Nutrition, Diet & Fasting - Fasting
The 24-hour fast
By Ryan Lowe. Last reviewed 3 August 2026. How we write this.
A full day without food, usually once or twice a week. Metabolically it is a genuinely different thing from a 16-hour fast - and the practical case for it still mostly comes down to the calories it removes.
A 24-hour fast means going from one meal to the same meal the following day - dinner to dinner is the usual arrangement, since it means never spending a whole waking day without having eaten. Water, black coffee and tea are typically allowed. Most people who do this do it once or twice a week rather than daily.
What actually changes at 24 hours
The timeline matters, because it is where a 24-hour fast genuinely separates from a 16-hour one. Liver glycogen - the body's readily available carbohydrate store - is largely depleted somewhere in the region of 24 hours without food, depending on how much was there to start with and how active the person is. Once it runs low, fat becomes the dominant fuel and ketone production rises measurably.
Insulin falls to its lowest levels of any normal daily pattern. Growth hormone rises, sometimes substantially - this is well documented and frequently over-interpreted, since a hormone rising is not the same as a benefit arriving. Noradrenaline rises too, which is part of why many people report feeling alert rather than sluggish once the initial hunger passes.
These are real, measurable changes. The honest gap is between measuring them and demonstrating that they translate into outcomes anyone would notice.
What the research shows
Most of the human evidence at this length comes from studies of alternate-day fasting and 5:2-style patterns, since those are built from repeated 24-hour-ish fasts, and that evidence is covered in its own lesson. In short: those patterns do produce weight loss, and the better meta-analyses find the advantage over ordinary calorie restriction is small and tends to shrink in longer trials.
For a once-weekly 24-hour fast specifically, the evidence base is thin. The clearest thing that can be said is arithmetic rather than metabolic: skipping a day's food once a week removes roughly a day's calories from the week, which is a meaningful deficit if it is not simply eaten back afterwards. Whether it is eaten back is the variable that decides the outcome, and it varies a lot between people.
What it is like, and why that matters
Hunger during a 24-hour fast is not linear. It tends to come in waves that track when someone normally eats, rather than building steadily, and many people find the wave passes. Headache, irritability and poor concentration are common on early attempts and usually diminish with repetition. Caffeine on an empty stomach makes all of that worse for some people.
This matters because the practical experience is what determines whether anyone keeps doing it, and adherence is what determines results - the same conclusion the diet trials keep reaching from a different direction.
Where the evidence stops
There is very little research on 24-hour fasting as a standalone weekly practice, as opposed to as part of a structured alternate-day protocol. Claims about autophagy at this duration are covered in their own lesson and are weaker than usually presented. There is no long-term outcome data.
Who should be careful
The same medication cautions as any fasting apply and they are sharper here: insulin and sulfonylureas can cause dangerous hypoglycaemia in someone who is not eating, and doses may need adjusting by a prescriber rather than by the person fasting. Pregnancy, breastfeeding, being underweight, a history of eating disorders, and type 1 diabetes are all reasons not to do this without medical advice. Read the fasting safety lesson.
Educational content only and not medical advice. Fasting while taking insulin or other glucose-lowering medication carries a real risk of hypoglycaemia - speak to your prescriber first.
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Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.
