Though a bad diet is a direct risk factor for diabetes (Hu et al.,2001),research now suggest that this behavior is a major factor as well:

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Smoking, especially heavy smoking, is a major risk factor for diabetes. This behavior not only affects the lungs but also impairs insulin sensitivity and leads to a higher chance of developing type 2 diabetes (Willi et al., 2007).

Though a Bad Diet Is a Direct Risk Factor for Diabetes