The prevalence of type 2 diabetes has been escalating throughout the world and usual food consumption is regarded as one of the most important environmental determinants for chronic diseases. Thus, the role of type and quality of carbohydrates on the etiology of type 2 diabetes is still poorly understood. The present review discusses the available epidemiological evidence regarding the effect of the type and source of carbohydrate of the usual diet and the risk of type 2 diabetes in adults. Although epidemiological investigations support the hypothesis for a potential risk effect of high glycemic index and low fiber content diets for diabetes, the results are controversial and the benefit may be related to the magnesium content of the structure of the grains, suggesting the relevance of taking into account the food sources instead of nutrients on investigations of diet and risk of chronic diseases. The available evidence suggests that eating a diet rich in whole grain cereals and vegetables and low in refined grains, sucrose and fructose contents is beneficial in the prevention of diabetes. Thus, more clinical trials are required to support the associations and establish biologically plausible pathways.