Effect of intermittent fasting and refeeding on insulin action in healthy men

Summary

This is one of the cleanest human studies on what fasting does to insulin sensitivity. Eight healthy young men (average age 25, BMI around 26) fasted for 20 hours every other day for 15 days. Before and after the protocol, the researchers measured insulin action with the gold-standard test in metabolic research: the euglycemic-hyperinsulinemic clamp, which directly tells you how much glucose insulin can move into tissues at a fixed concentration. After the 15-day intermittent-fasting block, insulin-mediated whole-body glucose uptake rose from 6.3 to 7.3 mg per kilogram per minute — about a 16 percent improvement, statistically significant. Adiponectin, a hormone that improves insulin signaling and tracks metabolic health, rose by more than 50 percent measured against the basal level. The men did not lose meaningful weight, so the change is not explained by fat loss. The study was the first in humans to show that intermittent fasting itself can directly improve how insulin works.