by Ben Best
Most scientific papers addressing the subject of the health hazards of cooking focus on carcinogens in cooked meat. One exception [BULLETIN OF ENVIRONMENTAL CONTAMINATION AND TOXICOLOGY 53:259-266 (1994)] describes the leaching of Fe, Cr & Ni into food which has been cooked in stainless steel. In THE 120-YEAR DIET Roy Walford mentions toxic agents created from cooking egg yolks. There are undoubtedly many peroxidized lipids produced from cooking of unsaturated fats which are harmful in ways other than by carcinogenesis, but since I have not located literature on the subject this essay will focus on mutagens.
The most mutagenic known agents in cooking are heterocyclic amines, found in "well-cooked" beef, pork, chicken and fish muscle (typically cooked at temperatures just above 200ºC). Mutagens are also found in "barbacued samples, cooked at higher temperatures over an open flame". The nature of these substances are mostly undetermined, but it is known that they are not heterocyclic amines. [FOOD AND CHEMICAL TOXICOLOGY 32 (7):595-603 (1994)].
Concerning cooking methods, one paper states that "The relative mutagen activity of cooking processes with muscle meats show that frying, broiling and barbecuing produce more mutagenic activity and HAs [Heterocyclic Amines], where analyzed, and that stewing, steaming and poaching show little or no mutagenic activity...Microwave cooked meats are generally low in mutagenic activity, but a study by Barrington et. al. (11) generated appreciable mutagenic activity in sirloin steak." [CARCINOGENS 16(1): 39-52 (1995)].
Gas flames generate NO2, which can result in high levels of carcinogenic nitrosamines & nitropyrenes in foods cooked in gas ovens [CANCER LETTERS 25:239 (1985) and MUTATION RESEARCH 171:105 (1986)]. The inhaling of air in kitchens using gas-cooking may lead to the presence of dimethylnitrosamine in human urine [CANCER RESEARCH 46:5392 (1986)].
Acrylamide is a cancer-causing agent that is formed by heating certain amino acids (methionine, glutamine, cysteine and especially asparagine) with reducing sugars (fructose, glucose, lactose, sucrose, etc.) [NATURE; Stadler,RH; 419:449-450 (2002)].
One thoroughly-referenced review (about 150 references) -- "Cooking Procedures and Food Mutagens: A Literature Review" [FOOD AND CHEMICAL TOXICOLOGY 31(9):655-675 (1993)] -- also focuses on the mutagenicity of cooked meats due to heterocyclic amines. Non-enzymatic glycosylation is a mechanism of aging, but it is evidently also a mechanism for the creation of mutagenic heterocyclic amines in cooked meats.
The necessary mutagenic ingredients are: high temperature of extended duration, free amino acids, creatine (or creatinine) and sugar. The creatinine, glycogen and free amino acids in meat cause the sugar to catalyze the cross-linking of the free amino acids to form the heterocyclic amines (a Maillard reaction). Few mutagens were formed if glucose was not present and "Bovine liver and kidney contained low concentrations of creatine and shared no mutagenic activity after frying". Moreover, in the cooking of "other protein-rich foods such as cheese, tofu, beans, shrimp and tissue from inner organs, the mutagenic activity was negligible." In model systems where there was protein without free amino acids, "no mutagenic activity was detected". ("Mutagenic activity" nowadays is usually detected by bacterial assays -- the Ames test.)
Concerning temperature and duration, "Domestic frying and broiling are usually performed at temperatures between 150ºC and 200ºC for 5-15 minutes, oven roasting often takes place at 125-200ºC for 30-60 minutes, whereas stewing and microwave cooking involve temperatures in the range of 100ºC to 150ºC. However, most frying and broiling experiments cited in the literature have been performed at temperatures of 200ºC or above.
"In model systems also, the mutagenic activity is related to the temperature in the range of 150ºC to 200ºC. When a mixture of glycine, creatine and glucose was heated at 140ºC for 15 minutes, no significant mutagenic activity was produced, but at 160ºC and 180ºC, mutagenic activity was detected."
I conclude from the above that microwave cooking is one of the safest ways to cook. This is in agreement with my intuitive sense that heat that penetrates somewhat uniformly is far less damaging than high temperature applied to the surface of food in order to cook the inside in an acceptable time-frame. Cooking with fats, of course, results in the production of free radicals & other carcinogens.
Aside from mutagens, cooking also results in non-enzymatic browning (also referred to as glycation or the "Maillard" reaction). Oxidized glycation products are known as Advanced Glycation End-products (AGEs). AGEs contribute to diabetic neuropathy, atherosclerosis, diabetic retinopathy, diabetic nephropathy (kidney failure) and aging [CLINICAL DIABETES; Peppa,M; 21(4):186-187 (2003)]. About one-third of absorbed dietary AGEs (from browned foods, particularly fried poultry skin) are excreted, while the rest is presumably incorporated into body tissue [PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (USA); Koschinsky,T; 94(12):6474-6479 (1997)]. As with creatinine, AGE excretion is entirely dependent upon glomerular filtration, and can serve as an index of kidney function. [JOURNAL OF BIOLOGICAL CHEMISTRY; )]. The type of cooking can make a great difference, with significantly more AGE formation from frying, broiling & grilling as compared to steaming, poaching or boiling [JOURNAL OF THE AMERICAN SOCIETY OF NEPHROLOGY; Uribarri,J; 14(3):728-731 (2003)]. (For more on the effect of ingested AGEs, see INGESTION OF ADVANCED GLYCATION END-PRODUCTS (AGES).)
I try to keep my cooking to a minimum. The most practical use of cooking is to kill microorganisms in meat to make it safe to eat. I only eat meat occasionally -- meat contains too many pesticides and too much saturated fat which elevates LDL cholesterol (increasing vulnerability to cardiovascular disease.) Otherwise, cooking simply reduces the nutritional content of foods while making food more palatable. For a person seeking to practice CRAN (Caloric Restriction with Adequate Nutrition) -- or anyone wanting to keep weight down -- food is already too appetizing.
I believe that adequate nutrition can be achieved without cooking or meat. A diet containing lots of fresh fruits & vegetables that includes low-fat dairy products & nutritional supplements (including essential fatty acids and whey protein) will supply ample nutrition without the time-wasting cooking and high level of pesticides & saturated fats.