May even our cooking habits hurt us?
Members of the raw food community believe that heating and preparation of food is somehow bad for you. The levels of some vitamins, notably vitamin C, are indeed reduced by cooking. On the other hand, cooking destroys many noxious agents, especially microbes, and even many plant toxins. Cooking also makes our food easier to digest. There is even a scientific hypothesis that cooking was necessary for the development of the intelligent Homo sapiens, because it freed some of the energy previously needed for digestion to be used in building a bigger brain. Human brains are voracious; a resting adult’s brain uses 25% of the total energy consumed by the body. Therefore it is rather difficult to tell what the overall result of cooking is or to claim that one or the other way is always best. Probably the advantages should be rather evaluated case by case.
In spring 2002, the Swedish Food Agency detected high acrylamide concentrations in many fried, deep-fried or baked products. The highest concentrations were found in potato chips, french fries and crispbread, all of which are heated to relatively high temperatures. Acrylamide has been used for many technical purposes; so there was a considerable amount of animal and occupational toxicology data available. However, the possible effects of long-term low-level exposure were unknown. The most important concerns were the possibilities of cancer causation, and nerve damage. The latter possibility was not considered as truly relevant on the basis of previous data, but many groups have since tried to assess the possible risk of carcinogenicity.
The formation of acrylamide seems to be based on the so called Maillard reaction which is in fact essential in all baking and frying. The crucial factor is the amino acid, asparagine, which especially in potatoes is present at high concentrations. The Maillard reaction is responsible for giving the nice brown colour to baked and fried products, and also some of the aroma. One of the products is acrylamide, and it is possible that this would cause a slight increase in cancer risk.
One of the problems in the evaluation is that there is no good and accurate information on the consumption of potato chips and French fries in different youth groups. One could assume that also individual differences are huge, and this kind of information is essential if one wishes to perform epidemiological studies to assess the risk. Meanwhile the food-processing manufacturers are now developing production methods decreasing acrylamide formation to as low a level as possible. It is not known very well how much acrylamide is formed in home cooking.
When meat or fish are heated, mutagenic compounds are formed. The best studied of these compounds are the heterocyclic amines. Some of these have been shown to be carcinogenic in rodents. The doses used in animal studies have been so high that it has not been possible to calculate their relevance to human risk. In smoked and charcoal grilled/broiled products there are also many polyaromatic hydrocarbons (PAH), which are also carcinogenic in animals at high doses. Mutagenicity in urine associates better with the levels of heterocyclic amines rather than the polyaromatic hydrocarbons after consumption of grilled or roasted meat. The formation of mutagenic compounds can be reduced by lowering the oven temperature and not letting the surface to char.
Comparison with exposure to the same compounds via lungs (smoking, air pollutants) is difficult. It is true that the amounts of mutagenic compounds in food are relatively high. On the other hand they are attacked by many metabolising enzymes already in the gut and the liver, which may detoxify them. The decrease of stomach cancer clearly visible in the cancer statistics over the last 50 years has been attributed to the decreased consumption of heavily smoked, salty or microbiologically spoiled food, after the introduction of improved hygiene and better refrigeration. This is an example of the importance of benefit – risk analysis, because heat and salt may decrease microbiological risks and outbreaks of food poisonings, but they may increase cancer risk.
Hygienic handling of food materials and cleanliness are very important for food safety. A crucial question with meat products, especially chicken, is salmonella. The situation in Finland in this respect is very good, but unfortunately this does not apply even throughout Europe not to speak of other parts of the world. Therefore hens and chicken have to be prepared well and when handling the raw carcass, one has to take care that bacteria have no possibility of contaminating other foods in the kitchen. This means thorough washing of hands and kitchenware after handling raw chicken. It is recommended that the same knife should not be used at all for slicing bread or other foods not heated before consumption.
In sweet foods such as cakes the most important bacterium is Staphylococcus. Uninterrupted refrigeration is an essential part of food safety even in the domestic kitchen. This is because microbes proliferate in a logarithmic manner, e.g. they double in number every half hour. This means that one microbe has multiplied to 4096 (212) microbes in six hours. Refrigeration might slow down the proliferation so that doubling is only once in two hours, resulting only in 8 (23) microbes in 6 hours. This illustrates the dramatic difference in keeping food in a refrigerator as compared to room temperature.
To improve food hygiene also irradiation has been proposed to kill the microbes. Nowadays irradiation is in general use only to sterilize certain spices. Consumer opposition to irradiation is so strong that it is not plausible that its use would increase. There have also been suspicions that if allowed, it would be misused to “freshen up” old food products. On the other hand, regular cooking at home by heating causes more biochemical changes in the food possibly affecting health more than irradiation. Irradiation is like light; its effects will not remain after the source has been extinguished.
The role for human health of carcinogenic or toxic substances formed during cooking is unclear at the moment. A few of these substances cause cancer in animals. The doses in animal experiments have, however, been very high, and it is a real problem to estimate their relevance to real human exposure levels that are often thousands of times lower. Some of these effects are based on in vitro tests, and then the relevance assessment is even more uncertain.
It is possible and even likely that part of the cancer risk attributable to food is due to these compounds, but the proportion of the risk cannot be estimated at the moment. It seems to be clearly lower than the cancer risk associated with energy consumption (too many calories) or possibly that attributable to animal fat and red meat. On the other hand, it may well be much greater than the risk posed by the food additives and pesticide residues much feared by the general public.
The most important safety factors of food are cleanliness and hygiene, because many microbes compete for food with humans, and they proliferate very quickly. Bacteria may also use chemical weapons to defend their interests. Cooking, especially frying, results in the formation of new substances but their role in health and disease is not clear at present.
Notes and references
- See the chapter "Mutagenesis, carcinogenesis and other scary words – what do they really mean?".
- See the chapter "Is cancer more common today than it was in the past?".
- In vitro means literally ”in glass”, i.e in a test tube. It is used for studies utilizing microbes or animal or human cells kept viable under artificial conditions.
One level up: Is man defiled by what goes to his mouth?
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