During a fast, most people burn roughly their resting calories—about 0.9–1.1 kcal per kg per hour—with small shifts across 24–72 hours.
Short Fast (≤24 h)
36–48 Hours
72 Hours+
12–18 h Time-Restricted
- Eats in a shorter window.
- Burn tracks resting needs.
- Ketones begin to rise.
Everyday window
24–36 h Short Fast
- RMR stable or slightly up.
- Glycogen mostly depleted.
- Stay hydrated, add electrolytes.
Occasional reset
48–72 h Extended
- Burn can taper a bit.
- Move gently; sleep well.
- Plan a balanced refeed.
Advanced
Calories Burned During A Fast: The Baseline
Your body keeps the lights on around the clock. Even without meals, you’re burning energy to breathe, pump blood, run the brain, and regulate temperature. Exercise adds on top, but the background burn is steady. A handy yardstick here is the metabolic equivalent, or MET. By convention, 1 MET equals about 1 kilocalorie per kilogram per hour and matches quiet sitting; it’s also defined as ~3.5 ml O2/kg/min. This lets you estimate resting burn with simple math: weight (kg) × 1 ≈ kcal per hour at rest (Compendium definition).
Quick Math For Resting Burn
Use body weight in kilograms. A 70-kg person burns ~70 kcal per hour at rest. Over 24 hours, that’s near 1,680 kcal. Short fasts don’t erase this baseline; in many studies it holds steady, and in some cases bumps slightly for a day or two.
Estimated Resting Calories During A Fast
The table uses the 1 MET convention and shows per-hour and 24-hour totals. Real life varies a touch with age, body composition, sleep, and temperature.
| Body Weight (kg) | Per Hour (kcal) | 24 Hours (kcal) |
|---|---|---|
| 50 | 50 | 1,200 |
| 60 | 60 | 1,440 |
| 70 | 70 | 1,680 |
| 80 | 80 | 1,920 |
| 90 | 90 | 2,160 |
| 100 | 100 | 2,400 |
Real days aren’t perfectly still. You stand, walk to the sink, change posture, maybe take a gentle stroll. Those add a few hundred calories on top of the baseline. If you’re mapping a plan, it helps to pair this math with a simple calorie deficit plan so the numbers connect to goals without guesswork.
What Short Fasts Do To Metabolism (24–48 Hours)
Classic lab work in healthy adults shows resting burn staying level, and in some trials nudging up, across one to two days without food. A widely cited study found no drop after a 48-hour fast in women, with a slight upward trend that matched earlier work in men (British Journal of Nutrition, 1990). Another trial linked a small rise in resting energy on short-term starvation to higher norepinephrine levels (Am J Clin Nutr summary cited widely). The take-home: across a day or two, burn doesn’t crash.
Why The Number Doesn’t Tank Immediately
Early on, your body leans on glycogen, raises circulating fatty acids, and shifts toward ketones. Catecholamines help keep energy output steady while you’re not eating. A medical review in the New England Journal of Medicine notes this “metabolic switch” starting around the half-day mark in humans, with ketones rising as fasting time extends (NEJM review).
What Longer Fasts Change (72 Hours And Beyond)
Once you stretch past two to three days, many people start to see a modest taper in energy outlay. Work in men and women across 12, 36, and 72 hours tracked hormones, heart rate, and substrate use; the pattern suggested stability early, then signs of adaptation later (British Journal of Nutrition, 1994). Reviews summarizing this literature describe a small drop emerging around the third day for many, with magnitude shaped by body size, sex, and prior diet.
Weight Loss Also Shifts The Baseline
As pounds come off, the resting number trends down since a smaller body costs less to run. Changes in fat-free mass drive much of this. That means a fast that leads to steady weight loss will eventually show a lower daily burn, even when you’re not eating.
How To Estimate Your Day’s Burn During A Fast
Start with 1 MET math. Convert your weight to kilograms (lb ÷ 2.2). Multiply by 1 to get calories per hour at rest. Log light movement in METs to refine the day’s total.
Step-By-Step Estimate
- Resting base: weight (kg) × 1 = kcal per hour.
- Hours resting or sleeping: count near 1.0 MET.
- Standing, puttering, easy chores: use 1.3–2.5 METs.
- Short easy walk: use ~2.0–3.0 METs.
Those MET values come from the standard compendium used by clinicians and researchers; it defines 1 MET as ~1 kcal/kg/hour and ~3.5 ml O2/kg/min, and lists common activities with assigned METs (Compendium site).
Light Movement During A Fast (70-kg Example)
Here’s how gentle activity layers onto the baseline for a 70-kg person. Calories per hour use MET × body weight.
| Activity | MET | kcal/hour (70 kg) |
|---|---|---|
| Resting / Sitting | 1.0 | 70 |
| Standing | 1.3 | 91 |
| Slow Walk (≈3 km/h) | 2.0 | 140 |
| Easy House Tasks | 2.5 | 175 |
| Leisure Walk (≈4 km/h) | 3.0 | 210 |
How Fasting Length Changes The Picture
12–18 Hours: Eating Window Shrinks
Most people see little change in total burn across the day. You’re still moving, still thinking, still sleeping. Ketones start to rise in this span while the background energy cost stays near the usual rate (NEJM review).
24–36 Hours: Short Fast
Studies report stable resting energy with some hints of a small bump. Norepinephrine can climb, which keeps the system ticking along. That’s why the “metabolism shuts off” line doesn’t match lab data in the first day or two (BJN 1990).
48–72 Hours: Extended Stretch
Now the curve can dip. Some trials show a modest reduction in resting burn as the body leans harder on fat and ketones and starts conserving a bit (BJN 1994). The range varies by the person; a lean endurance athlete won’t match a larger, less active desk worker.
How To Keep Muscle While You Fast
Short, gentle movement helps. Light walks, a few sets of body-weight work, and regular sleep keep signals pointed at muscle retention. Protein lands when you eat again; plan a balanced meal that includes lean protein, fibrous carbs, and fluids. That mix feeds recovery without a rebound binge.
Putting Numbers To Work
One-Day Sample Tally (70-kg Person)
- Sleep: 8 h × 1.0 MET → ~560 kcal
- Desk time: 10 h × 1.1 MET → ~770 kcal
- Standing / chores: 3 h × 1.8 MET → ~378 kcal
- Two short walks: 2 h × 2.5 MET → ~350 kcal
Estimated total → ~2,058 kcal. Swap a walk for a nap and the total slides down; add a brisk stroll and it rises. That’s the daily dial you can turn during a fasting day.
Common Myths, Plain Answers
“Metabolism Stops When You Don’t Eat”
No. Short fasts leave resting burn near baseline, with research showing stability through 24–48 hours and only small shifts beyond that window (BJN 1990).
“You Burn More Just By Not Eating”
Not really. You still burn a lot, but the main driver is your resting needs and any movement you do. The thermic effect of food does disappear while you’re not eating, which trims a small slice of daily burn, yet the background cost keeps rolling.
Safety Notes In Plain Language
If you live with a medical condition, take medications, or are pregnant or breastfeeding, fasting plans need medical oversight. Hydration and electrolytes matter on longer stretches. Refeed with balanced meals to avoid swings.
Recap
Across a day without food, calories burned line up with your resting needs plus any light movement. Stretch the fast to 48 hours and you’re still near baseline. Push toward three days and a gentle taper may show up. If your goal is fat loss, steady habits and sustainable movement do the heavy lifting, while fasting can be one tool. If you want a primer on methods, see our intermittent fasting basics.