Important Health Notice
FastBurn is a tracking tool, not medical advice. Intermittent fasting is not safe for everyone. Do not use this app if you are pregnant or nursing, under 18 years old, have a history of eating disorders, or have a medical condition — without first consulting your doctor. Extended fasts (24h+) carry real health risks including electrolyte imbalance, hypoglycemia, and refeeding syndrome. Always consult a healthcare professional before starting any fasting regimen.
What Makes FastBurn Different
Most fasting apps sell you a fantasy. FastBurn tells you the truth.
Research-Grounded Descriptions
Every metabolic phase description in the app cites what the research actually says — including effect sizes, study limitations, and where the science is still uncertain. No cherry-picked studies, no exaggerated claims.
Real Safety Warnings
Other apps celebrate 72-hour fasts with confetti. FastBurn shows you threshold warnings at 24h, 36h, 48h, and 72h with specific risks at each stage. We auto-cap at 72 hours because going beyond that without medical supervision is dangerous.
At-Risk User Blocking
Health screening during onboarding identifies users who should not attempt extended or aggressive fasting protocols. Those plans are locked — not upsold. We'd rather lose a subscriber than put someone in the hospital.
Refeeding Guidance
After extended fasts (24h+), what you eat first matters. FastBurn provides post-fast refeeding guidance — what to eat, what to avoid, and how to ease your digestive system back in safely.
8 Plans from Beginner to Advanced
Every plan shows the fast-to-eat ratio, difficulty level, and who it's appropriate for. Start easy, progress only when ready.
12:12 — Beginner
12h fast / 12h eat. The easiest entry point. You're probably already close to this if you don't snack after dinner. A natural starting place to build the habit without stress. Free plan.
14:10 — Easy
14h fast / 10h eat. A gentle step up. Skip the late-night snack and push breakfast back an hour. Most people can sustain this daily without difficulty. Free plan.
16:8 — Moderate
16h fast / 8h eat. The most popular IF protocol worldwide. Enough fasting time to enter fat-burning phase, short enough to be sustainable long-term. The sweet spot for most people. Free plan.
18:6 — Intermediate
18h fast / 6h eat. A tighter window that maximizes time in the fat-burning metabolic phase. For those who've mastered 16:8 and want to push further. Premium plan.
20:4 Warrior — Advanced
20h fast / 4h eat. Inspired by the Warrior Diet. A 4-hour eating window requires careful meal planning to hit nutritional needs. Not for beginners. Premium plan.
23:1 OMAD — Advanced
23h fast / 1h eat. One Meal A Day. All daily calories in a single sitting. Requires experience with shorter fasting windows first. Includes enhanced monitoring. Premium plan.
24h Eat-Stop-Eat — Expert
24h fast / eat normally next day. A full day without food, done 1–2 times per week. Triggers safety warnings and includes refeeding guidance. Not for daily use. Premium plan.
36h Monk Fast — Expert
36h fast / eat normally after. Extended fast with significant metabolic impact. Requires health screening, triggers multiple safety checkpoints, and includes detailed refeeding protocol. Premium plan.
5 Phases Your Body Moves Through
FastBurn shows you which metabolic phase you're in as your fast progresses. Each description is honest about what the research supports — and what it doesn't.
- Fed State (0–4 hours) — Your body is digesting and absorbing nutrients. Blood sugar and insulin are elevated. Energy comes from the food you just ate. This is not a fasting state — your body is processing its most recent meal.
- Catabolic Phase (4–12 hours) — Blood sugar normalizes, insulin drops. Your body begins tapping into glycogen (stored glucose) in the liver and muscles. You may feel slightly hungry as your body transitions fuel sources. This is the early fasting phase most people sleep through.
- Fat Burning (12–18 hours) — Glycogen stores are depleting. Your body increasingly switches to burning stored fat for energy via lipolysis. Growth hormone may increase. This is where most of the metabolic benefit of intermittent fasting occurs, according to current research.
- Ketosis (18–24 hours) — The liver begins converting fatty acids into ketone bodies as the primary fuel source. Mental clarity may improve for some people (individual variation is significant). Autophagy — cellular cleanup — is believed to increase in this window, though human data is still limited.
- Deep Ketosis (24+ hours) — Ketone levels continue to rise. Autophagy is more pronounced. However, electrolyte imbalance risk increases significantly. Muscle protein breakdown becomes a concern. This phase requires hydration, electrolytes, and ideally medical awareness. FastBurn shows safety warnings throughout this phase.
Built-In Guardrails at Every Stage
Threshold Alerts
Automatic safety warnings at 24h, 36h, 48h, and 72h. Each alert explains the specific risks at that duration — electrolyte depletion, hypoglycemia, refeeding syndrome — and recommends when to stop.
72-Hour Hard Cap
FastBurn automatically ends any fast at 72 hours. Fasting beyond 3 days without medical supervision is dangerous. We don't offer a way to override this — by design.
Health Screening
During onboarding, FastBurn asks health-relevant questions. Users who indicate pregnancy, eating disorder history, or being under 18 are blocked from extended and aggressive fasting plans. Not upsold — blocked.
Refeeding Guidance
After any fast over 24 hours, FastBurn shows specific guidance on how to break your fast safely. What to eat first, what to avoid, portion sizing, and how long to wait before eating normally. Refeeding syndrome is real — we take it seriously.
Measure What Matters
Weight Logging
Log weight entries in kg or lbs with optional notes. Track trends over weeks and months. Weight is noisy day-to-day — FastBurn shows the trend, not the noise.
Hydration Tracking
Log water intake during fasts. Dehydration is the most common fasting side effect — and the most preventable. Track glasses, get reminders, stay safe.
Fast History & Stats
Total fasts completed, success rate percentage, current streak, longest fast, and average duration. Real numbers that show your actual progress — not gamified vanity metrics.
Live Fasting Timer
Real-time countdown with elapsed time, total duration, progress bar, and current metabolic phase. One glance tells you everything: where you are, where you're going, and how long until you get there.
Apple Health + Apple Watch
HealthKit Sync
Weight, water intake, and fasting data sync to Apple Health automatically. One source of truth for your health data. HealthKit integration is entirely optional — you choose what to share, and you can revoke access at any time.
Apple Watch Companion
Real-time fasting timer on your wrist. See time remaining, current metabolic phase, and progress without pulling out your phone. Glanceable, minimal, and always up to date.
Honest App. Honest Pricing.
Free
- 3 fasting plans (12:12, 14:10, 16:8)
- Live fasting timer
- 5 metabolic phase tracking
- Basic fast history
- Safety warnings at all thresholds
- No ads, no tracking
Coming soon
FastBurn Pro
- All 8 fasting plans unlocked
- Custom plan builder
- Apple Watch companion
- Apple Health sync
- Weight logging (kg/lbs)
- Hydration tracking
- Advanced stats and streaks
- Refeeding guidance for extended fasts
Coming soon
Your Health Data Never Leaves Your Device
Local-Only Storage
All fasting data, weight logs, hydration entries, and history are stored locally on your device. Nothing is transmitted to our servers. Apple Health sync is optional and controlled entirely by you — we never read from HealthKit without your explicit permission.
RevenueCat for Purchases Only
If you subscribe to FastBurn Pro, purchase validation is handled by RevenueCat using an anonymous device identifier. No email, no name, no health data is shared. No analytics SDKs, no ad networks, no third-party tracking of any kind.