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Since most of the monozygotic twins of patients with insulin dependent diabetes do not have the condition environmental factors are presumably important in producing it. The theory that early feeding with cows’ milk may be concerned has still to be disproved and there are known, apparently protective, associations between the incidence of diabetes in children and factors which might lead to more early life infections, such as deprivation and increased population density. The possibility of environmental pancreatic beta cell toxins seems to have attracted less attention but a study in Yorkshire (R C Parslow and colleagues,Diabetologia 1997;40:550-6) has pointed the finger at nitrates in drinking water. Over a period of 16 years a new diagnosis of diabetes was recorded for almost 1800 children under 16 in Yorkshire. Water is distributed in zones each supplying water of the same chemical composition to up to 50 000 people. Water analysis data were available for 148 such zones and it was possible to match up the water supply zones and the addresses of the children with diabetes. Demographic data were also collected. The previously reported inverse relationships between the incidence of childhood diabetes and childhood population density, deprivation scores, and proportion of non-white residents were confirmed. After statistical adjustment for these factors there was a 15% increase in incidence of childhood diabetes in areas with water nitrate concentrations above 14.85 mg/l. Nitrosamines may be toxic to beta cells and these authors suggest that the conversion of ingested nitrate to nitrosamines gives biological plausibility to their finding. Within the European Community the maximum acceptable drinking water concentration of nitrate is 50 mg/l and it is recommended that levels should be kept below 25 mg/l. In this study some 30% of over 9000 water samples contained more than 25 mg/l of nitrate. The estimated adult daily intake of nitrate in food in the UK is about 60–80 mg. More needs to be known about water and food nitrate intakes in children.