Occupational metal exposures, smoking and diabetes

Dear Sir,

Yang et al. investigated the effect of occupational exposure to metals and smoking on the risk of diabetes and prediabetes in male workers [1]. The authors adopted Poisson regression analyses to estimate the interaction of metal exposures and smoking on the prevalence of diabetes. Adjusted prevalence ratio (PR) (95% CI) of workers with >40 pack-years of smoking and with smelting/refining work (the highest metal exposures) for diabetes was 3.6 (2.4–5.4). In patients with prediabetes, significance of PR and significant interaction disappeared. I have some queries on their study.

First, Kuo et al. conducted a systematic review on the association between environmental chemicals and type 2 diabetes, by considering obesity and metabolic syndrome [2]. From the view point of consistency, temporality, strength, dose–response relationship and biological plausibility, arsenic, cadmium, mercury, persistent organic pollutants, phthalates and bisphenol A showed no clear association with type 2 diabetes. Although the difference of the level of exposure and the type of metal would affect the risk of diabetes, Yang et al. should conduct quantitative evaluation on metal exposure and also consider the relationship between specific metal and diabetes with a sufficient number of samples.

Second, Le Boudec et al. assessed the association between smoking cessation and incidence of type 2 diabetes and impaired fasting glucose by a 5.5-year follow-up study [3]. They concluded that adjusted odds ratio of recent quitters and long-term quitters for the incidence of type 2 diabetes and for impaired fasting glucose did not become significant. Pan et al. conducted a meta-analysis of prospective studies on smoking behaviours and diabetes risk [4]. Active and passive smoking were associated with increased risks of type 2 diabetes. About smoking cessation, the risk of diabetes increased in new quitters but decreased substantially as the time since quitting increased. Subjects with impaired fasting glucose can be considered as a state of prediabetes, and there was no significant effect of smoking cessation on the risk of diabetes and prediabetes, which should be confirmed by further study.

Finally, Moon conducted a cross-sectional study on the association of heavy metals, including lead, mercury and cadmium, with diabetes [5]. Blood lead, mercury and cadmium had no significant relationship with diabetes by adjustment for age, sex, region, smoking, alcohol consumption and regular exercise, and the sum of heavy metal mixture with prevalent diabetes did not significantly relate with diabetes. The level of metal exposure would be lower than workers with occupational exposure, and independent variables for predicting diabetes and prediabetes should be handled quantitatively to confirm the association.

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