What happens to your body in a 48-hour fast?
By 48 hours, your body has usually switched to burning fat. The catch: in healthy men on water only, average blood sugar fell below the level the NHS usually calls low.
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What happens in a 48-hour fast?
In the first day, your body burns the sugar stored in your liver. Then it switches to fat, and to ketones, a fuel your liver makes from fat. That switch usually happens 12 to 36 hours in.
Think of a car with a small tank and a big reserve tank. Liver sugar is the small tank.
By hour 48 you are usually running on the reserve, and your brain uses ketones as well as sugar. The reserve keeps you going, but your blood sugar still drops, which is why the next section matters.
From the first day to day 3: what studies measured
- 1
12 to 24 hours
Liver sugar runs low. Blood sugar usually falls by a fifth or more.
- 2
12 to 36 hours
The switch to fat and ketones usually happens here.
- 3
48 hours
In 9 healthy men, hunger rose from 43 to 77 out of 100.
- 4
72 hours
In 6 healthy men, ketones rose from 0.1 to 2.3 mmol/L.
Sources: Longo 2014; Anton 2018; Solianik 2016; McDougal 2018
The 48-hour men also felt angrier. Their other moods didn’t change.
Never fasted for a whole day? Start with a 24-hour fast and see how you feel.
How much weight do you lose in a 48-hour or 3-day fast?
About 3 to 4% of your weight in two small studies of healthy men: 2.8% in 48 hours and 3.7% in 72 hours. Nine men went 48 hours on water and dropped from 89.3 kg to 86.8 kg on average. Six men fasted for 72 hours.
| Measure | 48 hours (9 men) | 72 hours (6 men) |
|---|---|---|
| Weight | 89.3 kg to 86.8 kg | Down 3.7% |
| Blood sugar | 4.81 to 3.74 mmol/L | About 4.7 to 3.9 mmol/L |
| Hunger | 43 to 77 out of 100 | Not reported |
Sources: Solianik 2016; McDougal 2018
Neither study measured how much of that weight was fat. So don’t judge the fast by the scale on day 3.
Fasting helps weight loss simply because you eat less. Most diets, fasting included, bring short-term loss followed by regain.
A year-long trial followed 100 adults with obesity. Eating a quarter of their usual food every other day did no better than a daily calorie cut: 6.0% lost against 5.3%.
Pick the plan you can keep up. Compare options in intermittent fasting for weight loss.
Is a 48-hour fast safe?
Watch your blood sugar. The NHS says low blood sugar is usually below 4 mmol/L. The 48-hour men averaged 3.74.
The 72-hour men wore sugar monitors. They spent 18% of the fast below about 3.9 mmol/L on average. Both groups were healthy, checked first, and the 72-hour men stayed in a research unit.
Low sugar can make you hungry, dizzy, anxious or irritable, sweaty, shaky, tired or weak, or confused. It can also cause tingling lips, a pounding heartbeat or blurred vision. If side effects keep up and get in the way of your day, stop and ask a health professional.
| If you | What the guidance says |
|---|---|
| Are pregnant, breastfeeding or a teenager | Don’t fast. |
| Are underweight or have had disordered eating | Only with a health professional’s supervision. |
| Take insulin or other diabetes medicine | Talk to your healthcare team first. |
| Are having cancer treatment | Speak to your medical team first. |
| Have gout | The NHS says avoid crash diets. Ask your GP first. |
Sources: Dietitians Australia; Diabetes UK; NHS
Want to go past 2 days? Dietitians Australia says fasts of several days need a doctor or dietitian supervising.
How to do a 48-hour fast
One simple way: stop after dinner one evening and eat again at dinner two days later. Pick two calm days, since hunger and anger both rose in the study.
Before and during your 48-hour fast
- Do a 24-hour fast first and see how you feel.
- Drink plenty of water. Tea, coffee and sugar-free squash are fine too.
- Take insulin or other diabetes medicine? Talk to your healthcare team before you fast.
- If side effects keep up and get in the way of your day, stop and ask a health professional.
- Break the fast with veg, fruit, wholegrains and lean protein.
Sources: Dietitians Australia; Diabetes UK
Never stop or change a prescribed medicine without your prescriber.
Fasting to lose weight? Read how to keep muscle while you lose weight.
When to speak to a GP
Low blood sugar is rare in people without diabetes. Check your numbers against blood sugar levels by age. Book an appointment if:
- You take insulin or other diabetes medicine, such as gliclazide, and want to fast.
- You don’t have diabetes but get symptoms of low blood sugar, such as shaking, sweating, a fast heartbeat and feeling confused.
- 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