What does intermittent fasting do to your insulin sensitivity?

Intermittent fasting improves fasting insulin, fasting blood glucose and HOMA-IR (a measure of insulin resistance) across multiple meta-analyses of randomised trials. The caveat is significant: most of the underlying studies enrolled an average of 38 participants and lasted an average of three months. Reviewers rate the evidence for these outcomes as low to very low.

The strongest indication that fasting actually does something to insulin sensitivity, and not merely through weight loss, comes from a controlled crossover study in men with prediabetes. For five weeks they ate all their meals before 15:00 within a six-hour window. Without losing so much as a single gram, both their insulin sensitivity and the pancreatic response to blood glucose improved. This suggests that the timing of eating itself matters, independently of how much you eat.

The time of day genuinely makes a difference. When you consume most of your calories early and avoid late evening meals, post-meal blood glucose regulation appears to improve further. This is consistent with the circadian rhythm of metabolism: insulin sensitivity is naturally higher in the morning.

In older adults with insulin resistance, the 5:2 diet (two fasting days per week) improved blood markers for carbohydrate and fat metabolism after eight weeks. The effect was comparable to a standard healthy diet, not spectacularly better. All common forms of fasting are considered safe and do not demonstrably lead to disordered eating.

For people undergoing cancer treatment, a different consideration applies. In animal research, fasting also improves insulin sensitivity during chemotherapy, but in humans there are hardly any controlled studies. Fasting during cancer treatment should never be done without medical supervision.