intermittentfasting

The switch is already in you

Stop counting calories. Start using the clock.

Somewhere between twelve and sixteen hours after your last meal, your body stops running on the food you ate and starts running on you. Insulin drops. Ketones climb. Repair genes wake up. That switch is free, it is already installed, and almost nobody ever flips it.

These guides show you how — with the trials, the numbers, and a two-week plan you can start tomorrow morning by doing nothing at all.

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Four numbers from four real trials

Not testimonials, not theory. Peer-reviewed randomised trials, with the citation under each one so you can go and read it yourself.

550 kcal a day, eaten without noticing That is how much intake fell in adults given only a 4- or 6-hour window — “without calorie counting.”1
2.3 kg extra lost, just by eating earlier Same diet, same calories — one group simply closed the window at 3pm and lost significantly more.2
18 h the fast that switched on repair genes An 8am–2pm window raised expression of the autophagy gene LC3A and the longevity gene SIRT1.3
0 kg lost — and it still worked Insulin sensitivity, blood pressure and oxidative stress all improved with body weight deliberately held constant.4

What happens while you are not eating

The hours you skip are not empty. This is the sequence your body runs once the food stops arriving — and the reason a clock beats a calorie counter.

  1. Hours 0–4

    Storing

    Insulin is up, doing its job: moving sugar out of your blood and into storage. While it is elevated, fat stays locked where it is. Every snack restarts this clock — which is precisely why grazing all day is the hard way to do anything.

  2. Hours 4–12

    Draining the tank

    Liver glycogen runs down. Insulin falls, and as it falls the lock comes off. Your body starts pulling fatty acids out of storage to cover the gap. Most people never get here, because most people eat again first.

  3. Hours 12–16

    The switch

    Glycogen is spent, and the liver starts turning fat into ketones. This is the flip that the New England Journal of Medicine called a metabolic switch from glucose-based to ketone-based energy — and it is the whole reason the sixteen-hour mark is worth reaching.5

  4. Hours 16–18

    Housekeeping

    In an eighteen-hour fast, researchers measured ketones climbing alongside raised expression of LC3A — a gene involved in autophagy, the process by which cells clear out and recycle damaged parts — and SIRT1, tied to stress resistance and ageing.3

  5. Tomorrow

    You do it again

    And that is the entire protocol. No shopping list, no supplement, no app subscription. You already own everything required to run this, and the only cost is a breakfast you probably weren't enjoying anyway.

Start here

Six guides. Pick the one that answers the question stopping you right now.

Method

The 16:8 method

The schedule that gets you to the switch every single day, and how to pick a window that fits around the life you already have.

 Method

The 5:2 approach

Two low days a week, five days off. Same results as daily dieting — and in one trial, a bigger improvement in insulin.

 Practical

What actually breaks a fast

Black coffee, milk in tea, sweeteners, supplements. Clear answers so you stop second-guessing and get on with it.

 Practical

Seven beginner mistakes

The avoidable ways week one goes wrong — and the small fixes that keep you going long enough to see the change.

 Training

Fasting and exercise

Eight weeks of 16:8 plus lifting: fat down, strength and muscle held. Here is how to set up the day.

 Safety

Who should not fast

Fasting is not for everyone. Read this first if you are pregnant, take medication for diabetes, or have any history of disordered eating.

Pick your schedule

They all work. The best one is the one you will still be doing in March.

Method Pattern Best for What it gives you
16:8 Daily 8-hour eating window Almost everyone starting out Reaches the metabolic switch daily
14:10 Daily 10-hour window Week one, and shift workers Cut weight, blood pressure and lipids in metabolic syndrome
eTRE Early window, closing mid-afternoon Anyone who can move dinner earlier The strongest results in the literature
5:2 Two low days weekly People who hate daily rules Equal weight loss, better insulin markers
6:18 / 4:20 Very short daily window Experienced fasters only ~3% of bodyweight in eight weeks

Fourteen days. That is all I am asking for.

A day-by-day plan for your first fortnight: choosing a window, getting through day three, handling a fasted morning meeting, and reaching the point — usually somewhere in week two — where morning hunger simply stops showing up.

