What can I have during fasting?

Strictly during fasting: water, plain black coffee, plain tea, electrolytes (sodium, potassium, magnesium with no calories or sweeteners). NOT during fast: coffee with cream/sugar, “bulletproof coffee” (breaks the fast metabolically), bone broth (contains calories and triggers insulin), juice, milk, anything with calories or significant sweetener content. Some IF schools allow tiny amounts of fat (1 tsp coconut oil) without breaking the fast — this is contested.

Will fasting cause muscle loss?

Some, but minimal at typical IF protocols (16-18 hours). The body protects muscle during shorter fasts via human growth hormone elevation and ketosis-derived energy. Mitigation: get adequate protein (0.7-1g per pound body weight) in your eating window, maintain resistance training, eat to satiety in your window. Extended fasts (over 24 hours) start risking more muscle loss; that's why short daily IF is preferred over multi-day fasting for body composition.

How long until I see results?

Adaptation period: 1-2 weeks of mild hunger, headaches, low energy as your body shifts metabolic flexibility. After 2-4 weeks: steady-state hunger normalized, mental clarity often improves, weight loss begins (modest — 1-2 lbs/week typical). Body composition changes: 2-3 months of consistent practice. Insulin sensitivity improvements: measurable in blood work after 6-8 weeks. Don't evaluate IF's effect after 1 week — your body is still adapting.

Should I work out fasted?

Light to moderate cardio: yes, fine fasted, may even improve fat oxidation. Heavy resistance training or high-intensity intervals: pre-fast or in your eating window for best performance. Sport-specific: depends — endurance athletes often do well training fasted (carb-fasted training improves fat utilization); strength athletes typically need fed training. Listen to your body — fasted workouts are an acquired adaptation.

The cellular process where damaged proteins and organelles are recycled — “cellular cleanup.” Triggered by fasting (low insulin signals nutrient scarcity, activates autophagy genes). Research suggests significant autophagy benefits start around 24-48 hours of fasting; some begins earlier at 16-18 hours. Long-term implications for healthy aging are studied but not yet conclusive. The autophagy promise is one motivation for longer extended fasts.

How do I break a long fast?

Start small with easily-digestible foods: bone broth, eggs, avocado, nuts. Avoid heavy meals or large carb bombs immediately — can cause refeeding syndrome (electrolyte shifts, GI distress). For fasts over 24 hours: 2-3 small meals over the first 4-6 hours, gradually returning to normal portions. Many people overeat after a long fast and undo the benefits; eat to comfortable satiety, not to compensate for missed meals.

Health & Fitness · Free tool

Fasting Timer

Start a free fasting timer showing elapsed, remaining, and eating window for any schedule. No sign-up, works instantly in your browser.

Updated June 2026

Fasting schedule

Started at —

Not started

00:00:00

16:00:00 remaining to 16h target

Target: 16:00:00 (16h)

Estimate only — consult a doctor or RD for medical advice.

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What it does

Intermittent fasting (IF) became one of the most-adopted dietary protocols of the 2020s, with research support for several health benefits including improved insulin sensitivity, modest weight loss, and metabolic-flexibility improvements. The dominant protocols are time-restricted eating (TRE) windows: 16:8 (16-hour fast, 8-hour eating window — the most popular, accessible, and research-supported), 18:6 (more aggressive, often skips breakfast entirely), 20:4 (Warrior Diet — eat one big meal in 4 hours), OMAD (One Meal A Day, ~23:1), and longer protocols like 5:2 (eat normally 5 days, fast 2 days at 500-600 cal). Most research support clusters around 16:8 and 18:6 — the more aggressive protocols (OMAD, 5:2) have less long-term safety data.

The timer starts when your last meal ended and counts down through your fasting period, then signals when the eating window opens. Display: elapsed time fasting (with a phase label — 0-12 hr glucose burning, 12-16 hr fat burning starts, 16-24 hr ketosis emerges, 24-48 hr deeper autophagy, 48+ hr extended fasting territory), remaining time until eating window opens, and eating-window duration. Customizable to any fast/eat ratio. Useful for tracking consistency over weeks (most benefits emerge after 2-4 weeks of regular adherence) and spotting compliance drift.

