A vast majority of studies are routinely using overnight or 6h of fasting before metabolic testing of glucose homeostasis in mice. Other studies have empirically been using shorter fasting time (<6h). We aimed here to determine the shortest fasting time required for optimal insulin responsiveness while minimizing metabolic stress.
A time-course of fasting up to 24h (0, 2, 4, 6, 12 and 24h) was performed in C57Bl/6J male mice. Body weight, metabolic parameters and insulin tolerance were measured in each experimental group. Organs were collected at the same time point on a separate occasion and glycogen and metabolic gene expression measured in liver and skeletal muscle.
Our data show that blood glucose levels do not significantly change during a 6h fast, while plasma insulin levels decrease to similar levels between 2h and 6h fast. During overnight (12h) and 24h fast, a robust decrease of blood glucose and plasma insulin is observed along with a profound depletion in liver glycogen content. Insulin tolerance was comparable between baseline and 6h fast while 4h and 6h fast was associated with a greater depletion of liver glycogen than 2h fast impacting the glucose counter-regulatory response. Fasting induced progressive weight loss which was attenuated at thermoneutrality. Above 4h, fasting induced major body weight loss (>5%) and significant changes in catabolic gene expression in liver and skeletal muscle.
Collectively, these data suggest that 2h fasting appears optimal for the assessment of insulin tolerance in mice as this duration minimizes major metabolic stress and weight loss.