Is intermittent fasting safe?

For most healthy adults, time-restricted eating in the 14:10 to 18:6 range is well tolerated and supported by a growing body of research (Mattson 2019, NEJM). It is not recommended for children, pregnant or breastfeeding people, those with a history of eating disorders, type 1 diabetes, or people on insulin or sulfonylurea medications without medical supervision.

Will I lose muscle on intermittent fasting?

Not if you eat enough protein and train. Meta-analyses show that 16:8 with adequate protein (1.6 g/kg or more) and resistance training preserves lean mass as well as standard calorie restriction. Spread your daily protein across 2 to 3 meals inside your eating window.

What can I drink during fasting, and does coffee break a fast?

Zero calorie drinks only: water, plain black coffee, plain tea (green, black, herbal), sparkling water, and electrolytes with no sugar or sweeteners. Plain black coffee does not raise insulin meaningfully and does not break a fast. Adding cream, milk, sugar, honey, juice, diet soda, or sweeteners (even artificial) is debated and may blunt some benefits. Bulletproof coffee with butter and MCT oil (200 to 400 kcal) breaks a fast in a strict sense but may still preserve some fat oxidation. For pure autophagy, stick to water, black coffee, and plain tea.

Do I need to eat keto to do intermittent fasting?

No. Intermittent fasting works on any macro split. Some people prefer lower carb intake to reduce hunger during the fast, but a balanced diet (40% carbs, 30% protein, 30% fat) works equally well for most people.

When does autophagy start during a fast?

Detectable autophagy in human studies starts around 16 to 18 hours of fasting and ramps significantly between 24 and 48 hours (Bagherniya 2018 review). Daily 16:8 hits the early autophagy window briefly; 18:6 and 20:4 spend meaningfully more time there.

What time should I stop eating for 16:8, and do sleeping hours count?

For a 16:8 schedule with a noon-to-8 PM eating window, your last meal time is 8 PM, after which you fast 16 hours until noon. Sleeping hours absolutely count toward your fast, which is why overnight fasting does most of the work: an 8 PM last meal plus 8 hours of sleep already covers half of a 16-hour fast. Below roughly 16 hours you are mostly clearing your last meal and tapping liver glycogen. Real fat oxidation, mild ketosis, and the early edge of autophagy show up between hours 12 and 18 for most adults, which is why 16 or more hours is the common target.

Can women do intermittent fasting?

Yes, but the evidence is mixed for very aggressive protocols. Most women tolerate 14:10 and 16:8 well. Daily 20:4 or OMAD has been associated in some reports with menstrual irregularity, hair shedding, and sleep disruption. Start gentle and back off if cycles, sleep, or mood change for the worse.

Will intermittent fasting slow my metabolism?

Short and medium-term studies (up to 12 months) do not show metabolic slowdown beyond what is expected from any weight loss. Your basal metabolic rate falls slightly as you lose body mass, but the resting energy expenditure adaptation seen in some IF studies is similar to daily calorie restriction.

Do I still need to count calories?

Not necessarily. Most people eat 5% to 15% fewer calories on 16:8 without conscious effort (Patterson & Sears 2017). If progress stalls after 4 to 6 weeks, then tracking calories inside your eating window helps you find the actual deficit.

Is OMAD (one meal a day) safe?

OMAD is the most aggressive daily protocol and is not appropriate for everyone. It is hard to hit protein and micronutrient targets in one sitting. Adherence drops to roughly 35% in long-term tracking. If you try OMAD, prioritize protein (40 g or more), vegetables, and a multivitamin, and limit it to 4 to 6 days per week rather than daily.

How fast will I see results on intermittent fasting?

Most people drop 1 to 4 lb in the first 2 weeks (largely water and glycogen). Real fat loss settles to roughly 0.5 to 2 lb per week depending on protocol and adherence. Expect a visible body composition change at week 6 to 8, not week 1.

Can I work out while fasted?

Yes. Most people tolerate moderate cardio and lifting in the second half of a 16:8 fast. Performance on long, high-intensity sessions may dip after week 1 to 2. Schedule heavy training sessions inside or right before your eating window for best results.

Free fasting tool

Intermittent Fasting Calculator

When does your eating window open?

Intermittent fasting splits your day into a fasting window and an eating window. The most popular is 16:8 (fast 16 hours, eat within 8), with 18:6, 20:4, and OMAD as stricter options. Fat burning ramps up around 12 hours in as glycogen drops, and autophagy begins near the 16 to 18 hour mark. Set your last meal and protocol below for exact window times and a live countdown.

