Fasting and appetite: what really happens in your body after 24 hours
Metabolic physiologist James Betts explains that most people never actually reach the fasted state — they spend nearly their whole waking life digesting. Real fasting means plain water only, because even a small amount of food interrupts the slow build-up of ketones and autophagy. A 10-hour eating window is roughly where the benefits start to accumulate.
Overview
James Betts, professor of metabolic physiology at the University of Bath, unpacks what fasting actually means once you look at the physiology rather than the marketing. Being 'fasted' is not the same as 'not currently eating': scientists call it the post-absorptive state, when nothing in your body still shows a trace of your last meal.
Carbohydrates clear within a couple of hours, but fats can take five to eight, which means someone eating dinner at 9 p.m. and breakfast at 7:30 a.m. may only be truly fasted for a couple of hours. He walks through the fuel shift — carbohydrate to fat on day one, then ketones appearing on day two or three — and describes his own five-day study fast, where hunger peaked on the Tuesday and then vanished entirely.
The Bath Breakfast Project found no meaningful health advantage to eating breakfast, though skipping it made people spontaneously less active in the morning. On practicalities, he argues the '600 calories on fasting days' compromise quietly undoes the mechanism it was meant to enable, and that olive oil during a fast breaks the fast too. Fasted training may amplify the same adaptation signals, while hard sessions are better fuelled. His bottom line: fasting works, but mostly as one workable way to eat less overall — not as a category of its own.
Key quotes
5Most people eating a typical meal pattern are actually in the fed state for their entire waking lives.
That small amount of food, while it seems harmless, has probably interfered with the proposed benefit of fasting.
By day two I was really very hungry, but that just completely went away by Wednesday morning.
Having a window of 10 hours or more is probably where you need to be to start accumulating that benefit.
Olive oil is lipid, is fat. It absolutely is breaking your fast.
Key ideas
9Nutrient timing is the real question
Humans eat in discrete lumps while the body's demand runs continuously. How the body bridges those gaps — storing, using or clearing nutrients — is what nutrient timing describes.
Rodent studies only take us so far
Mice eat almost continuously, are nocturnal and become hyperactive when fasted. Humans get lethargic instead, so meal-timing findings from rodents translate poorly.
Fasted means post-absorptive, not 'not eating'
The fasted state means no measurable trace of your last meal anywhere in your physiology. Carbohydrates clear in about two hours; fats can take five to eight.
Most people never reach the fasted state
Dinner at 9 p.m. and breakfast at 7:30 a.m. leaves roughly two hours truly fasted. With snacks, many spend their entire waking life in the fed state.
The fasting family tree
Intermittent fasting is the umbrella: alternate-day fasting, 5:2, and time-restricted eating, where the window is pinned to specific hours of the day.
Hunger peaks, then disappears
In a Monday-to-Friday study fast, hunger was worst on day two and had gone by Wednesday morning. Paradoxically, a stricter cut can be easier than a partial one.
The Bath Breakfast Project
Six weeks of eating versus skipping breakfast produced no meaningful difference in standard health markers. Lean people who skipped it lost weight; breakfast did not aid weight loss.
Carbohydrate, then fat, then ketones
You carry only 1,000–2,000 calories of carbohydrate but tens of thousands as fat. By day two the liver starts converting fat into ketones for the brain.
Fasting works — but so does eating less
Studies show weight loss and better insulin sensitivity, yet it is unclear how much is fasting itself rather than simply a practical way to cut overall intake.
Practical takeaways
6- 1
Count your real fasting hours 14:30
Work out the gap between your last bite and your first one — including any evening snack. Most people find it is far shorter than they imagined.
- 2
All-or-nothing is often simpler 19:30
A clean fast removes the grey area of 'how much am I allowed'. Many people find a full fast easier to follow than a 600-calorie version.
- 3
Plain water is the strict definition 25:00
In research settings a fast means water only. For everyday use, unsweetened tea or coffee is generally seen as workable rather than disqualifying.
- 4
Start at a 10-hour eating window 41:00
Roughly ten hours of eating and fourteen without is where benefits typically begin to accumulate. Longer intervals tend to do more, up to what you can sustain.
- 5
Match training intensity to fuel 47:30
Routine, steady sessions may work well in a fasted state. Save the hard, high-quality efforts for when you are fed, so you can genuinely push.
- 6
Decide the first meal in advance 55:00
Hunger erodes good judgement, so plan what and how much you will eat before the fast begins rather than choosing at the end of it.
Topics & chapters
14Quick-fire round
Breakfast is not the most important meal, meal timing does matter, and the starvation-mode myth gets dismantled.
What nutrient timing means
Why time is the most studied variable in science, and how the body bridges the gaps between meals.
Why rodents mislead us
Faster metabolism, continuous eating and nocturnal rhythms make mouse data a shaky basis for human meal-timing advice.
Defining the fasted state
Post-prandial versus post-absorptive, and how long different macronutrients keep your metabolism perturbed.
The permanently fed life
How a normal eating pattern leaves almost no genuinely fasted hours, and how that differs from our ancestors.
The types of fasting
Alternate-day fasting, 5:2 and time-restricted eating, and what actually separates them.
Why partial fasts blunt the effect
Ketosis and autophagy build slowly and can be interrupted in seconds by a small snack.
Five days without food
Betts on his own study fast: hunger on Tuesday, gone by Wednesday, energised by Thursday as ketones rose.
What you can consume
Water only in the lab, caffeine as a grey area, and why electrolytes matter on longer fasts.
The breakfast experiments
Six-week randomised trials in lean and larger-bodied participants, and their surprisingly flat results.
How your fuel switches
From carbohydrate to fat on day one, and the arrival of ketone bodies on day two or three.
Health effects and eating windows
Insulin sensitivity, inflammation, autophagy and gut diversity — plus where the 10-hour window comes from.
Training in a fasted state
Why the same run can send stronger adaptation signals when fasted, and when to eat first instead.
Consistency, olive oil and breaking the fast
Whether your body prefers routine, why olive oil breaks a fast, and how to plan the meal that ends one.