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Does burnt food cause cancer?

Does burnt food cause cancer? Someone said to me that burnt food was "as dangerous as a cigarette", which is a pretty big claim, so I've been searching the web and some research databases, looking for evidence.

There's very little on the web about it, besides a lot of idle speculation on messageboards. This ScienceNews article from 2005 says that the US government now lists certain chemicals found in "meats when they're cooked too long at high temperature" as carcinogenic. It also says:

Finally, the report notes that while inconclusive, published studies in people "provide some indication" of human risks from eating broiled [grilled] or fried foods "that may contain IQ and/or other heterocyclic amines." The National Cancer Institute conducted one of those suggestive studies. It compared the diets of 176 stomach cancer patients and another 503 cancerfree individuals. Overall, people who regularly ate their beef medium-well or well-done faced more than three times the stomach cancer risk of those who ate their meat rare or medium-rare, according to a 1997 report of the research.

More information about this is in a very helpful summary by the USA National Cancer Institute. Note that one of the studies quoted looked at cooking at 200ºC or 250ºC, which is much hotter than ordinary baking/roasting. However, that is the kind of temperature you use to cook a pizza...

Statistics like "three times the cancer risk" always sound scary, but you need to ask, three times what? We need to know how the risk compares against other things. More on that later.

I found a messageboard thread on which someone said "You can put tomato sauce on it. I heard it helps lessen the production of carcinogen which causes the cancer." This is a big mistake. Fruits like tomatoes or cherries do contain antioxidants which counteract the formation of the carcinogens, but only during the cooking process, mixed in with the meat (e.g. in a burger mixture). Putting ketchup on afterwards will make zero difference.

I also found a journal article discussing the increased cancer risk from barbecued food especially (Lijinsky W, (1991), Mutation Research 259 (3-4): 251-261). It suggested that the reason for the risk was that fat will drip off the meat, then burn at high temperatures when it hits the coals, forming the cancer-causing substances that then mix in with the barbecue smoke and may then coat the outside of the meat being cooked. This explanation was proposed to explain their finding that the chemicals were mainly found in fattier foods cooked over burning logs.

Other relevant journal articles:

  1. One found a similar connection: the highest concentrations found in the Italian diet were in pizzas cooked in wood-burning ovens, and in barbecued beef and pork. Ludovici M et al (1995), Food Additives and Contaminants 12 (5): 703-713)
  2. One found that the Indian tradition of cooking with homemade clay-stoves, called "Chulha", created a lot of smoke containing the problematic chemicals.(Bhargava A et al (2004), Atmospheric Environment 38 (28): 4761-4767) This was said to increase the risk for people who cook with them - remember that inhaling carcinogens is typically much more dodgy than swallowing them, because the route into the body is more direct.

The relative risk? Is a barbecued steak as dangerous as a cigarette, as certain internet message boards might lead you to believe? Clearly not: many people eat well-cooked meat, yet nine-out-of-ten cancer deaths can be attributed to smoking. (The nine-out-of-ten figure comes from a study of USA deaths in 1995: source.) Scientists can calculate guideline statistics such as the "incremental cancer risk", an averaged-out measure of the risk from something. For cigarettes it's 0.079 (source); for burnt meat it's somewhere between 0.00001 and 0.00038 (source). So the risk is somewhere between 200 and 8000 times lower - there's no comparison between one cigarette and one burnt steak.

My conclusions:

  1. Regularly eating burnt or barbecued meat, especially meat that's been cooked at high temperatures for a long time, is relatively risky behaviour. But don't panic: it's not comparable to smoking.
  2. For non-meat food the research is less clear-cut: it's not obvious whether all smoke-cooked or overcooked food carries risks. Certainly if you don't eat it regularly there's nothing to worry about.
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