The second purpose of this study is to determine whether serum sphingolipid metabolites associate significantly with the process and severity of type 2 diabetes. The serum concentrations of sphingolipid metabolites of 20 subjects with type 2 diabetes will be compared to age-, sex- and BMI-matched series of healthy and pre-diabetic subjects. Investigators speculate that the serum concentration of sphingolipid metabolites are positively related with the progression of diabetes. In order to discover why serum sphingolipid metabolites correlates with the progression of diabetes, detailed information on HbA1c, duration of diabetes history and insulin resistance defined by homeostatic model assessment (HOMA-IR) will be analysed. These three parameters may affect the serum concentrations of sphingolipid metabolites.