Fasting is ancient – but only modern cell biology explains why it benefits the body. One central reason is autophagy: the cellular self-cleaning that kicks in during longer eating breaks such as a 24-hour fast. Its mechanisms were discovered by a Japanese researcher who won the Nobel Prize for them. This guide explains the science, the benefit for liver and kidneys, and how to fast safely.
Yoshinori Ohsumi – the Nobel laureate behind autophagy
The Japanese cell biologist Yoshinori Ohsumi received the 2016 Nobel Prize in Physiology or Medicine – alone – "for his discoveries of mechanisms for autophagy." In the 1990s he used baker's yeast to identify the genes that control this self-cleaning process, laying the foundation for an entire field of research. Today we know autophagy is essential for the renewal of our cells.
What is autophagy?
The term comes from the Greek for "self-eating." The cell selectively breaks down its damaged components – misfolded proteins, broken organelles – and reuses the building blocks. Think of it as a cellular waste-collection and recycling plant. It is triggered above all by nutrient scarcity: when insulin falls and less energy is available, the cell switches into cleaning mode.
What happens in the body during a 24-hour fast
In the first hours after the last meal, the body uses stored sugar (glycogen). Once these stores are depleted after about 12–16 hours, it increasingly switches to burning fat and forms ketone bodies. At the same time insulin drops and pathways such as mTOR are turned down – the conditions under which autophagy ramps up. Important context: the exact timing in humans is not firmly established; much data comes from cell and animal studies. Still, longer fasts from about 16–24 hours are considered favourable.
How fasting supports the liver
The liver benefits especially. An eating break lowers liver fat and improves insulin sensitivity – both key to fatty liver. Fasting also gives the liver a rest from constantly processing food, and autophagy helps renew damaged liver-cell components. Combined with liver-friendly foods during eating phases, it's an effective lever.
How fasting supports the kidneys
For the kidneys too, studies (mostly in animals) show protective effects of autophagy: it helps kidney cells cope with stress. A moderate eating break can lower the metabolic load. The decisive factor, however, is drinking enough – the kidneys need fluid to work. Anyone prone to stones should ensure good hydration. With existing kidney disease: fast only under medical supervision.
How to fast for 24 hours properly
- Method: easiest is dinner to dinner – so you still eat once each calendar day. A planned break beats simply "not eating."
- Drink: plenty of water, unsweetened herbal tea and black coffee (no sugar or milk) are allowed and even important.
- Electrolytes: for weakness or headache, a pinch of salt, magnesium and potassium help – especially in longer fasts.
- Break it gently: start with a light meal (e.g. vegetable soup, some protein) rather than a large, sugary feast.
- Frequency: a 24-hour fast once or twice a week is manageable for many – not a permanent state.
- Listen to your body: with strong discomfort, dizziness or palpitations, end the fast.
Who should not fast (caution & when to see a doctor)
Fasting is not for everyone. Please do not fast without medical advice in case of: pregnancy and breastfeeding, underweight or eating disorders, in children and adolescents, in type 1 diabetes or when taking blood-sugar-lowering medication or insulin (risk of hypoglycaemia), in advanced kidney or liver disease and with certain long-term medications. When in doubt, always consult a doctor.
Frequently asked questions
When does autophagy begin? No exact time is proven in humans; longer fasts from about 16–24 hours are considered favourable.
Can I drink coffee while fasting? Yes – black and unsweetened. Water and unsweetened tea are also ideal and important.
How often should I fast for 24 hours? For many, once or twice a week is a good, sustainable rhythm.
Will I lose muscle while fasting? With short fasts the effect is small; ensure enough protein on eating days.
Does fasting help against fatty liver? Yes – through weight loss and reduced liver fat it's an effective building block; see our fatty liver guide.
Suitable companions for fasting – electrolytes, minerals and more – can be found in our recommendations below.
