How many hours have you been fasting?
Tap the number to edit — results update instantly
Hours fasted
0
hours
tap to edit · any number
For educational purposes only — not medical or nutritional advice. Consult a healthcare provider before any extended fast.
👆
Tap your hours above
Enter how long you've been fasting to see your current biological phase
What can I consume?
Does it break the fast?
✓ Fine during fast
✓
WaterAny amount. Zero calories.
✓
Black coffeeNo milk, no sweetener. No insulin response.
✓
Plain teaAny type. Nothing added.
✓
Sparkling waterZero calories, no response.
✓
Salt in waterAddresses sodium depletion. Doesn't break fast.
✗ Breaks fast
✗
Milk or creamFat and protein raise insulin.
✗
Any sweetenerMany raise insulin without calories.
✗
Juice (any kind)Sugar. Ends fasted state immediately.
✗
BCAAsAmino acids activate mTOR.
✗
Any foodAny caloric intake ends the fast.
Bone broth contains protein and technically breaks a strict metabolic fast, though some use it during extended fasts for electrolyte support and comfort.
Why do I have a headache or cramps right now?
Why fasts fail and how to prevent it
Most fasting discomfort — headaches, fatigue, muscle cramps, brain fog — is not hunger. It's electrolyte depletion. When insulin drops during a fast, your kidneys release more sodium, and potassium and magnesium follow. Replacing these three minerals prevents nearly all common fasting symptoms.
What to do right now
Set your fasting hours above to see personalized electrolyte guidance for your current phase.
Reference table by fast duration:
| Duration | Sodium | Potassium | Magnesium | What to do |
|---|---|---|---|---|
| 12–24 hrs | 1,000–2,000 mg | 200–300 mg | 200 mg | A pinch of sea salt in water 1–2 times. |
| 24–36 hrs | 2,000–3,000 mg | 300–400 mg | 300 mg | Salt water 2–3x daily. Consider a potassium supplement. |
| 36–72 hrs | 3,000–4,000 mg | 400–600 mg | 400 mg | Critical. Salt water regularly + magnesium glycinate at night. |
The single most important tip: If you have a headache during a fast, add a pinch of sea salt to a glass of water and drink it. Wait 20 minutes. In most cases the headache resolves completely — because it was never from hunger.
How much protein should I eat to break my fast?
protein bolus timing protocol (popularized by Thomas DeLauer)
A large single protein bolus after an extended fast sustains muscle protein synthesis for 12+ hours — more effective than spreading protein across multiple smaller meals. Enter your details below.
90
grams
protein target for your first meal
At 175 lbs after a 36-hour fast, aim for 90 grams of protein in your first meal.
Each row below shows how much of that single food alone would reach your target. Choose one source, or combine smaller portions from several to hit 90 grams total.
Should I stop or push through?
Signal reference — green, yellow, red
✓
You reached your goal hours
Planned completion. Break it confidently.
✓
Clear, stable, and calm
The fasting zone. Continue or stop — both are correct.
!
Mild headache
Try salt in water first — wait 20 minutes. Resolves in most cases. If not, break the fast.
!
Hunger around hours 13–16
The metabolic valley. Temporary. Often passes in 1–2 hours.
✗
Dizziness or weakness
Break the fast. These signals override goals.
✗
Heart palpitations or confusion
Break immediately. Eat something. No goal is worth this.
⚠ Severe chest pain, fainting, or persistent palpitations: call emergency services immediately. Do not wait.
Advertisement
Women Fast Differently
A guide to fasting with your hormonal cycle
Why standard fasting advice doesn't always apply to women
If fasting feels harder for you than it seems to for others, there's a biological reason for that. Most fasting research was conducted on men. Research by Dr. Mindy Pelz and others shows that women who fast the same way every day throughout their monthly cycle can disrupt the hormonal signals that regulate estrogen and progesterone — potentially leading to irregular cycles, worsened PMS, and energy issues.
The key insight: different cycle phases have different fasting tolerances.
Days 1–10 — Power Phase
Longer fasts supported
Estrogen is building. Your body tolerates 16–24 hour fasts well in this window. Good time for extended fasting goals.
Days 11–15 — Ovulation
Keep fasts short
Estrogen and testosterone peak. Your body needs more fuel. Keep fasts under 15 hours. Avoid extended fasting this week.
Days 16–19 — Manifestation
Moderate fasting only
Progesterone is building. 13–16 hour fasts are reasonable. Avoid anything longer than 16 hours.
Days 20–28 — Nurture Phase
Minimal fasting
Progesterone and estrogen both elevated. This is a nurturing phase — focus on nourishment, not restriction. 13 hours max.
- Postmenopausal women have more flexibility — hormonal cycling is no longer a factor. Longer fasts can be practiced more consistently, though electrolyte management remains important.
- If you don't track your cycle, a general rule: err toward shorter fasts (16 hours) more consistently, rather than aggressive extended fasts.
- Signs fasting is disrupting your hormones: irregular periods, increased PMS symptoms, persistent fatigue, or difficulty sleeping after fasting.
This framework is based on research by Dr. Mindy Pelz (Fast Like a Girl). Her complete 30-day fasting cycle protocol and meal plans are in the book.
Save or share your fasting guide
Open · screenshot · send to anyone starting out
Common questions about fasting
Everything you need to know about extended fasting
What is the metabolic valley and why does it happen?