Free, no upsell, nothing to buy at the end of it. Just the plan.

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Read the contents first →

The evidence, if you want it

Every claim on this site traces back to a randomised trial. Here are the ones worth your time.

Fat loss

It works without counting anything

Adults with obesity ate freely inside a 4-hour or a 6-hour window for eight weeks. No calorie targets, no food rules. Both groups lost around 3% of body weight and cut insulin resistance and oxidative stress.1

Timing

Earlier is better still

Add an early window to a diet and you beat the diet alone: 2.3 kg more lost over 14 weeks, diastolic blood pressure down 4 mmHg, and — the authors measured this too — better mood.2

Metabolic

Benefits with no weight lost at all

The cleanest study in the field fed men with prediabetes enough to keep their weight identical. Insulin sensitivity, beta-cell response, blood pressure and oxidative stress all improved anyway.4

Heart

On top of the medication you already take

Nineteen adults with metabolic syndrome, most on a statin or a blood-pressure drug, moved to a 10-hour window for 12 weeks. Weight, blood pressure and atherogenic lipids all came down.6

Muscle

Lose the fat, keep the strength

34 trained men, eight weeks of 16:8 alongside their normal lifting. Fat mass fell; fat-free mass and maximal strength on bench and leg press held steady.7

Cells

Repair genes, switched on

An 18-hour fast raised morning ketones and increased expression of LC3A, an autophagy gene, and SIRT1, linked to stress resistance and ageing.3

References

  1. Cienfuegos S, Gabel K, Kalam F, et al. Effects of 4- and 6-h Time-Restricted Feeding on Weight and Cardiometabolic Health: A Randomized Controlled Trial in Adults with Obesity. Cell Metab. 2020;32(3):366–378.e3. doi:10.1016/j.cmet.2020.06.018
  2. Jamshed H, Steger FL, Bryan DR, et al. Effectiveness of Early Time-Restricted Eating for Weight Loss, Fat Loss, and Cardiometabolic Health in Adults With Obesity: A Randomized Clinical Trial. JAMA Intern Med. 2022;182(9):953–962. doi:10.1001/jamainternmed.2022.3050
  3. Jamshed H, Beyl RA, Della Manna DL, Yang ES, Ravussin E, Peterson CM. Early Time-Restricted Feeding Improves 24-Hour Glucose Levels and Affects Markers of the Circadian Clock, Aging, and Autophagy in Humans. Nutrients. 2019;11(6):1234. doi:10.3390/nu11061234
  4. Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metab. 2018;27(6):1212–1221.e3. doi:10.1016/j.cmet.2018.04.010
  5. de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541–2551. doi:10.1056/NEJMra1905136
  6. Wilkinson MJ, Manoogian ENC, Zadourian A, et al. Ten-Hour Time-Restricted Eating Reduces Weight, Blood Pressure, and Atherogenic Lipids in Patients with Metabolic Syndrome. Cell Metab. 2020;31(1):92–104.e5. doi:10.1016/j.cmet.2019.11.004
  7. Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. 2016;14(1):290. doi:10.1186/s12967-016-1044-0

Why I write this

Nicholas Denver
Editor · intermittentfasting.com

Because the mechanism is beautiful and hardly anyone knows about it. Intermittent fasting is a pattern of when you eat, not what you eat — which means there is nothing to buy, nothing to weigh, and nothing to give up permanently. The intervention is a clock.

I am a writer rather than a clinician, so I do the thing writers can do: read the trials and tell you what they found in a language you can act on. What they found is genuinely exciting. Fat loss without arithmetic. Blood pressure down alongside the drugs already doing that job. Insulin sensitivity improving in people who lost no weight whatsoever. Repair genes turning on overnight.

Nobody is going to run a marketing campaign for skipping breakfast, because there is no product at the end of it. That is exactly why it needs saying loudly. More about the site →

Start tomorrow morning

This is general information, not medical advice. Fasting is not appropriate for everyone, and for some people it is actively harmful. If you are pregnant or breastfeeding, have type 1 diabetes or take insulin or other glucose-lowering medication, have a history of disordered eating, are underweight, or are under 18, do not start a fasting protocol without talking to a doctor first. See who should not fast.