What is 18:6 fasting?
Fast 18 hours, eat in 6. Say 1pm to 7pm. For 20:4, eat in 4, like 3pm to 7pm. In one small study of 8 men, though, the fast wasn’t the hard part.
On this page
What is 18:6 intermittent fasting?
You eat all your food within 6 hours, then go 18 hours without food. In a US trial, the 18:6 group ate from 1pm to 7pm.
20:4 is the same idea with a 4-hour window. That trial’s 20:4 group ate from 3pm to 7pm, so each fast ran 20 hours.
| Plan | Eat between | Fast for |
|---|---|---|
| 18:6 | 1pm and 7pm | 18 hours, 7pm to 1pm |
| 18:6, early | 7am and 1pm | 18 hours, 1pm to 7am |
| 20:4 | 3pm and 7pm | 20 hours, 7pm to 3pm |
| 16:8, a gentler start | 10am and 6pm | 16 hours, 6pm to 10am |
Sources: Cienfuegos 2020; Sutton 2018; Diabetes UK
Most of the fast happens while you sleep. On a 1pm start, breakfast goes and lunch is your first meal. On 20:4, lunch goes too.
The early version suits morning eaters. In one study, men ate at 7am, 10am and 1pm, then nothing until breakfast.
Pick the row that fits your work days and your weekends.
Is 18:6 or 20:4 better for weight loss?
Neither, in the one trial that tested both. After 8 weeks, each group lost 3.2% of their weight. A group that ate as normal barely changed.
| Result | 18:6 (1pm to 7pm) | 20:4 (3pm to 7pm) |
|---|---|---|
| Weight | Down 3.2% | Down 3.2% |
| Food eaten | About 550 kcal a day less | About 550 kcal a day less |
| Fat | Down 1.4 kg | Down 2.8 kg |
| Lean mass, which includes muscle | Down 1.5 kg | Down 0.8 kg |
Source: Cienfuegos 2020
Why did it work? People ate about 550 calories a day less, without counting them. NICE says weight loss needs you to take in fewer calories than you use.
Keep it in proportion. The trial had 58 adults, mostly middle-aged women with obesity. Its authors say 3% falls short of the 5% loss that counts as a meaningful change for health.
The 18:6 group also lost more lean mass. To protect yours, see keeping muscle while you lose weight and protein for weight loss.
Comparing all the fasting types? See intermittent fasting for weight loss.
Is an 18 hour fast hard?
Less than you might expect. In a tiny study, 8 men ate in 6 hours with dinner before 3pm. They had prediabetes, meaning blood sugar above normal but not yet diabetes.
They rated eating within 6 hours as harder than fasting for 18. The hard part was the 6-hour eating window, fitting a day’s food into it.
All but one adjusted within about 2 weeks. Afterwards, they thought an eating window of about 8 hours would work for most people.
How the first weeks went on 18:6 and 20:4 in one trial
- 1
Week 2
Mild dizziness, feeling sick, headaches and diarrhoea peaked.
- 2
Week 3
Those had gone by week 3.
- 3
Ongoing
Constipation and dry mouth showed up by week 2 and lasted for some or most of the trial.
Source: Cienfuegos 2020
What can you drink? The trial asked people to drink plenty of water. They could also have black tea, coffee and diet fizzy drinks.
Diabetes UK lists non-alcoholic drinks only while you fast. Hunger is the main side effect, and you may feel weak at first.
If 6 hours feels too tight, start with 16:8 fasting instead.
Is 18:6 or 20:4 fasting safe?
It isn’t for everyone, and long-term evidence is thin. NICE has no recommendation on intermittent fasting. It has asked for more research.
| If you are | The advice |
|---|---|
| Pregnant or breastfeeding | Intermittent fasting isn’t suitable. |
| A teenager | Not appropriate for teenagers. |
| Underweight, or have had an eating disorder | Not for you, unless a health professional supervises. |
| Taking insulin or other diabetes medicine | Speak to your healthcare team first. Some medicines can make blood sugar drop too low. |
| Having treatment for cancer | Speak to your medical team before you fast. |
Sources: Dietitians Australia; Diabetes UK
On diabetes medicine? Your team can tell you what to change to keep you safe while you fast.
Both 18:6 and 20:4 mean eating in under 8 hours. One study linked that to a 91% higher risk of dying from heart and circulatory disease.
The BHF says that study can’t show cause and effect. It asked about just 2 days of eating, and other researchers hadn’t checked it. So it doesn’t prove harm, and nothing yet proves long-term safety.
Never stop or change a prescribed medicine without your prescriber.
How to start 18:6 fasting
How to build up to 18:6
- Cut out night-time snacks first.
- Find a way of fasting that feels right for you, and be okay with changing it if you need to.
- Eat a nutritious meal in your window, with plenty of vegetables and fruit.
- Drink plenty of non-alcoholic drinks while you fast.
- If side effects carry on and get in the way of your day, stop and get advice.
- If 6 hours is too tight, an eating window of about 8 hours is a fair place to stay.
Sources: Dietitians Australia; Diabetes UK; Sutton 2018
Need meals that fit a short window? Try the weight loss meal plan.
Thinking of a whole day off food? Read the 24 hour fast guide first.
When to speak to a GP
Book an appointment if:
- You take insulin or other diabetes medicine and want to try 18:6 or 20:4. Speak to your healthcare team first.
- You eat very little, have very strict food routines or avoid social events with food. Or you make yourself sick, take laxatives, exercise too much or worry a lot about your weight and shape. The NHS says see a GP as soon as you can.
- Along with those habits, your mood changes, or you feel cold, tired or dizzy. Or you get pain, tingling or numbness in your arms and legs, a racing heart, or you faint or feel faint.
- Your weight is very low or very high for your age and height, your periods stop, or other signs of puberty are late, or you have bloating, constipation or diarrhoea alongside those habits.
- Side effects of fasting carry on and get in the way of your day.
Worried about your eating? You can also call the Beat helpline on 0808 801 0677.