What happens when you fast for 24 hours?
Usually between 12 and 36 hours in, your body switches to burning fat. Five days needs a doctor or dietitian watching. Over a year, fast days did no better than eating a bit less daily.
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What happens in a 24-hour fast?
Your body uses up the sugar it stores in your liver. Then it switches to burning fat, and your liver turns some of that fat into another fuel called ketones. This switch usually comes between 12 and 36 hours after you stop eating.
Think of your liver sugar as a small fuel tank, and fat as the backup. How full the small tank was at the start, and how active you are while fasting, decide when you switch. So at 24 hours you may have switched, or be close.
Dietitians Australia says fasting helps weight loss simply because you eat less food.
Expect hunger. You may also feel weak and think less clearly, which may pass as your body adapts.
| Fast | What it looks like | What the research says |
|---|---|---|
| 24 hours | Dinner to dinner, once or twice a week. | Works for weight loss because you eat less. Not shown to beat other diets. |
| 36 hours | A whole day with no food, such as dinner Monday to breakfast Wednesday. | Done every other day for 4 weeks, healthy, middle-aged adults ate 37% fewer calories. |
| 5 days | Several days with little or no food. | Only with a doctor or dietitian watching over you. |
Sources: Dietitians Australia; Stekovic 2019; Anton 2018
Never fasted before? Dietitians Australia suggests you start by cutting out night-time snacks. Or try a gentler daily routine with 16:8 fasting.
What happens in a 36-hour fast?
A 36-hour fast means skipping one whole day of food. Stop after dinner on Monday and eat again at breakfast on Wednesday.
In one trial, healthy, middle-aged adults went a whole day without food every other day for 4 weeks. They ate 37% fewer calories on average and lost body fat.
The team saw no harmful effects, even in people who had done it for over 6 months. But it didn’t compare fasting with a normal calorie cut.
Want to try? Do a single 24-hour fast first and see how you feel.
Does a 24-hour fast help you lose weight?
Only if you eat less overall. In a year-long US trial, 100 adults with obesity had fast days, ate a bit less every day, or changed nothing.
These weren’t full 24-hour fasts. Fast days meant a quarter of normal food, every other day. Compared with the no-change group, fasters lost 6.0% of their weight and daily dieters 5.3%.
That’s the catch: fast days won nothing extra. Fasters ate more than planned on fast days, and more of them quit: 38% against 29%.
Their bad cholesterol (LDL) was also higher at 12 months than in the daily dieters.
So pick the plan you can keep up. Compare the options in the best diet to lose weight, and see how to keep muscle while you lose weight.
Is a 5-day fast safe?
Not on your own. Dietitians Australia says fasts of several days should only be done with a doctor or dietitian supervising.
Side effects are common, even with doctors on hand. One US clinic checked 768 stays of 2 days or more on water only. People on medicines they couldn’t stop weren’t allowed to fast.
| Side effect | Share of stays |
|---|---|
| Tiredness | 48% |
| Trouble sleeping | 34% |
| Feeling sick | 32% |
| Headache | 30% |
| Feeling close to fainting | 28% |
Source: Finnell 2018
Most were mild. But by day 5, about 1 in 5 had at least one rated severe. The authors are linked to the clinic.
NICE counts eating little or nothing for more than 5 days as a risk sign in patients. Add one more, such as a BMI (weight for height) under 18.5 or taking insulin. Then NICE sees a high risk of problems when eating starts again.
Thinking about 5 days? Talk to your GP first.
How to do a 24-hour fast
A dinner-to-dinner 24-hour fast
- 1
Eat dinner
Have a normal evening meal, then stop.
- 2
Drink through the day
Water, tea, coffee or sugar-free squash. Drink plenty.
- 3
Watch how you feel
You may feel weak or foggy at first. If it keeps up and gets in your way, stop and ask a health professional.
- 4
Break the fast
Dinner again, with veg, fruit, wholegrains and lean protein.
Sources: Diabetes UK; Dietitians Australia
Before your first 24-hour fast
- Cut out night-time snacks first, then build up to a full day.
- Don’t fast if you’re pregnant, breastfeeding or a teenager.
- If you’re underweight or have had an eating disorder, only fast with a health professional’s support.
- Take insulin or other diabetes medicine? Talk to your healthcare team before you fast.
- Having cancer treatment? Ask your medical team first.
- Eat normally after the fast. Fasting works for weight loss because you eat less overall.
Sources: Dietitians Australia; Diabetes UK
Never stop or change a prescribed medicine without your prescriber.
For the days you do eat, use a weight loss meal plan.
When to speak to a GP
Low blood sugar is rare in people without diabetes. See your blood sugar levels by age. Book an appointment if:
- You take insulin or other diabetes medicine, such as gliclazide, and want to fast. Low blood sugar is more likely on insulin and some other diabetes medicines.
- You don’t have diabetes but get shaking, sweating, a fast heartbeat and confusion.
- You eat very little, have very strict food habits or worry a lot about your weight and shape. The NHS says see a GP as soon as you can.
- Side effects of fasting carry on and get in the way of your day.
- You want to fast for several days.
You can also call the eating disorder charity Beat on 0808 801 0677.
Call 999 if someone with very low blood sugar isn’t responding normally or is unconscious, and:
- there’s no glucagon injection (an emergency medicine for very low blood sugar), or you don’t know how to use it
- you’ve given one and they haven’t recovered within 10 minutes
- they’ve been drinking alcohol