Humans are unique amongst the animal species because we eat cooked food. Cooking has been extensively studied in the nutritional sciences because it is known that heat application to meat and plant foods can alter their nutritional properties. Heating certain plant foods can increase the bioavailability of many nutrients because plants possess cell walls that are made of non-digestible carbohydrates. These cellulose walls encase the nutrients, but they are not able to be digested by human enzymes. Cooking is able to breakdown the cellulose structure and release the nutrients for digestion. For example, the bioavailability of β-carotene from cooked carrots is higher than from raw carrots. However, cooking can also be detrimental to nutrient quality of foods if the heat is excessive. This is because cooking with high temperatures can cause chemical reactions which an result in the formation of potentially harmful products.
To investigate the role of heating food in the development of health conditions, researchers1 fed 62 healthy 18 to 24 year old French volunteers diets containing lightly cooked (steamed) or highly cooked (standard) foods. The diets contained similar energy levels, with mixed foods, and were designed to provide all of the basic nutrients necessary for health. The standard diet contained food that had been grilled, fried, roasted and also contained commercial foods that had high heat treatment during manufacture. The steamed diet contained some raw foods as well as foods that were steamed only. After just one month, those subjects ingesting highly cooked standard diet had developed reduced insulin sensitivity compared to those eating lightly steamed foods. In addition, subjects consuming the standard diet had lower plasma levels of n-3 long chain fatty acids, vitamin E and vitamin C.
The two diets in this study were designed specifically to differ in the concentrations of Maillard reaction products (MRP). The Maillard reaction occurs during heating and chemically involves reactions between reducing carbohydrates and an amino acid. The browning seen on grilled and fried foods is the result of these Maillard reactions occurring, and the rate of reactions tend to increase with increasing heat. The chemicals produced during Maillard reactions are numerous and many contribute to the taste and aroma of grilled and fried foods, which is why they appeal to many people. However, some of the MRP from cooking are known to be carcinogens such as acrylamide and heterocyclic amides. Research suggests that chronic intakes of high heat treated foods may result in cardiovascular complications and increase the chance of developing diabetes.
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