Effects of Different Fasting Durations on Glucose and Lipid Metabolism in Sprague Dawley Rats

Overnight fasting of varying length is common practice when studying glucose and lipid metabolism in rats. However, prolonged fasting may influence insulin sensitivity, and it is unknown to which extent different fasting durations affect postprandial metabolism in rats. The purpose of this study was to investigate the effect of different fasting durations (6-, 12-, or 18-h) on fat tolerance and glucose tolerance in male Sprague Dawley rats. We also aimed to examine the effect of two test fats with different fatty acid composition on postprandial triglycerides. We conducted a fat tolerance test, where butterfat or rapeseed oil was administered in a crossover design (experiment 1), and an oral glucose tolerance test (experiment 2). Regarding the fat tolerance test, we found no effects of fasting duration on triglycerides or free fatty acids, whereas the 18-h fast resulted in reduced glucose and insulin area under the curves. We did not find differential effects of butterfat and rapeseed oil on the outcomes. We found decreased fasting insulin and homeostasis model assessment of insulin resistance (HOMA-IR), and increased beta-hydroxybutyric acid concentrations after 18-h fast compared with shorter fasting durations. Regarding the oral glucose tolerance test, both 12-h and 18-h fast resulted in greater peak insulin concentrations than 6-h fast, and peak glucose concentrations were higher after 18-h than 12-h fast. We found no effects of fasting on the insulin sensitivity index. In conclusion, extending the fasting duration had an impact on glucose metabolism in rats, but did not appear to influence fat tolerance.