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What 16:8, 18:6 and 20:4 mean, how to start gently, what to eat in your eating window, what to drink while fasting – and who shouldn’t fast.
Intermittent fasting means you eat within a set window each day and take a longer break from food afterwards. Here’s what the three common rhythms mean, how to ease in, what goes on your plate and what you can drink during the break. It’s an eating rhythm, not a promise – and it isn’t right for everyone.
Intermittent fasting doesn’t ban any food. It sets when you eat: within a window during the day, followed by a longer break without food – much of it overnight.
This guide is about daily fasting by the hour. Other forms, such as 5:2 (eating very little on two days a week) or fasting for several days, are a different thing.
The first number is the hours without food, the second is your eating window. Together they add up to 24 hours.
More hours aren’t better. Pick the rhythm that fits your day and feels good. You never need to go beyond 16:8.
Skip intermittent fasting if any of these apply to you:
Diabetes or medication? Talk to your doctor first. With some diabetes medicines, long breaks from food can lead to hypos. Some other medicines need to be taken with food. Check with your doctor before you start – the same goes for any long-term illness.
If you feel dizzy, shaky or sick while fasting, eat something. Stopping early isn’t failing.
Intermittent fasting changes when you eat. What you eat matters just as much. In your window you should get full and get enough nutrients – fasting doesn’t mean eating as little as possible.
For a rough idea of how much you need in a day, use the calorie calculator. You eat that amount when fasting too – just in fewer hours. Eating less than 1,200 kcal a day (women) or 1,500 kcal (men) over a longer period isn’t a good idea.
For more ideas, see intermittent fasting recipes: meals with at least 20 g protein and 400 to 800 kcal per serving. Training in your eating window? Read eating before and after exercise.
Anything with calories ends the break: milk or oat milk in your coffee, juice, fizzy drinks, smoothies and alcohol. Drink enough over the day – during the break you miss the glass you’d usually have with a meal.
On coffee: the European Food Safety Authority (EFSA) says up to 400 mg of caffeine a day doesn’t raise safety concerns for healthy adults. That’s about four cups of filter coffee.
In the SmarTasty app you start a fast on Today under “Body”. Pick 16:8, 18:6 or 20:4 and when it began, for example since your last meal. When the time is up you get a notification – at night it stays quiet. Your history shows every fast and your average length. More about intermittent fasting in SmarTasty.
There are lots of studies on intermittent fasting, but most are short and small. It’s often hard to tell what comes from fasting and what comes from eating differently in a shorter window. Solid data over many years is missing.
So we don’t promise anything. See intermittent fasting as a rhythm that may suit your day, not as a strict regime. If it doesn’t feel good, stop. Three meals a day is just as fine.
Water, unsweetened tea and black coffee. Milk in your coffee, juice, fizzy drinks, smoothies and alcohol all have calories and end the break.
16:8 is the most common way to start. But begin smaller: a 12-hour break first, then add an hour a week. Whether you stay at 14 or 16 hours is up to you.
The length of the break: 16, 18 or 20 hours without food, with an eating window of 8, 6 or 4 hours. More hours aren’t better, and 20:4 is hard for most people to stick to.
As much as you’d normally eat in a day, just in fewer hours. Fasting doesn’t mean eating less than you need. Eating under 1,200 kcal a day (women) or 1,500 kcal (men) over a longer period isn’t a good idea.
Yes. Many people put their workout inside their eating window so they can eat afterwards. If you feel dizzy, eat something.
Not under 18. While you’re still growing, you need regular energy and nutrients. The SmarTasty app doesn’t suggest fasting to anyone under 18.
Anyone who is pregnant or breastfeeding, has or has had an eating disorder, is underweight or is under 18. If you have diabetes or take medication, only after talking to your doctor.