Start fasting timer

Backed by Mattson 2019 NEJMReal-time countdownNo signup

Live previewSample

16:8 schedule

16h

fasting

Eat 12:00 PM · 8:00 PM

Glycogen12hBurning

Autophagy16hActive

Deep ketosis18hRamping

Next meal in

04:22:51

Live countdown

Autophagy at 16h

Protocol

Pick how long you fast each day or week.

16:8 (LeanGains). The most studied and most popular protocol. Touches autophagy at the tail of the fast. High adherence in trials.

Schedule

Set when your eating window starts. Times use your local clock.

Eating window starts

Typical: 12:00 PM for 16:8, 1:00 PM for 18:6, 4:00 PM for 20:4. With 16:8 eating window times of noon to 8:00 PM, that 8:00 PM close is what time to stop eating for the day.

Wake-up time

For reference. Helps you visualize the fast against your day.

About you

Tunes your TDEE estimate and weekly fat-loss projection.

Units

Weight (lbs)

Gender

Activity level

Primary goal

Your fasting schedule

Pick a protocol, set your eating window, and enter your weight. Results show here.

Side by side

Every IF protocol, ranked.

Adherence rates come from long-term tracking studies. Weekly fat-loss ranges assume reasonable protein and no overcompensation in the eating window.

Why adherence matters: The best protocol is the one you can stick with for 12 weeks. 16:8 has the highest long-term adherence in research, which is why most clinicians start there.

Saved locally

Saved schedules

Up to 8 entries stored in this browser only. Click any item to reload its inputs.

Save your first schedule to see it here

Fill in the calculator above and click Save schedule.

Method

Three steps to your fasting plan.

Pick a protocol, anchor an eating window, and let the calculator track your phase in real time.

Step

Pick a protocol

Choose 16:8 if you are starting out. Step up to 18:6 or 20:4 once 16:8 feels easy. OMAD and Eat-Stop-Eat are for experienced fasters.

Fast + Eat = 24 hours (per day)

Step

Anchor an eating window

Pick the start time of your eating window. Most people choose 12 PM (skip breakfast) for 16:8, or 1 PM for 18:6. Lock it in your local clock.

Window: 12:00 PM to 8:00 PM (8h eating)

Step

Read your phase

The calculator shows where you are in the fasting timeline: glycogen depletion, ketosis, autophagy, or HGH peak. A live countdown tracks the next eating window.

Phase = f(hours into fast)

Back to calculator

Hour by hour

What happens during a fast.

Your body cycles through predictable stages: digestion, glycogen depletion, ketosis, autophagy, and growth hormone elevation. The timing is well-mapped in human metabolic research. Fat burning starts during a fast once liver glycogen runs low, roughly 12 hours in, and how long you fast for autophagy matters too, since cellular cleanup only becomes detectable near the 16 to 18 hour mark. Because sleep counts toward fasting, the overnight hours quietly cover much of the work before you ever feel hungry.

0 to 4 h
Digesting
Insulin is up and glucose from your last meal is being absorbed and stored. Liver and muscle top up glycogen. No fasting benefits yet.
Cahill 2006, Annu Rev Nutr (fuel metabolism review)
4 to 8 h
Postabsorptive (early)
Insulin drops, glucagon rises. Body switches from storing energy to releasing it. Glycogenolysis (liver glycogen breakdown) kicks in to maintain blood sugar.
Berg, Tymoczko & Stryer, Biochemistry, 7th ed.
8 to 12 h
Glycogen depletion
Liver glycogen running low. Body shifts toward fat oxidation. Free fatty acids in the blood rise. Some people feel hungry, some feel sharper.
Mattson 2019, NEJM (review of fasting metabolism)
12 to 16 h
Fat burning ramps up
Mild ketosis begins. Ketones (beta-hydroxybutyrate, acetoacetate) start replacing glucose as brain fuel. Hunger waves typically peak then subside.
Mattson 2019, NEJM; Cahill 2006
16 to 18 h
Autophagy begins
Detectable autophagy: cellular cleanup machinery activates and starts recycling damaged proteins and organelles. Linked to longevity and cellular health.
Mizushima 2008, Cell; Bagherniya 2018 review
18 to 24 h
Deep ketosis + HGH
Ketones supply roughly 60% of brain energy. Growth hormone elevated 5x to 20x baseline. Insulin sensitivity improves overnight.
Hartman 1992, J Clin Endocrinol Metab (HGH spike); Cahill 2006
24 to 36 h
Deeper autophagy
More pronounced autophagy. Some stem cell activity (Cheng 2014 mouse data, partial human evidence). Beyond 24h, hydration and electrolytes matter.
Cheng 2014, Cell Stem Cell; Longo & Mattson 2014, Cell Metab
36 to 72 h
Prolonged fast
Significant ketosis (often greater than 3 mmol/L). Hormonal changes more pronounced. Should be done with medical supervision and electrolyte support.
Stewart & Fleming 1973 (early prolonged fast research)

