Comment on Bonnet et al. Association Between Handedness and Type 2 Diabetes: The E3N Study. Diabetes Care 2015;38:e199

I read with interest the short report by Bonnet et al. (1). It does not appear that the results were adjusted for potential age differences between the exposure groups. It would be helpful to know if there were any age differences between the right-handed, left-handed, and mixed-handed individuals. There are substantial cohort effects in handedness (2), with an increased acceptability of left-handedness in recent decades. Older people may be more likely to be mixed-handed compared with younger individuals as older left-handed people are more likely to have had schooling that required writing with the right hand. Age is one of the most important risk factors for type 2 diabetes. Could a confounding effect by age have resulted in a spurious association between handedness and type 2 diabetes risk?