For more than 65 years, doctors have told their patients that they could lower blood cholesterol levels and prevent heart attacks by substituting polyunsaturated fats from plants for saturated fats from animals.
A 2016 review of research done more than 40 years ago showed that while this lowers blood cholesterol levels, it is associated with increased risk for heart attacks and premature death. Since the majority of the scientific literature has shown that polyunsaturated fats in vegetables are healthful and help to prevent heart attacks, we have to find an explanation for 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 focused on 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 cause the formation of plaques that lead to heart attacks. Societies with the highest blood levels of polyunsaturated fats have the highest heart attack death rates, as found by a 2005 study published in the Annals of the New York Academy of Sciences. Another study, published in BMC Medicine in 2012, found that heated polyunsaturated fats are associated in humans and animals with increased risk for cancer.
The most healthful vegetable oils are those that are still in plants, as they have not been heated and processed and are still paired with fiber, minerals, vitamins, and other nutrients. You can get these most healthful oils by eating nuts, avocados, beans, sunflower seeds, and other plant matter that is high in fats.
Processing oils to remove them from their plant sources and to stabilize them can make them less healthful.
The process called partial hydrogenation, which forms trans fats, is most harmful, and these oil products have been removed from a large portion of 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.
A 2006 study published by Foodservice Research International found that these heated polyunsaturated oils are full of toxic oxidized aldehydes. Processed coconut oil, which is high in saturated fats, produces the lowest levels of aldehydes, while heated 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 oils, as well as 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 plenty 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 sautéing. Olive oil should not be used for high temperature cooking.
- Avoid heating polyunsaturated oils extracted from vegetables and seeds to high temperatures, as this can form toxic levels of aldehydes. The higher the heating temperature, the more toxic products 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, which are known carcinogens and can also increase your risk of developing diabetes.
Gabe Mirkin, M.D., has been a practicing physician for over 50 years. He is board-certified in sports medicine, allergy and immunology, pediatrics, and pediatric immunology. This article was originally published on DrMirkin.com. Subscribe to his free weekly Fitness & Health newsletter.