Timing varies by person, recent diet, training, and prior fasting experience. Times above are typical for adults coming from a mixed diet.

The science

What the research actually shows.

Four mechanisms drive the most-discussed IF benefits. Each is anchored in peer-reviewed metabolic research.

Hour 8 to 16

Glycogen depletion

Once liver glycogen drops, your body has to make energy from fat. Free fatty acids climb, fat oxidation rises, mild ketosis starts. This is the metabolic switch behind IF fat-loss claims.

Marker

Insulin ↓ Glucagon ↑ FFAs ↑ Ketones ↑

Liver glycogen depleted around hour 10 to 14 (Cahill 2006)

Hour 12 to 18

Nutritional ketosis

Ketone bodies (beta-hydroxybutyrate, acetoacetate, acetone) rise as the brain switches partially to ketone fuel. Reported benefits: stable energy, less hunger, mental clarity for some.

Marker

BHB > 0.5 mmol/L = mild ketosis

Most adults hit mild ketosis by 14 to 16 hours of fasting

Hour 16 to 36

Autophagy

Cellular self-cleaning. Damaged proteins and organelles are tagged and recycled. Mostly mapped in animal models; human evidence is more limited but consistent for the 16 to 48 hour window.

Marker

mTOR ↓ → AMPK ↑ → autophagy

Bagherniya 2018 systematic review found measurable autophagy markers from ~16h

Hour 18 to 24

HGH elevation

Growth hormone rises 5x to 20x during prolonged fasts. Helps preserve lean mass during weight loss and supports recovery. Strongest evidence is in 24+ hour fasts (Hartman 1992).

Marker

HGH x5 to x20 baseline (24h fast)

Largest HGH spikes seen in fasts longer than 24 hours

Pick honestly

Which IF protocol is right for you?

The right protocol depends on your goal and how much restriction you can sustain. Adherence beats intensity over months. For most people doing intermittent fasting for weight loss, the best fasting window for beginners is 16:8, which is gentle enough to keep and well studied. The real 18:6 vs 16:8 question comes down to tolerance: 18:6 trims two more hours off your eating window for slightly faster results, but only if you can hold it without overeating later.

14:10 (gentle)

Beginner

A gentle on-ramp. Most adults already do close to 14:10 naturally. Few benefits beyond a calmer eating routine.

Schedule: Eat 9 AM to 7 PM, fast overnight.

Adherence

85%

Weekly loss

0.3–0.8 lb

16:8 (LeanGains)

Beginner

The most studied and most popular protocol. Touches autophagy at the tail of the fast. High adherence in trials.

Schedule: Eat 12 PM to 8 PM, fast 8 PM to 12 PM next day.

Adherence

78%

Weekly loss

0.5–1.5 lb

18:6

Intermediate

Pushes you firmly into autophagy territory every day. Harder to socialize around, but adherence is workable.

Schedule: Eat 1 PM to 7 PM, fast 7 PM to 1 PM next day.

Adherence

60%

Weekly loss

0.8–2 lb

20:4 (Warrior)

Advanced

The Warrior Diet. Four hour eating window forces concentrated, large meals. Hard to hit protein and micronutrients.

Schedule: Eat 4 PM to 8 PM, fast 8 PM to 4 PM next day.

Adherence

40%

Weekly loss

1–2.5 lb

OMAD (23:1)

Expert

One Meal A Day. Maximum daily autophagy depth. Hard to hit protein in one sitting. Not recommended for women of reproductive age long-term.

Schedule: Eat one large meal (e.g., 5 PM to 6 PM), fast the rest of the day.

