Seventy percent of North American adults will become diabetic or pre-diabetic because they eat too much fat, carbohydrates and protein. Of course, eating too much sugar and other refined carbohydrates increases diabetes risk, but so does eating too much meat (Am J of Epidem, Oct, 2017;186(7):824-833) or too much fat from any source (Diabetes, Nov 18, 2019). North Americans eat far more fat than they need, primarily from the vegetable oils that are added to just about every packaged food or fast food meal you can buy, and from our tendency to cook most of our foods in fat. It is harmful to take in a lot of fats from vegetable oils because excessive fat intake can cause high insulin levels and insulin resistance, which can cause diabetes.
Vegetable oils are largely healthful in plants such as avocados, corn, coconut, and various nuts and seeds, but the process of extracting oils from plants usually uses heat that converts the healthful oils to harmful oxidized fats that can cause inflammation. Inflammation increases risk for heart attacks, certain cancers, and other diseases. The processed oils are further oxidized when they are exposed to air, light or heat. Oxidized fats have free radicals that damage DNA to increase risk for cancers and other diseases. Also, when foods are cooked in vegetable oils at high temperatures, they form advanced glycation endproducts (AGEs) that increase risk for heart attacks and certain cancers. Vegetable oils that are high in polyunsaturated omega-6s can form harmful trans fats when you cook them (Circulation, 2017;136:e1-e23).
Classification of Fats
Today there is controversy among scientists about health benefits or harms from fats. Fats are usually classified as:
• monounsaturated fats – healthful
• polyunsaturated omega-3s – healthful
• polyunsaturated omega-6s – possibly healthful but more likely neutral
• saturated fats – probably harmful
• trans fats – harmful
Based on Ancel Keys‘s original studies in the early 1950s, many scientists thought all polyunsaturated fats were healthful and would prevent heart attacks if they were substituted for saturated fats in the diet. However, scientists have now gone over Dr. Keys’s original data and have shown that the supposed prevention of heart attacks offered by polyunsaturated fats was really only offered by omega-3 polyunsaturated fats and not by the omega-6 polyunsaturated fats (BMJ, Feb 4, 2013;346:e8707 and BMJ, 2016;352:i1246). They also found that if the saturated fats were replaced with refined carbohydrates, there was no heart-attack-reducing benefit whatever. People tend to replace saturated fats with sugars and other refined carbohydrates that may be more harmful than the saturated fats. Fatty triglycerides that you make in your own body from excess sugar and other refined carbohydrates are more dangerous than the fats you eat in your food (Lancet Diabetes & Endocrinology, August 6, 2014).
Cooking with Vegetable Oils
I believe that everyone should limit deep-fried foods (foods cooked by submerging in vegetable oil at high temperatures). For deep-frying, choose an oil with the highest smoke point, such as avocado, sunflower seed or peanut oil. The smoke point of an oil is the temperature at which it stops shimmering and starts smoking. The smoke point of oils runs from a low 325F to very high 520F. This is the temperature at which the fat breaks down to form harmful oxidized oils and harmful sugar-protein complexes (AGEs), indicated by browning of the food. Low-smoke-point oils such as olive oil can be used for brief sauteing of moist foods such as onions, green peppers, garlic and celery, or stir-frying combinations of food that include plenty of vegetables. The low-smoke-point oils can also be used uncooked for salad dressings, dips and sauces, or in recipes with a high water content such as soups.
Olive oil is high in monounsaturated fat, and several studies have shown that people who replace saturated fats with monounsaturated fats have lower rates of heart attacks and deaths from heart attacks (N Engl J Med, April 4, 2013;368:1279-1290). Choose an olive oil in a dark bottle to block light, and store it sealed in a cool, dark place to delay oxidation. It should have a fresh taste; a rancid or stale taste means that it is already oxidized and harmful. The smoke points for olive oils are low, ranging from 325F (extra-virgin olive oil) to 465F (refined olive oil), so it should not be used for deep frying.
Canola oil contains more than twice as much healthful monounsaturated fat than polyunsaturated fat, and is very low in saturated fat. It has a low smoke point of 428-446°F, similar to olive oil, so it should not be used for high-temperature frying. In one study, people who followed diets using canola oil had lower total and bad LDL cholesterol levels than those who ate a typical Western diet high in saturated fat (Nutr Rev, Jun 2013;71(6):370-385). Canola oil has received some bad press because of its plant source (GMO rapeseed) and use of solvents in processing, but nutritionists usually rank it as one of the most healthful vegetable oils.
Reasonable amounts of coconut oils appear to be neither harmful nor healthful. Coconut, palm and palm kernel oils contain mostly saturated fats that raise LDL cholesterol, which is associated with increased risk for heart attacks, but nobody has shown that people who eat coconut oil are at increased risk for heart attacks. Some studies show that people who eat large amounts of coconut oils are at reduced risk for heart attacks, but these studies were done in populations that report low rates of heart attacks overall, such as India, the Philippines, and Polynesia. As of today, studies show that coconut oils raise the harmful LDL cholesterol (Nutr Rev, Apr 2016;74(4):267-280), but also raise blood levels of the healthful HDL cholesterol (BMJ Open, 2018;8(3):e020167). Coconut oil has a low smoke point of 350°F, so it should not be used for deep frying.
Other Vegetable Oils
Avocado oil is primarily a monounsaturated fat with a very high smoke point (520F). Sunflower oil is high in monounsaturated fat and has a smoke point of 450F. Grapeseed oil is high in omega-6 polyunsaturated fats and has a smoke point of 420F. Peanut oil, with a smoke point of 450F, contains significant amounts of monounsaturated fat but also has polyunsaturated omega-6s. Wikipedia has a chart of smoke points of various cooking oils and fats
Vegetable oils are safe when eaten in plants. When oils are removed from plants, they can form harmful oxidized fats that increase risks for cancers and heart attacks when they are stored for a long time, exposed to air or light, or heated during processing or cooking. Oxidized fats and trans fats are formed in increasing amounts when the oil is cooked at any temperature higher than 212F. Oils that have become rancid (oxidized) in storage or from cooking will usually have a bad taste or smell and should be discarded.
Most scientists feel that vegetable oils with omega-3 polyunsaturated fats and monounsaturated fats are healthful, that saturated fats are probably harmful, and that omega-6 polyunsaturated fats are neutral. Quantities of all types of fats should be limited. The small amounts of oil used in most home recipes are likely to be perfectly healthful, but do not believe promoters who suggest that you should add oils such as coconut oil to your food or coffee for supposed health benefits. Excess fat from any source is unhealthful.