Diet guide
Intermittent fasting, explained
Intermittent fasting changes when you eat rather than what you eat. Here's how the main schedules work, what a real day looks like, and how to tell if it suits you.
Intermittent fasting means going longer stretches without food, either every day (like 16:8, where you eat within eight hours) or on a couple of days a week (like 5:2). It helps some people eat less without counting calories. It isn't magic, though. In studies it tends to work about as well as ordinary calorie cutting, and what you eat in your window still matters.
16:8, 5:2 and other schedules
- 12:12. Twelve hours without food overnight, say 8pm to 8am. Many people already do this without calling it fasting. It's the easiest place to start.
- 16:8. A 16-hour fast and an 8-hour eating window, often noon to 8pm. This is the most popular form of time-restricted eating. 14:10 is a gentler version.
- 5:2. You eat normally five days a week and have about 500-600 calories on two non-consecutive days, for example Monday and Thursday.
- Alternate-day fasting. Every other day is a very low-calorie day. It's harder to stick to and most people drop out or switch.
- 24-hour or longer fasts. These are tougher, carry more risk, and are not something to try on your own if you have any health condition.
Does it work?
Fasting works mainly by shrinking the time you have to eat. Skip the late-night snacks and the mid-morning pastry and you'll often end up in a calorie deficit without tracking anything. That's the real appeal: one rule instead of an app full of numbers.
When researchers compare fasting with plain calorie cutting, weight loss is usually similar. Some studies find small improvements in blood sugar and insulin, but results vary and most trials last months, not years. Claims that fasting "resets your metabolism" or switches on some special fat burning go beyond the evidence. And if you eat a whole day's worth of food plus extra in your window, you won't lose weight.
After a month, it's working for you if:
- you're losing around 0.5-1 kg (1-2 lb) a week, or your waist is shrinking;
- the hunger at your old breakfast time has faded;
- you're not bingeing when the window opens;
- you're sleeping normally and have energy for work and exercise.
A day of eating on 16:8 and on 5:2
Here's what a 16:8 day with a noon to 8pm window can look like:
- Morning (fasting): water, black coffee or plain tea. Most people find the first hour after waking is the hardest.
- Lunch, around noon: a filling first meal, like a chicken, bean and vegetable wrap with a pot of yogurt, or two eggs on wholegrain toast with avocado and a piece of fruit.
- Snack, around 3:30pm: a handful of nuts and an apple, or hummus with vegetable sticks.
- Dinner, by 7:30pm: salmon or tofu with roasted vegetables and brown rice or potatoes. Then the kitchen is closed.
If evenings are your hard time, try an earlier window, such as 8am to 4pm. Some research suggests eating earlier in the day fits your body clock better, but the schedule you can actually keep matters more.
A 5:2 fast day at about 500-600 calories might be two boiled eggs and black coffee in the morning, then grilled chicken or fish with a large plate of vegetables in the evening. Drink plenty of water, keep the day quiet if you can, and eat normally the next day. Normally doesn't mean double.
What to eat in your eating window
An eight-hour window full of fizzy drinks, crisps and takeaway pizza isn't a healthy diet just because it ends at 8pm. You're eating fewer meals, so each one needs to count.
- Get protein at every meal (eggs, fish, chicken, beans, tofu, Greek yogurt). It keeps you full through the next fast.
- Fill half your plate with vegetables or salad.
- Choose slow carbs like oats, wholegrain bread, beans and potatoes with skins, rather than white bread and sweets.
- Break your fast with a proper meal, not biscuits. A sugary first bite often turns into grazing all afternoon.
The basics in healthy eating basics apply, and fasting pairs well with a Mediterranean-style diet.
How to start: your first two weeks
Days 1-4: start with 12:12. Finish dinner by 8pm and don't eat until 8am. The main change for most people is no late-night snacking.