Around hours 13–17, both blood sugar and ketone levels are simultaneously low. Glucose stores are nearly depleted but ketone production hasn't fully ramped up — creating a brief window of low available fuel. You may feel hunger, brain fog, or irritability. This is the hardest moment of any extended fast, and the number one reason people quit unnecessarily. It passes within 1–2 hours once the liver fully shifts to ketone production. Knowing it's coming is what makes the difference between quitting and pushing through.
Does black coffee break a fast?
No. Black coffee contains no calories and does not raise insulin. It does not break a metabolic fast. Some research suggests it may enhance fat oxidation. The strict rule: no milk, no cream, no sweetener of any kind — including zero-calorie sweeteners, which can still trigger an insulin response in some people. Plain black only.
When does ketosis start during a fast?
Detectable ketones (0.5 mmol/L or higher) typically appear between hours 18 and 24. The exact timing depends on your last meal — a high-carbohydrate meal delays entry while a low-carb meal accelerates it. People who regularly follow a ketogenic diet often enter ketosis in 12–14 hours. By hours 24–36, deep ketosis is established. A blood ketone meter gives the most accurate confirmation.
When does autophagy start during fasting?
Autophagy begins initiating around hour 24 and becomes significantly elevated between hours 36 and 72. It is the cellular process of breaking down and recycling damaged proteins and organelles — sometimes described as the body running its cleanup crew. The 2016 Nobel Prize in Medicine was awarded to Yoshinori Ohsumi for discovering its mechanisms. Peak autophagy occurs beyond 72 hours. Protein intake, especially BCAAs, suppresses autophagy by activating mTOR.
Will fasting cause muscle loss?
Not significantly during typical 24–72 hour fasts. Human Growth Hormone rises significantly above baseline during deep ketosis — an evolutionary mechanism designed to preserve muscle during food scarcity. Ketone bodies also have direct anti-catabolic effects. A protein-forward first meal at the end of the fast further protects and rebuilds muscle. Significant muscle loss only becomes a concern with very prolonged fasting or inadequate protein in the refeeding period.
What should I eat to break my fast?
Lead with protein. For fasts under 24 hours: eggs, Greek yogurt, or chicken work well. For fasts of 24–72 hours: start gently with a small protein-forward meal (or a scoop of clean whey protein (affiliate)), wait 30–60 minutes, then eat your full refeeding meal. The key principle: do not break an extended fast with large amounts of simple carbohydrates or sugary foods. After prolonged insulin suppression, a large carbohydrate load can cause nausea, fatigue, and reactive hypoglycemia.
What is the difference between a 16-hour and a 36-hour fast?
A 16-hour fast delivers reliable fat burning and early ketosis and can be done daily — the most common intermittent fasting protocol. A 36-hour fast delivers deep ketosis with a significant hormone spike, the beginning of meaningful autophagy, and more substantial metabolic benefits, but requires electrolyte management and is typically done once or twice per week at most. A 72-hour fast adds peak autophagy and cellular renewal. Each level adds biological depth but requires more careful preparation.
Does fasting affect women differently than men?
Yes — significantly. Most early fasting research was conducted on men. Research shows that women who fast aggressively throughout their full monthly cycle can disrupt estrogen and progesterone production, leading to hormonal imbalance. Different phases of the menstrual cycle have different fasting tolerances: days 1–10 support longer fasts, while the ovulation window and luteal phase require shorter or no fasts. The Women's section above has the complete framework.
What are electrolytes and why do they matter during fasting?
Electrolytes are minerals — primarily sodium, potassium, and magnesium — that regulate critical body functions. When insulin drops during a fast, your kidneys excrete more sodium, and potassium and magnesium follow. This is why fasting headaches, muscle cramps, fatigue, and brain fog are almost always electrolyte depletion rather than hunger. A pinch of sea salt in water addresses sodium immediately. Most fasting discomfort that makes people quit is electrolyte-related and entirely preventable.
What is HGH and why does it increase during fasting?
Human Growth Hormone is produced by the pituitary gland and plays a key role in muscle preservation, fat metabolism, and cellular repair. During deep ketosis, it rises significantly above baseline — an evolutionary response to food scarcity that protects muscle while burning fat. This is one of the primary reasons extended fasting is more effective for body recomposition than simple caloric restriction: the hormonal environment actively preserves muscle while accelerating fat oxidation.
Can I exercise while fasting?
Yes. Light to moderate exercise — walking, Zone 2 cardio, yoga — pairs well with any phase of fasting. High-intensity exercise during the early phases is harder but manageable. During deep ketosis, energy typically stabilizes and exercise performance normalizes. Strength training during a fast is effective for body recomposition due to the elevated hormonal environment. Avoid heavy lifting during the metabolic valley at hours 13–17. Electrolytes are especially important during fasted exercise.
About this site — data sources and methodology
FastingPhases.com is an independent educational resource built on peer-reviewed research. Phase timings are derived from NCBI StatPearls, Endocrine Reviews (Oxford Academic), and published fasting research. Content reflects frameworks discussed publicly by Thomas DeLauer, Dr. Jason Fung, and Dr. Mindy Pelz — this site is not affiliated with, endorsed by, or sponsored by any of them. All phase timings are approximate and vary by individual metabolism, prior diet, activity level, and hormonal status. This site is for educational purposes only and is not a substitute for medical advice.