Intermittent fasting methods compared: 16:8, 5:2, OMAD and alternate days
This article is general information, not medical advice. It is not a substitute for a doctor, dietitian or other qualified professional who knows your circumstances. If you have a medical condition, are pregnant, or are changing your diet or exercise significantly, talk to one first.
Intermittent fasting is any eating pattern that alternates set periods of eating with periods of not eating, or eating very little. The main approaches are daily time-restricted eating such as 16:8, the 5:2 diet, and alternate-day fasting. Research so far suggests they can help weight loss about as well as ordinary calorie reduction, mainly by helping some people eat less, rather than through any special effect. The fasting timer tracks daily fasts; this guide compares the methods and their limits.
What intermittent fasting is
Fasting periods are counted from the end of your last meal or snack. Traditional fasting is part of many religions, but the modern interest comes from diets that use fasting as a way to cut calories, and from research into how meal timing affects metabolism. Intermittent fasting does not specify which foods to eat, only when. That simplicity is its appeal: some people find a clear rule such as no food after 8 pm easier to keep than counting calories.
The main methods
- Time-restricted eating: a daily fasting window followed by an eating window. Common versions are 14:10 (14 hours fasting, 10 hours eating), 16:8, 18:6 and 20:4. Finishing dinner at 8 pm and eating again at noon is a 16:8 day. Much of the fast happens during sleep, which is why this is the easiest version to start with.
- One meal a day (OMAD or 23:1): the strictest daily version, with all food eaten in about an hour. It makes it hard to eat enough protein and nutrients and does not suit most people.
- The 5:2 diet: normal eating on five days of the week, and about 500 to 600 calories, roughly a quarter of usual intake, on two non-consecutive days.
- Alternate-day fasting: fast days, either with no food or with up to about a quarter of normal energy needs, alternate with normal eating days.
- Occasional 24-hour fasts once or twice a week, sometimes called eat-stop-eat.
What the research shows
Most human studies of intermittent fasting have found weight loss, typically in the range of a few percent of body weight over weeks to months, and trials that compare it with continuous calorie restriction generally find similar results. Some studies show improvements in insulin resistance, blood pressure and cholesterol, again broadly similar to other ways of losing weight. Long-term evidence is limited, and dropout rates vary widely, so the best method is the one you can stick to. There is no particular hour at which fat burning switches on: whether you lose weight still depends on total energy intake over time. The calorie calculator estimates your daily needs if you want to check your eating window against them.
Eating and drinking well
During a fast, water, black coffee and plain tea are generally fine; anything with meaningful calories, including milk in coffee, sugary drinks and snacks, ends it. Stay hydrated, since some of your usual fluid normally comes from food; the water intake calculator gives a rough daily target. In the eating window, aim for normal, balanced meals rather than making up for lost time with large portions. Getting enough protein is harder with a short window, especially for older adults and people who train; How much protein do you actually need? covers the amounts. Early side effects such as hunger, irritability, headaches and poor concentration are common and usually ease within the first few weeks.
Who should not fast
Intermittent fasting is not recommended for everyone. Avoid it, or talk to a doctor first, if you:
- are pregnant, trying to conceive or breastfeeding;
- are under 18, or are older and frail or underweight;
- have, or have had, an eating disorder;
- have diabetes, especially if you take insulin or other medicines that lower blood sugar, because fasting can cause dangerous lows;
- take medicines that need to be taken with food, or have another long-term health condition.
Stop and seek advice if fasting makes you dizzy, faint or unwell, or if it starts to feel like an obsession.
Tracking your fasts
The fasting timer offers 14:10, 16:8, 18:6, 20:4, 23:1 and custom schedules. Press Start after your last meal and a ring fills until you reach the goal; if you forgot to start it, you can edit the start time. Ending a fast logs it, and the history shows your streak of days with a completed fast, how often you hit the goal and your average fast length. It keeps counting if you close the tab because the start time is saved in your browser, but it cannot send reminders, and nothing is sent to a server. If weight is the goal, pair it with the calorie deficit calculator to see how a realistic deficit translates into weekly change.
Sources and further reading
The claims in this guide rest on these references, which were checked when the guide was last updated. Spotted an error? The contact page says how to report it.