Introduction
Previous studies have identified drinking water quality and compounds found in drinking water as potential environmental risk factors for the development of type 1 diabetes.1 High levels of mercury, arsenic and acidity (low pH) of water have been previously associated with an increased incidence of type 1 diabetes.1–4 Park et al 5 found that total urinary arsenic was associated with insulin sensitivity but not β-cell function measures, suggesting that low-level arsenic exposure may influence diabetes risk through impairing insulin sensitivity in adults.
The potential toxic effects of nitrate-nitrite-nitrosamine on β cells function have remained a controversial issue over the past two decades.6 Parslow et al 2 in the UK, Kostraba et al 7 in the USA and Moltchanova et al 8 in Finland reported that the incidence of type 1 diabetes was positively associated with high nitrate levels in drinking water. They suggested that low-level nitrate exposure through drinking water may play a role in the etiology of type 1 diabetes, perhaps as a promoter through the generation of free radicals. In contrast, other studies have concluded that there is a lack of convincing evidence that nitrate levels in drinking water are a risk factor for childhood type 1 diabetes.9–11
The data from other ecological studies suggest that the incidence rate of type 1 diabetes is significantly lower when the concentrations of zinc and magnesium in the domestic drinking water are in the range 22.27–27.00 mg/L (incidence rate ratio (IRR) 0.76; 95% CI 0.59 to 0.97) and >2.61 mg/L (IRR 0.72; 95% CI 0.58 to 0.91).12–14 It has been also suggested that zinc, magnesium, calcium, chromium and to some extent copper, have a protective effect against developing type 1 diabetes.11 12 15 16 Zinc is involved in cellular antioxidative defense and mostly found in the secretory vesicles of β-cells in the pancreatic islets. Magnesium is an essential cofactor helping insulin to bind to the insulin receptor. Therefore, lower serum zinc and magnesium are significantly associated with type 1 diabetes .17 While all of these studies point to a possible connection to various elements found in water, the impact of water quality on the incidence of type 1 diabetes is still inconclusive .18
The Canadian province of Newfoundland and Labrador (NL) is a well-suited location to study environmental triggers associated with type 1 diabetes (figure 1). The province has a distinctive geography, and a stable population in terms of its size and geographic distribution during the study period. NL has the second highest incidence rate of type 1 diabetes globally, after Finland (in 2011 with 57.6/100 000; 0 to 14 yearage group).19 For the period between 1987 and 2005, the incidence of type 1 diabetes in the NL population was 35.08/100 000 (95% CI 32.54 to 37.62), with a steady increase in rates over this time period.20 For 2007–2010, the incidence rose over to 49.9/100 000 (95% CI 42.2 to 57.6).21 The incidence increased significantly in all age groups, with the highest rates of type 1 diabetes in the 2007–2010 period for ages 5–9 (IRR 59.1; 95% CI 45.0 to 76.3).21 NL also demonstrates significant geographical variations in incidence rates of type 1 diabetes, with some areas having half the incidence rate as compared with other regions.
Approximately 70% of the provincial population has access to publicly supplied water.22 The NL Department of Environment and Climate Change23 provides regular water quality monitoring and public reporting for all public water sources.24 Public water supply systems treat water to ensure it is free from any microbiological contamination and measures 39 chemical and physical parameters of the Guidelines for Canadian Drinking Water Quality.25 Using longitudinal data on both incidence rates of type 1 diabetes and the components identified in the testing of public water supply, we sought to explore the relationship between various components found in drinking water and the development of childhood-onset type 1 diabetes.