temperature cooking.
• Heating polyunsaturated oils extracted from vegetables and seeds to high temperatures can form toxic levels of aldehydes. The higher the heating temperature, the more toxic products are formed.
• Avoid deep-fried foods or limit them to occasional treats. For deep-frying, I recommend using peanut oil (high in monounsaturated fats) rather than any of the polyunsaturated vegetable oils. In addition to the damage to vegetable oils caused by high temperatures, foods that are deep-fried in any type of oil or fat will contain advanced glycation end products (AGEs) that are known carcinogens and can increase risk for diabetes.
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