A study of almost 107,000 women, ages 50-79, followed for an average 18 years, found that one serving or more of fried chicken a week was associated with a 13 percent higher risk of death during the study period, and a serving of fried fish or shellfish per week was associated with a seven percent greater risk of death (BMJ, Jan 23, 2019;364:k5420). In this U.S. study, the Women's Health Initiative, those who ate the most fried foods also ate the least healthful diets: they ate fewer vegetables, fruits and whole grains, and more sugar-sweetened beverages, red meat, processed meat, trans fats and salt. They also tended to be younger, less educated, more likely to smoke, less likely to exercise, and more likely to be overweight and/or diabetic. The authors adjusted for these factors when they computed the death rates of those who ate more fried foods compared to those who ate less.
Another study followed more than 75,000 healthy men and women for 6-13 years and found that those who ate meat, fish or chicken cooked at high temperatures or over a flame (grilling/barbecuing, broiling, or roasting), two or more times a week, suffered a marked increase in becoming diabetic (Diabetes Care, Mar 12, 2018 and Aug 2017;40(8):1041-1049). The association between high temperature cooking and diabetes remained for fish, chicken and red meat individually and preferring well-done meat appeared to increase risk also. Eating French fries has also been associated with increased risk for premature death, heart attacks and some cancers (American J of Clin Nutr, July 2017;106(1):162-167). However, a study from Spain found no association between fried foods and risk of death (BMJ, Jan 24, 2012;344:e363), possibly because their subjects did not eat in restaurants where the oil is reheated and reused many times (typical in the U.S.), which increases the formation of advanced glycation endproducts (AGEs) and trans fats that have been shown to increase risk for heart attacks, diabetes and cancers.
How Foods Cooked at High Temperatures Can Harm You When you cook with water, the sugars in foods combine with the water to form end products that have not been shown to be harmful. On the other hand, when sugars or carbohydrates (chains of sugars) are cooked with proteins or fats at high temperatures and without water, the sugars bind to the proteins and DNA to form chemicals called advanced glycation endproducts (AGEs). High-temperature cooking methods that do not use water include deep-frying, grilling, barbecuing, broiling, roasting, baking and toasting. Browning during cooking is a sign that AGEs are being formed.
AGEs have been shown to turn on your immune system to cause inflammation (Curr Diabetes Rev, May 2008;4(2):92-100; J Am Diet Assoc, Jun 2010;110(6):911–16.e12), that prevents your cells from responding to insulin, which can lead to diabetes or make it harder to control existing diabetes (Diabetes Care, January 2014;37:88-95). Many animal studies have shown that a diet high in AGEs prevents cells from responding to insulin, raises blood sugar levels and raises insulin levels, which can cause or worsen diabetes, while restricting AGEs helps to lower blood sugar levels. AGEs also increase risk for heart attacks and cancers (Cancer Causes & Control, 2012, 23:405-420).
Deep-frying is particularly unhealthful because in addition to the formation of AGEs, hot oil oxidizes and hydrogenates fats to convert healthful unsaturated fatty acids such as linoleic acid to harmful trans fatty acids such as trans linoleic acid (J Food Prot, 2001;64:1062-6).
How To Reduce Your Exposure to AGEs
• Reduce intake of animal-derived foods that are high in fat and protein because they form the most AGEs during cooking.
• Limit foods that have been browned in the cooking process, including grilled, broiled, roasted, toasted and baked foods.
• Use water-based cooking methods whenever possible: steaming, simmering, blanching, boiling and so forth. Water prevents the sugars from attaching to proteins and fats. Cook for shorter durations, at lower temperatures, and where possible, include acidic ingredients such as lemon juice or vinegar (J Am Diet Assoc, Jun 2010;110(6):911-16).
• Eat a wide variety of vegetables, whole grains and beans. These foods are usually cooked with water so they are low in AGEs.
• Include uncooked vegetables, fruits and nuts in your diet. Fresh fruits are associated with reduced susceptibility to diabetes, even though they may have a high sugar content (PLoS One, April 11, 2017) Raw nuts are preferable to roasted nuts, which are a source of AGEs.
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