These compounds have been linked to human cancer of the oesophagus, stomach, bladder and possibly lung.16 Beer as well as nitrite-cured meat products, especially bacon after frying, and salt-dried or smoked fish, are major sources of these compounds. Incidentally, the mainstream smoke from one cigarette contains up to 65µg volatile nitroso amines and the side stream contains up to 1000µg,13 a healthy lifestyle thus constitutes more than just eating correctly. It is difficult to estimate what the level of exposure to these compounds is, particularly since it is known that nitroso amines are efficiently metabolized in the liver. Nevertheless, these compounds have induced liver and oesophagus cancer in experimental animals.12,13 Intestinal bacteria can also catalyse the formation of nitroso amines and this has been linked to gastric cancer. Nitrosated amides are direct acting carcinogens, and cause tumours near to the site they are produced, whereas nitrosated amines require hydroxylation and can initiate tumours at distal sites.2 In one study, increased consumption of red meat caused a 3-fold increase (from 40 to 113µ g/day) in N-Nitroso compounds in the faeces of eight volunteer males who were subjected to low and high meat diets, but white meats did not seem to induce similar effects. The increase is high, when one considers that smoking 40 cigarettes a day produces an exposure to approximately 30µ g/day of tobacco-specific carcinogenic N-Nitroso compounds.17
Table 4.2. The occurrence of heterocyclic amines in animal products. Concentrations are in µg/kg. (Adapted from reference 14