Foods Cooked at High Temperatures Can Be Unhealthy
Several studies have found that advanced glycation end products (AGEs) may have a powerful negative effect on your metabolic health. AGEs are harmful compounds that accumulate naturally as you age and are created when certain foods are cooked at high temperatures.
Typically, AGEs form when sugars combine with certain amino acids (the building blocks of proteins) or fats - a process called glycation.
AGEs are found naturally in many foods, especially animal-derived products that are rich in protein and fat. Various chemical reactions increase AGE formation, as when certain foods are cooked at high temperatures. Certain processes used by food companies to add flavor or color to their products also increase AGEs.
More AGEs form when meats are grilled, roasted, seared, fried, or baked (all dry-heat methods) at relatively high temperatures and for longer times compared to when they are steamed, poached, stewed, or boiled (methods that retain food moisture). Microwaving produces few AGEs because of its relatively short cooking times. The source of animal protein also affects the generation of AGEs, with beef generally having the most, and fish (unless broiled or fried) the least. The fat in meat tends to contain the most AGEs. Carbohydrate-rich plant foods such as fruit, vegetables, and many grain products have a low AGE content, even when grilled or roasted. However, grain products such as crackers, chips, and cookies are higher in AGEs because of their dry heat processing and added fats and sugars, but still have much less than meats. Frying potatoes also boosts AGEs.
How AGEs Affect Health
AGEs have been shown to activate the immune system to cause inflammation which prevents cells in your body from responding to insulin. This can lead to diabetes or make it harder to control existing diabetes. Several 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. AGEs also appear to increase the risks of cancer and heart disease. On the other hand, studies show that restricting dietary AGEs may reduce diabetes and kidney disease.
Deep-frying seems to be particularly unhealthy. 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.
A recent study emanating from the Women’s Health Initiative published in the British Medical Journal demonstrated an association of fried foods with higher risks of death. The study enrolled almost 107,000 women, ages 50-79, and followed them for an average of 18 years. They 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 7 percent greater risk of death.
In this study, 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 researchers adjusted for those factors when they computed the death rates of those who ate more fried foods compared to those who ate less.
Another recently published 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, had a much higher rate of developing diabetes. The association between high temperature cooking and diabetes was found for fish, chicken and red meat individually and a preference for “well-done” meat also appeared to increase risk. A study published in the American Journal of Clinical Nutrition found fried potatoes (i.e. french fries) to be associated with increased risk for premature death, heart attacks and some cancers. A study from Spain on the contrary found no association between fried foods and risk of death. But those researchers suggest that this was likely due to the use of healthier and cardiovascular protective oils such as olive and sunflower oil which are typical in the Mediterranean diet of that region.
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.
Avoid processed meats and select your meat carefully - see our previous article Red Meat - An Objective Review.
Limit meats that have been browned in the cooking process, including grilled, broiled, roasted.
When grilling or roasting, marinate your meat in garlic, red wine, lemon juice or olive oil. Use herbs such as rosemary, thyme, oregano and other spices which have high antioxidant capacity. These can reduce AGE formation by up to 50%.
Use water-based cooking methods when possible: steaming, simmering, blanching, boiling and so forth. Water prevents the sugars from attaching to proteins and fats. Cook for shorter duration, at lower temperatures, and where possible, include acidic ingredients such as lemon juice or vinegar.
Eat a wide variety of vegetables, whole grains and beans. These foods are low in AGEs, even when cooked at high heat.
Stay Active - regular exercise and an active lifestyle have been shown to reduce the amount of AGEs in the body