Practical guidance: most people hit steady-state benefits at 14-18 hour daily fasts (16:8 is the popular sweet spot). Below 12 hours, no metabolic switch occurs. Above 24 hours, you're in extended-fasting territory which has different physiology and isn't recommended without medical supervision for most people. Common pitfalls: (1) drinking coffee with cream/sugar — breaks the fast metabolically; only black coffee, plain tea, or water during fasting hours. (2) Overeating during the eating window — consuming 2,500 calories in 8 hours isn't weight loss; total calorie intake still matters. (3) Not getting enough protein in the compressed window — leads to muscle loss; aim for at least 0.7g protein per pound of body weight. (4) Caffeine timing — drinking coffee through most of fasting then trouble sleeping. Stop caffeine 8-10 hours before bedtime.

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How to use it

Pick your fasting protocol: 16:8, 18:6, 20:4, OMAD, or custom.
Set when your last meal ended (or click Start now).
Read elapsed time, current metabolic phase, and time until eating window opens.
Optionally save daily fasts to track consistency over weeks.

When to use this tool

Daily intermittent fasting tracking (16:8, 18:6 most common).
Tracking longer extended fasts (24-72 hour) under medical supervision.
Visual / mental motivation during fasting hours — seeing the countdown helps.
Comparing your actual eating window against intended (most people drift longer than they think).
Identifying which protocol matches your lifestyle and adherence pattern.

When not to use it

Pregnancy or breastfeeding — IF not recommended.
Type 1 diabetes or insulin-dependent type 2 — risk of hypoglycemia; consult endocrinologist.
History of eating disorder — IF can trigger relapse; work with a therapist instead.
Children, adolescents, anyone underweight or with high physical demands (athletes in heavy training).
Anyone on medications requiring food intake at specific times.

Common use cases

Onboarding a colleague who needs the same calculation/conversion
Verifying a number or output before passing it on
Quick use during a typical workday
Pre-decision sanity-check on inputs and outputs

Frequently asked questions

What can I have during fasting?: Strictly during fasting: water, plain black coffee, plain tea, electrolytes (sodium, potassium, magnesium with no calories or sweeteners). NOT during fast: coffee with cream/sugar, “bulletproof coffee” (breaks the fast metabolically), bone broth (contains calories and triggers insulin), juice, milk, anything with calories or significant sweetener content. Some IF schools allow tiny amounts of fat (1 tsp coconut oil) without breaking the fast — this is contested.
Will fasting cause muscle loss?: Some, but minimal at typical IF protocols (16-18 hours). The body protects muscle during shorter fasts via human growth hormone elevation and ketosis-derived energy. Mitigation: get adequate protein (0.7-1g per pound body weight) in your eating window, maintain resistance training, eat to satiety in your window. Extended fasts (over 24 hours) start risking more muscle loss; that's why short daily IF is preferred over multi-day fasting for body composition.
How long until I see results?: Adaptation period: 1-2 weeks of mild hunger, headaches, low energy as your body shifts metabolic flexibility. After 2-4 weeks: steady-state hunger normalized, mental clarity often improves, weight loss begins (modest — 1-2 lbs/week typical). Body composition changes: 2-3 months of consistent practice. Insulin sensitivity improvements: measurable in blood work after 6-8 weeks. Don't evaluate IF's effect after 1 week — your body is still adapting.
Should I work out fasted?: Light to moderate cardio: yes, fine fasted, may even improve fat oxidation. Heavy resistance training or high-intensity intervals: pre-fast or in your eating window for best performance. Sport-specific: depends — endurance athletes often do well training fasted (carb-fasted training improves fat utilization); strength athletes typically need fed training. Listen to your body — fasted workouts are an acquired adaptation.
What's autophagy?: The cellular process where damaged proteins and organelles are recycled — “cellular cleanup.” Triggered by fasting (low insulin signals nutrient scarcity, activates autophagy genes). Research suggests significant autophagy benefits start around 24-48 hours of fasting; some begins earlier at 16-18 hours. Long-term implications for healthy aging are studied but not yet conclusive. The autophagy promise is one motivation for longer extended fasts.
How do I break a long fast?: Start small with easily-digestible foods: bone broth, eggs, avocado, nuts. Avoid heavy meals or large carb bombs immediately — can cause refeeding syndrome (electrolyte shifts, GI distress). For fasts over 24 hours: 2-3 small meals over the first 4-6 hours, gradually returning to normal portions. Many people overeat after a long fast and undo the benefits; eat to comfortable satiety, not to compensate for missed meals.

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