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See when your body switches to burning fat

Every calorie app tells you how much you ate. FUEL SHIFT shows you where that energy actually is right now, across your bloodstream, your liver, your muscles and your fat, and counts down to the moment your body runs out of sugar and turns to your stores.

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No card. No app store. Works in your phone's browser.

7:00 AM FAT BURNING
Fat is your main fuel right now

Not a mockup. This is the app's own engine running a real day at 2,400 times speed. Watch the liver fill after each meal and drain overnight.

Why the scale stalls

A calorie count is a daily total. Your body works minute to minute.

You can hit your numbers perfectly and still spend most of your waking hours with fat release switched off. Not because you ate too much. Because of when.

What a calorie app sees

One number a day. 1,800 in, 2,400 out, a 600 deficit. It looks identical whether you ate it in one sitting or grazed across fourteen hours, and to your body those are not remotely the same day.

What is actually happening

Every time you eat, insulin rises. Insulin does two things at once: it opens your sugar tanks so they can fill, and it shuts fat release down. Graze all day and your body rarely gets the chance to reach its reserves.

The four tanks

Your body doesn't store energy in one place

It stores it in four, and they behave nothing like each other. Which one you're burning from decides whether today moved the needle.

A human figure with four glowing regions: the circulatory network in cyan, the liver in amber, the major muscle groups in violet, and the body's fat layer in orange Blood Liver Muscle Fat
1

Bloodstream about 120 kcal

A teaspoon of sugar dissolved in everything. Minutes of fuel. Your body defends this number harder than almost anything else, because your brain runs on it.

2

Liver about 500 kcal

The day tank. It tops your blood sugar back up between meals so you feel level. It drains slowly through the night, and it has to run down before your body leans hard on fat. This is the one that matters.

3

Muscle about 2,000 kcal

By far the biggest sugar store, and the one everyone misreads. Muscle is missing the enzyme needed to send sugar back into your blood, so it can only fuel the muscle holding it. It barely moves while you sit still. Train, and it empties fast.

4

Fat often over 100,000 kcal

The reserve. Months of fuel. Nobody runs out. But it only hands energy over at a limited rate, and only once insulin is out of the way.

Blood 120
Liver 500
Muscle 2,000
All three 2,620
Fat 150,000

That sliver on the left is every bit of sugar your body can hold. The bar underneath it is your fat. The whole game is getting from one to the other, and the only thing standing between them is how long it has been since you ate.

One day, four tanks

Every meal moves all of them at once

Here is a real day from the engine: breakfast at 7:30, lunch at 12:30, a walk at 5:30, dinner at 7. The shaded stretch is the only part of it where fat was the main fuel.

Liver glycogen Muscle glycogen Share of energy from fat Fat-burning window
How it works

Three minutes to set up. Then it runs itself.

1

Tell it about you

Height, weight, age and body fat, matched against line-art figures, a tape measure, or a number from a scan if you have one. It sizes your tanks from your lean mass, then asks how you have been eating and when you last trained so it knows how full they are today.

2

Log what you eat

Search the food library, punch in calories, or enter exact macros. Log a workout and it drains muscle glycogen properly. Two taps and you are done.

3

Watch the meter, and wait

Your tanks update every second. When the countdown hits zero you are burning fat, and the app tells you how much you could eat right now before any of it has nowhere to go but storage.

What you get

The whole picture, live

The fat-burn window

A countdown to the moment fat becomes your main fuel, projected 24 hours ahead from where you are right now. The single most useful number if you are trying to lose fat.

Storage headroom

How much you could eat this minute before the surplus starts becoming fat. It rises as your tanks empty and jumps after a hard session, which is exactly why carbs after training land differently.

Check a meal before you eat it

Enter something and see how much refills glycogen, how much gets burned off, how much stores as fat, and how far it pushes your fat-burn window back. Then decide.

A live meter, not a daily total

Four animated tanks that fill and drain in real time, with the fuel currently being burned broken out by source. You can watch a meal arrive.

History and PDF reports

Intake against burn, weight and body-fat trend, every eating gap over eight hours. Export any window as a PDF you can keep or hand to a coach.

