What happens in your body, hour by hour (and how sure we are)

Fasting stages without the hype: what is well measured and what is speculation.

Many apps show a clock saying "Autophagy starts!" at an exact hour. That does not exist: the timing varies with what you ate, how much you moved and your own body. Here are the stages with their level of evidence.

0–4 h · Digesting (solid evidence)

You are absorbing your last meal. Glucose and insulin rise and then fall.

4–12 h · Settling in (solid evidence)

Insulin drops. Your liver releases stored glucose (glycogen) and your body starts burning more fat.

12–18 h · Switching fuel (moderate evidence)

With less glycogen available, fat burning rises and the liver starts producing ketone bodies. In studies the rise shows up between 12 and 24 h, with big differences between people.

18–24 h · Mild ketosis (moderate evidence)

Ketones keep rising. How much depends a lot on your previous meals and physical activity.

24–48 h · Long fast (moderate evidence)

Liver glycogen is very low and ketosis is clear. Hunger usually comes in waves rather than rising steadily.

Over 48 h · Extended fast (limited evidence)

There are few human studies and risks increase. Only do it with medical supervision.

What about autophagy?

Autophagy (cells "recycling" their own parts) increases with fasting in animal and cell studies. In humans we still do not know when it starts or how much: there is no practical way to measure it. That is why we do not show it as a stage with a fixed time.

Sources

• Cahill GF Jr. Fuel metabolism in starvation. Annu Rev Nutr. 2006;26:1-22.
• Anton SD et al. Flipping the metabolic switch: understanding and applying the health benefits of fasting. Obesity. 2018;26(2):254-268.
• de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019;381:2541-2551.
• Bagherniya M et al. The effect of fasting or calorie restriction on autophagy induction: a review of the literature. Ageing Res Rev. 2018;47:183-197.

General information; not a substitute for a health professional.