“Burnt toast (and, for that matter, barbecued or smoked meats) contains increased levels of a known carcinogen (cancer causing chemical), benzopyrene… Benzopyrene [has] been shown to cause cancer in animals. Indeed, benzopyrene is sometimes used in laboratory experiments to induce a range of cancers in mice. The chemical is thought to damage DNA by attaching itself to particular sites on the molecule. The DNA does repair itself, but just one damaged area could lead to mutations when body cells subsequently divide and, ultimately, to cancer.”
There was a theory that chemicals called heterocyclic amines that are in burnt food could cause cancer. I just checked with one of my colleagues who studies nutrition and cancer and she told me that there have been lots of studies to check whether this is true and they have all shown that eating burnt food does not give you cancer.
Yes, eating too much burnt, barbequed or smoked food can increase the risk of developing bowel cancer. I wouldn’t worry about it though, just try not to burn it too much and you can always scrape off the black bits.