Fasting insulin

Oral glucose tolerance tests (OGTTs) were performed after at least a 3-day resting period following the MR session (described below). Six-hour fasted mice (food removed at ~07:30 h) received a glucose load of 1.5 g/kg by oral gavage, and blood glucose was monitored from the tail tip with a glucometer (Breeze, Bayer, Zürich, Switzerland) for 2 h. The area under the curve (AUC) in the glucose versus time plot was calculated to assess glucose clearance from the blood. An insulin tolerance test (ITT) was then performed after a minimum 3-day resting period by intraperitoneal injection of insulin (0.75 IU/kg, Humulin, Novo Nordisk, Bagsvaerd, Denmark, diluted in 0.9% NaCl). Blood glucose was monitored from the tail tip for 3 h and the area above the curve (AAC) was calculated for the glucose versus time plots. Commercially available enzyme-linked immunosorbent assay (ELISA) kits were used to measure fasting insulin (Mercodia, Uppsala, Sweden) and C-peptide (Crystal Chem, Downers Grove, IL, USA) levels in the plasma. Insulin sensitivity was estimated with the quantitative insulin sensitivity check index (QUICKI) as the inverse of the log 10 sum of fasting insulin (μIU/mL) and fasting glucose (mg/dL).