ABSTRACT
Nitrates and nitrites are chemical compounds naturally present in the environment that form part of the nitrogen cycle. Major sources of nitrate include mineral deposits, soil, seawater, drinking water or freshwater systems, and the atmosphere. Nitrate plays an important role in plant nutrition and functions and is therefore used widely in fertilizers. Nitrate can accumulate in vegetables (Mensinga et al., 2003; Lundberg et al., 2004; Camargo and Alonso, 2006; EFSA, 2008; Lundberg et al., 2008). Levels tend to be higher in leaves and lower in seeds or tubers, and thus leaf crops generally have higher nitrate concentrations; for example, levels of 2.797 and 1.496 ppm have been recorded in spinach and celery respectively. On the other hand, the concentration of nitrite in fresh vegetables is generally low, depending on the method of cultivation, the rate and timing of fertilizer application, light intensity, daytime temperature, and soil characteristics. Human exposure to nitrate is mainly exogenous, from the consumption of vegetables, and to a lesser extent of water and other foods. Vegetables account for approximately 80% of the nitrate consumed via food and 10%–15% of the nitrate consumed via water (EFSA, 2008). Nitrate and nitrite are also formed endogenously in mammals, including humans. Nitrate is secreted in saliva and then converted to nitrite by oral microora. In contrast, exposure to its metabolite nitrite is mainly from endogenous nitrate conversion.
