[Various toxic effects of nitrates and nitrites]

The main features of the long-term toxicity of nitrates and/or nitrites in man and experimental animals are reviewed; the possible formation of carcinogenic nitrosamines is dealt with in another paper of this series. It appears that the allowable daily intake of nitrates and nitrites for the healthy adult man has been evaluated chiefly from the no-effect levels in rats. First, the no-effect levels in rats are questionable, since some experiments have pointed out adverse effects of nitrates and nitrites with dosages near, or lower than, the so-called no-effect levels. Second, there is a risk that the allowable daily intake is outpassed in the diet of a man. Third, nutritional conditions, such as the vitamin A or C, or the iodine content of the diet, as well as other factors which are discussed, might alter theno-effect levels. From the cell metabolism viewpoint, nitrites are inhibitors of oxidative phosphorylation and of microsomal enzymes. Our present lack of knowledge does not yet allow us to explain the long-term toxicity of nitrates and nitrites in terms of cell metabolic disturbances.