Insulin and fasting

If fasting has a control switch, it's insulin. This hormone, made by your pancreas, tells your body whether to store energy or burn it. Understanding it explains why fasting works the way it does, and why it can help people whose bodies have started to resist insulin's message.

The storage hormone

When you eat, glucose from your food enters your blood, and your pancreas releases insulin in response. Insulin works like a key: it lets glucose into your muscle and fat cells, tells your liver to stop making glucose and to store some as glycogen, and tells your fat cells to hold on to their fat ¹. In short, high insulin means "store".

When insulin stops working well

In insulin resistance, your muscles, liver and fat respond less to insulin's key. To keep blood sugar normal, your pancreas compensates by making more. For years, that extra effort can hide the problem: blood sugar stays normal while insulin runs high. Eventually, if the pancreas can't keep up, blood sugar starts to rise, first to prediabetes and then to type 2 diabetes ¹.

What fasting does

The most interesting finding is that fasting may improve insulin sensitivity on its own, not just through weight loss. In a Danish study, healthy young men fasted for 20 hours every other day for two weeks while eating enough to keep their weight stable. Afterwards, insulin moved glucose into their cells about 16% faster ².

Timing matters

Your body clock makes you more sensitive to insulin in the morning and around midday than at night ⁵. The same meal raises blood sugar more at 9 p.m. than at 9 a.m. That may be why early eating windows look especially promising: in a four-day study, eating between 8 a.m. and 2 p.m. lowered average 24-hour blood sugar and its swings compared with eating between 8 a.m. and 8 p.m. ⁶.

Sources

1. Petersen MC, Shulman GI. Mechanisms of Insulin Action and Insulin Resistance. Physiol Rev. 2018;98(4):2133-2223. doi.org/10.1152/physrev.00063.2017
2. Halberg N, Henriksen M, Söderhamn N, et al. Effect of intermittent fasting and refeeding on insulin action in healthy men. J Appl Physiol (1985). 2005;99(6):2128-36. doi.org/10.1152/japplphysiol.00683.2005
3. Sutton EF, Beyl R, Early KS, et al. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metab. 2018;27(6):1212-1221.e3. doi.org/10.1016/j.cmet.2018.04.010
4. Grajower MM, Horne BD. Clinical Management of Intermittent Fasting in Patients with Diabetes Mellitus. Nutrients. 2019;11(4):873. doi.org/10.3390/nu11040873
5. Poggiogalle E, Jamshed H, Peterson CM. Circadian regulation of glucose, lipid, and energy metabolism in humans. Metabolism. 2018;84:11-27. doi.org/10.1016/j.metabol.2017.11.017
6. Jamshed H, Beyl RA, Della Manna DL, et al. Early Time-Restricted Feeding Improves 24-Hour Glucose Levels and Affects Markers of the Circadian Clock, Aging, and Autophagy in Humans. Nutrients. 2019;11(6):1234. doi.org/10.3390/nu11061234