Adherence

35%

Weekly loss

1.5–3 lb

5:2 (Fast Diet)

Intermediate

5 normal eating days plus 2 non-consecutive days at roughly 500 kcal (women) or 600 kcal (men). Different from time-restricted eating.

Schedule: Mon and Thu at 500 to 600 kcal; other days eat normally.

Adherence

58%

Weekly loss

0.8–1.8 lb

Eat-Stop-Eat

Advanced

Brad Pilon protocol. One or two 24 hour fasts per week (dinner to dinner), normal eating in between. Strong autophagy bursts.

Schedule: Dinner Sunday to dinner Monday; repeat once midweek.

Adherence

45%

Weekly loss

1–2 lb

Hydration

What you can drink during a fast.

Zero calorie liquids do not raise insulin and do not break a strict fast. Anything with calories does, even small amounts of cream or sugar.

Allowed

Water (plain or sparkling)
Plain black coffee
Plain tea (green, black, herbal)
Electrolytes with zero calories
Apple cider vinegar in water

Debated

Diet soda (artificial sweeteners)
Coffee with a splash of cream
Stevia or monk fruit
Bone broth (some calories, may help electrolytes)

These probably will not derail fat oxidation, but may blunt strict autophagy.

Breaks the fast

Milk, cream, oat milk, almond milk
Juice (even fresh)
Sugar, honey, agave
Sports drinks with sugar
Bulletproof coffee (200 to 400 kcal)

For caffeine timing relative to sleep, see our caffeine half-life and sleep guide. Most coffee inside your eating window will still affect sleep 8 to 10 hours later.

Support

Frequently asked questions.

Plain-English answers about fasting safety, autophagy timing, women and IF, and what actually breaks a fast.

Still have a question?

Tell us what we missed and we will add it here.

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Sources & reading

Where the numbers come from.

Methodology

Data sources

Fasting phase timing draws from human metabolic studies, especially the Mattson 2019 NEJM review and Longo & Mattson 2014 Cell Metabolism overview. Adherence rates come from long-term tracking studies; fat-loss estimates come from Patterson & Sears 2017.

01
de Cabo & Mattson, NEJM (2019)
Effects of Intermittent Fasting on Health, Aging, and Disease
02
Longo & Mattson, Cell Metab (2014)
Fasting: Molecular Mechanisms and Clinical Applications
03
Patterson & Sears, Annu Rev Nutr (2017)
Metabolic Effects of Intermittent Fasting
04
Bagherniya et al., Ageing Res Rev (2018)
The effect of fasting or calorie restriction on autophagy induction

Related guides

Long-form articles on protein timing, overnight fasting, cutting vs bulking, and caffeine clearance during your fast.

Editorial

Built by Hunzala Ashfaq, Founder

Updated May 18, 2026Cross-checked against peer-reviewed fasting research

Data Sources

Fasting windows use 14:10, 16:8, 18:6, 20:4, OMAD (23:1), 5:2, eat-stop-eat (24h once or twice weekly), and custom protocols. Phase indicator transitions at 0-4h (digesting), 4-8h (postabsorptive), 8-12h (glycogen depletion), 12-16h (mild ketosis), 16-18h (autophagy), 18-24h (deep ketosis + HGH), 24-36h (extended fast). TDEE is a quick weight-based estimate; use the full TDEE Calculator for precision. Results are educational, not medical advice.

Patterson RE, Sears DD, 2017 (Annu Rev Nutr) — Meta-analysis showing 5-10% spontaneous calorie reduction on 16:8 fasting protocols. Source of our weekly deficit estimates.
Mizushima N, 2008 (Cell) — Foundational review of autophagy biology. Basis for the 16-hour autophagy threshold in our phase timeline.
Cheng CW et al., 2014 (Cell Stem Cell) — Fasting and stem-cell regeneration research informing the 18-24 hour deep ketosis phase.
Hartman ML et al., 1992 (J Clin Endocrinol Metab) — Growth hormone response to fasting in healthy adults, cited in our HGH phase indicator.
Hall KD et al., 2011 (Lancet) — Energy balance model behind our 7,700 kcal/kg fat-loss math.

Related Guides

Built and maintained by Hunzala Ashfaq, founder of DrinkDigits. Last updated 2026-05-19. Calculations verified against official brand and public nutrition data.

This calculator is educational and for general reference only. It is not medical or dietetic advice. Consult a licensed healthcare provider for personalized nutrition guidance.

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Total daily energy expenditure with three formulas and six goal targets.

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