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What happens in your body during a fast?

Enter the time of your last meal, or drag the slider, to see hour by hour how the body adapts.

You are at 14 h of fasting

Relative levels, a simplified diagram. Real values vary a lot from one person to another (previous meal, activity, sleep, health).

Frequently asked questions about the first 3 days of fasting

What happens in the body after 16 hours of fasting?

After 16 hours, the liver's sugar store is well depleted, fat has become the main fuel and ketone bodies start to rise. Insulin is low and glucagon higher than after a meal (Cahill, 2006).

When does autophagy start?

We do not know in humans: there is not yet a reliable way to measure autophagy in the organs of a living person. In mice, a short fast increases it sharply, even in brain neurons (Alirezaei, 2010). The precise durations that circulate for humans (16 h, 72 h) are not proven.

How much weight do you lose in 3 days of fasting?

Mostly water at first: the sugar store in the liver and muscles is kept with water, which leaves with it. Fat loss is much smaller: with an expenditure of 2,000 kcal per day, it is at most 250 g (about half a pound) per day (2,000 ÷ 7,700 kcal per kilo of fat, our calculation). The water weight comes back as soon as you eat again.

Is a 72-hour fast dangerous?

For most healthy adults, no, but it is not advised during pregnancy or breastfeeding, for children, or with treated diabetes, eating disorders or underweight. Some medications make it risky. Ask a health professional before a first long fast.

With care: this page informs, it does not replace medical advice. Fasting is not advised during pregnancy or breastfeeding, with treated diabetes or eating disorders, or for children and teenagers. Beyond 24 h, talk to a health professional first.
Sources
  1. Cahill, "Fuel metabolism in starvation", Annual Review of Nutrition, 2006. pubmed.ncbi.nlm.nih.gov
  2. Ho et al., "Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man", Journal of Clinical Investigation, 1988. pubmed.ncbi.nlm.nih.gov
  3. Anton et al., "Flipping the metabolic switch: understanding and applying the health benefits of fasting", Obesity, 2018. pubmed.ncbi.nlm.nih.gov
  4. de Cabo and Mattson, "Effects of intermittent fasting on health, aging, and disease", New England Journal of Medicine, 2019. pubmed.ncbi.nlm.nih.gov
  5. Alirezaei et al., "Short-term fasting induces profound neuronal autophagy", Autophagy, 2010. pubmed.ncbi.nlm.nih.gov