Nitrosamines are a group of chemicals that can be ingested from food, or are formed from other chemicals in the gut. Nitrosamines are formed by the reaction of a secondary or tertiary amine with a nitrosating agent, which in foods is often nitrous anhydride formed from the nitrites that are present in the food. In the gut, this chemical process is catalysed by the acidic conditions of the stomach, leading to the accelerated formation of nitrosamines. Nitrosamines are damaging because they are associated with an increased risk of gastrointestinal cancer, and this is supported by studies showing the molecular mechanisms of nitrosamine carcinogenicity. One of the most potentially problematic routes for nitrosamine formation in humans is the consumption of processed meats containing high levels of sodium nitrite and sodium nitrate (nitrate is oxidised nitrite). Nitrates are also present in foods of plant origin as nitrates are used as fertilisers during growth, however intakes from plant sources might not be as high as intakes from animal sources.
Antioxidants in foods and in the gut may inhibit the formation of nitrosamines and this may limit the damage they are able to inflict on gut cells. The addition of ascorbic acid or other antioxidants to foods has been shown to limit nitrosamine formation and the consumption of ascorbic acid with nitrite and nitrate containing foods limits nitrosamine formation. Nitrates on plant foods may not be so damaging because the levels of nitrate are lower, but also because the plant foods they contaminate often contains high levels of antioxidants. The ability of vitamin C to inhibit the formation of nitrosamine compounds may explain the protective effects of vitamin C against gastrointestinal cancers. The ability of green tea, a potent source of flavonoid antioxidants, to reduce the risk of gastrointestinal cancers may also be explained by its ability for prevent the formation of nitrosamines from foods containing high levels of nitrites and nitrates. This may also explain the inverse association between fruits and vegetables and cancer risk.
RdB
Mirvish, S. S. 1994. Experimental evidence for inhibition of N-nitroso compound formation as a factor in the negative correlation between vitamin C consumption and the incidence of certain cancers. Cancer Research. 54: 1948-1951
