Body stages Stage 3 of 11
12–18 h
Fat mobilisation
Fat is being released from stores in larger amounts. Ketone levels are usually still low at this point.
What studies measured
Insulin-to-glucagon ratio markedly reduced; free fatty acids rising; ketones still near baseline.
How it may feel
Mealtime hunger that passes.
What to do
Water to thirst. A 16-hour window usually ends here, and that is fine.
Sources
- Deru 2021The Effects of Exercise on β-Hydroxybutyrate Concentrations over a 36-h Fast: A Randomized Crossover Study. Med Sci Sports Exerc, 2021.
- Service and O'Brien 2005Increasing serum betahydroxybutyrate concentrations during the 72-hour fast: evidence against hyperinsulinemic hypoglycemia. J Clin Endocrinol Metab, 2005.
- Ho 1988Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. J Clin Invest, 1988.
What studies measured in healthy adults, not a reading of your own body. Stop and seek help if you feel unwell; the signs to stop for are listed under Stop signs on the safety page.