Built for a phone

Runs in your browser. Nothing to install, no app store, no ads, no data sold. Add it to your home screen and it behaves like an app.

The question the app answers

"Can I eat yet?"

Not a rule, a readout. You get the actual state of your tanks, the countdown to fat becoming your main fuel, and the size of the meal that fits before any of it turns into storage. What you do with that is yours.

The science

A physiological model, not a vibe

Most apps multiply your weight by a number. This one simulates your metabolism in five-minute steps and shows its working.

Calibrated, not guessed

Resting burn uses Katch-McArdle from your lean mass, cross-checked against Mifflin-St Jeor. The fuel mix is fitted to measured respiratory-quotient data across fasting durations. Liver depletion follows isotope studies.

Tested against the literature

36 automated checks ship with the app and run against published human data: liver glycogen across a 72-hour fast, post-meal glucose peaks, muscle glycogen sparing, and whether eating at maintenance actually holds fat mass flat.

You can read all of it

Every parameter, every equation, every source and every known weakness is documented inside the app. Tap the question mark and read the model yourself. No black box.

Some of what it is built on

  • Cahill GF, Starvation in man. New England Journal of Medicine, 1970. The fasting fuel sequence and obligate brain glucose.
  • Owen OE et al., Brain metabolism during fasting. Journal of Clinical Investigation, 1967. How far ketones displace glucose.
  • Nilsson & Hultman. Liver and muscle glycogen through fasting and refeeding.
  • Mifflin MD et al. American Journal of Clinical Nutrition, 1990. Resting energy expenditure.
  • Boirie Y et al. PNAS, 1997. Slow versus fast dietary protein absorption.
  • Jeukendrup AE. Sports Medicine, 2010. Intestinal carbohydrate absorption ceilings.
  • Brooks GA. The crossover concept for substrate use during exercise.
  • Ainsworth BE et al. 2011 Compendium of Physical Activities, for MET values.
Straight answers

What this will not do

Every other page you have read today made a promise. Here is where ours stops.

Pricing

Try the whole thing for 7 days

No card, no trial-length feature crippling. If it has not shown you something useful in a week, walk away and nothing happens.

$9.99
per month after your free 7 days
No card to start · Cancel in two taps
  • Everything, from day one
  • The live four-tank meter
  • Fat-burn window and storage headroom
  • Check any meal before you eat it
  • Full history and PDF export
  • Your data stays yours, always exportable
Start free

We only ask for a card if you decide to stay. Cancel any time from inside the app and you keep access to the end of the period you paid for.

Questions

Before you start

Do I have to fast to use this?
No. Plenty of people use it just to understand what their meals are doing. But if fat loss is the goal, the gaps between meals are the lever the app is built around, and you will see why within a day.
Is this just another calorie counter?
It counts calories, because it has to. The difference is what it does with them: it routes every meal through a simulation of your bloodstream, liver, muscle and fat, so you can see where the energy went rather than just what it totalled.
How accurate is it?
It is calibrated against published physiology and ships with 36 automated tests that check it reproduces measured human data. But it is still a model of a body, not a measurement of yours. Log your weight regularly and it corrects itself against reality. The app is candid about its own weak spots, which are listed in the guide.
Do I need a smart scale, CGM or any hardware?
No. If you happen to own a glucose meter or CGM you can type a reading in and it will use it, but nothing requires it.
What happens to my data if I cancel?
It stays yours. Even with a lapsed subscription you can still sign in, export everything as a PDF, and delete your account. We do not hold your history hostage to get you to resubscribe.
Is there an iPhone or Android app?
It runs in your browser and is built phone-first. Add it to your home screen and it opens full screen like any other app. No store, no install, and it updates itself.
Can I use it to gain weight or maintain?
Yes. Set your goal to build or hold steady and the guidance flips to keeping your tanks topped up rather than running them down. The focus of the app is fat loss, but the physiology is the same either way.

Stop guessing what your body is doing

Set it up in three minutes. See your first fat-burn window tonight.

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