Days 5-10: push breakfast back an hour or two, to 13 or 14 hours. You'll notice hunger at your usual breakfast time. It comes in waves and often passes within 15-20 minutes if you drink some water or a coffee. You might feel irritable or get a mild headache, especially if you usually have coffee with milk and a pastry.
Days 11-14: if it's going well, try 16:8 on a few days a week. You don't have to do it every day. Many people do 14:10 on weekdays and relax at weekends.
For most people, the hunger and grumpiness ease within two to four weeks. If you're still starving, lightheaded or thinking about food all day after a month, this probably isn't your method.
Side effects, mistakes and when to stop
Common side effects in the first weeks are hunger, irritability, headaches, feeling cold, trouble concentrating late in the fast, and poor sleep if you go to bed hungry. These usually fade.
The mistakes that trip people up:
- Starting at 18 or 20 hours. Build up slowly.
- Overeating when the window opens. Plan your first meal so you're not grabbing whatever's nearest.
- Not drinking enough. Some of your usual fluid comes from food. Keep water nearby.
- Too little protein. Fewer meals can mean less protein, which matters for keeping muscle.
- Letting "cream in coffee" drift into a latte. A splash is small. A large milky coffee is a snack.
Stop, or loosen up, if you find yourself bingeing, feeling guilty or anxious about eating outside your window, fainting or feeling dizzy, or if your periods become irregular. Easing off is simple. Widen your window by an hour a week until you're back to about 12 hours overnight, which is a healthy habit to keep for good. It helps to know what makes a diet sustainable.
Who shouldn't fast without advice
- You take insulin or diabetes tablets such as sulfonylureas. Going without food can drop your blood sugar dangerously low. Your doctor may need to change your doses.
- You're pregnant, breastfeeding or trying to conceive.
- You have had an eating disorder. Fasting rules can trigger restricting and bingeing.
- You're underweight, frail or older with a small appetite.
- You take medicines that must be taken with food.
- You're under 18.
Who fasting suits, and who it doesn't
| Situation | What it means for you |
|---|---|
| Likely a good fit | You're not hungry in the morning anyway, you snack late at night, you hate counting calories, and your meals are fairly regular day to day. |
| Could work with some care | You do shift work, train hard, or have a family who eat breakfast together. Pick a window that fits real life, or try 5:2 instead. |
| Probably a poor fit | You get shaky or can't concentrate when you skip meals, you tend to binge after restricting, or your sleep is already poor. |
| Needs medical advice first | Diabetes on insulin or sulfonylureas, pregnancy or breastfeeding, any history of an eating disorder, being underweight or frail, or anyone under 18. |
Still weighing up your options? Read how to choose a diet or how to spot a fad diet.
Fasting questions people ask
What can I drink while fasting?
Water, sparkling water, black coffee and plain tea, including herbal tea. Milk, sugar, juice and smoothies add calories and break the fast. A small splash of milk in coffee is a gray area that most people find doesn't make a difference to results.
Is 16:8 fasting safe?
For most healthy adults it appears to be safe in studies lasting up to about a year. A 2024 observational study presented at an American Heart Association meeting linked eating within eight hours to a higher risk of heart-related death, but it could not show cause and effect. If you have diabetes, are pregnant or have had an eating disorder, check with a doctor first.
Does intermittent fasting work better than counting calories?
Usually not for weight loss. Studies tend to find similar results when people eat similar calories. The advantage is that some people find a time window much easier to stick to than logging every meal.
Can I exercise while fasting?
Yes, light and moderate exercise is usually fine. For hard or long sessions, many people feel better training near the end of the fast and eating soon after. Stop if you feel dizzy or faint.
How long does it take to see results?
Give it four to six weeks. Hunger usually settles in the first two to four weeks, and steady weight loss of around 0.5-1 kg (1-2 lb) a week is a good sign.
Sources
If you take insulin or other diabetes medicine, are pregnant or breastfeeding, are underweight, or have had an eating disorder, please talk to your doctor or a dietitian before you try fasting. Skipping meals on some medicines can cause low blood sugar.