Background and Aims

Excess nutrient intake is linked to diseases such as stroke, cancer, cardiovascular disease, insulin resistance (IR) and type 2 diabetes (T2D).  In humans, caloric excess contributes to the progression of IR to T2D. We performed longtitutidal studies on mice fed a high fat diet (HFD) to assess whole body glucose tolerance and insulin sensitvity of insulin-responsive tissues as the mice become IR. This study design allowed us to pinpoint key transitions in response to chronic calorie overload.

Materials and Methods

Age matched C57Bl6 mice were placed on different lengths of high fat diet (HFD). Body weight and composition were assessed via dual energy x-ray absorptiometry. Blood insulin was measured via insulin ELISA. Glucose tolerance was measured by glucose tolerance tests, and in vivo glucose uptake into adipose and muscle was assessed using ³H-2-deoxyglucose. We performed ex vivo glucose uptake assays to determine insulin senstivity in  white adipose tissue, white muscle (EDL) and red muscle (soleus). Insulin signalling was assessed via western blotting using phospho-specific antibodies.

Results and conclusions

Reduced glucose tolerance in mice was observed after very short periods (3d) of high fat feeding, which was sustained up to 42d without any significant worsening. This was accompanied by elevated blood insulin in the fed and fasted states. Defects in insulin-stimulated glucose transport into isolated muscle was observed at the earliest time point (5d HFD) and similar defects measured at both 14 and 42d HFD. In white adipose tissue, a slight defect in insulin-stimulated glucose transport was observed at 3d HFD. At 7d HFD but not 14d, this defect was overcome with higher doses of insulin. This indicates that a significant loss of insulin sensitivity occurs in white adipose tissue between 7 and 14d HFD. We are undertaking further studies to examine the molecular mechanisms underlying loss of insulin responsiveness in muscle and fat tissue.