Vegetable-borne nitrate and nitrite and the risk of methaemoglobinaemia

Abstract

High levels of nitrate in vegetables are frequently reported. The potential hazard of vegetable-borne nitrate is from its conversion to methaemoglobin-producing nitrite before and/or after ingestion. Methaemoglobin cannot bind oxygen and produces a leftward shift in oxygen-dissociation curve, causing hypoxaemia. Infants under 3 months old are particularly susceptible to methaemoglobinaemia. Older infants and children are also at risk. Adults are not thought to be at risk of vegetable-borne nitrate or nitrite induced methaemoglobinaemia. This view should now change if the high nitrate levels in some vegetables and the effects of storage and food processing on its conversion to nitrite are taken into consideration. In fresh, undamaged vegetables, the nitrite concentrations are usually very low. Under adverse post-harvest storage conditions, nitrite concentrations in vegetables increase as a result of bacterial contamination and endogenous nitrate reductase action. Nitrite accumulation in vegetables is inhibited under frozen storage because endogenous nitrate reductase is inactivated. Pureeing releases endogenous nitrate reductase, increasing nitrite concentrations in vegetables. Oral reduction of nitrate is the most important source of nitrite. In order to maximise the health benefits from eating vegetables, measures should be taken to reduce the nitrate and nitrite exposures while maintaining the recommended vegetable intake. Excessive use of nitrogen fertilizers should be avoided so as to reduce nitrate build up in soil or vegetables. Vegetables must be stored and processed properly to prevent bacteria contamination. Removal of stem and midrib results in a decrease of nitrate content in lettuce and spinach. Peeling of potatoes and beetroot decreases the nitrate content. Nitrate levels in some vegetables can decrease after cooking in water or blanching. Home prepared infant food containing vegetables should be avoided until the infant is 3 months or older.