5:2 Intermittent Fasting, 4:3 and Alternate-Day Fasting
Published 22 October 2026 · Last reviewed 22 October 2026 · Figures checked against official sources (how we check)
Short answer: 5:2 intermittent fasting means eating as usual on five days a week and about 500 kcal (women) or 600 kcal (men) on two days. The 4:3 method uses three fast days, and alternate-day fasting uses every other day. In trials, all three lost about as much weight as cutting calories every day, and 4:3 and alternate-day fasting sometimes a little more.
5:2 intermittent fasting splits the week into five ordinary days and two low-calorie fast days. A fast day is usually about 500 kcal for women and 600 kcal for men. Those numbers are a widely used convention, not an official limit. Each is about a quarter of a typical day’s food. The 4:3 method and alternate-day fasting use the same idea with more fast days.
How 5:2 intermittent fasting, 4:3 and alternate-day fasting work
A fast day here is a low-calorie day, not a day of no food. The other days have no calorie target, though eating extra on them to make up for the fast days shrinks the weekly deficit.
| Method | Fast days | Other names you may see |
|---|---|---|
| 5:2 | 2 a week (500 calories 2 days a week is the common version) | intermittent fasting 5/2 or 2/5, “5 days on 2 days off” |
| 4:3 | 3 a week | intermittent fasting 4/3, “3 days a week” |
| Alternate-day | every other day (3 or 4 a week) | every other day fasting, ADF |
The 5:2 and alternate-day planner uses about 500 kcal (women) or 600 kcal (men) for every fast day, whichever method you choose.
Some people do alternate-day fasting with no food at all on fast days. Dinner at 8 pm on Monday, nothing on Tuesday, then breakfast at 8 am on Wednesday is a 36-hour fast, often written 36/12. Any fast over 24 hours, this one included, needs a doctor’s advice first.
A single fast day, or intermittent fasting once a week, is a gentler version that none of the trials below tested.
“Intermittent fasting 5 days a week” can also mean a daily eating window on weekdays only, with weekends off. That pattern is a time-restricted schedule, and our guide comparing daily windows sets out how 16:8, 18:6, 20:4 and OMAD differ.
What did the 5:2, 4:3 and alternate-day trials find?
The main trials compared fast days with cutting calories every day:
| Study | Who and how long | Result |
|---|---|---|
| 5:2-style trial (2011) | 107 women, 6 months, fast days about 650 kcal on 2 days a week | Same weight loss as cutting calories every day |
| Alternate-day trial (2017) | 100 adults with obesity, 1 year, fast days at 25% of energy needs | Same weight loss as daily calorie cutting (6.0% vs 5.3% at 12 months); 38% dropped out vs 29% |
| 4:3 trial (2025) | 165 adults with overweight or obesity, 12 months, 80% less food on 3 days | About 2.9 kg more weight lost than daily cutting with the same weekly deficit |
| Review of 99 trials (2025) | 6,582 adults across many fasting methods | Alternate-day fasting lost about 1.3 kg more than daily cutting, mainly in trials under 24 weeks |
The alternate-day trial also tracked what people ate. They ate more than planned on fast days and less than planned on the days between. LDL cholesterol was also higher in that group after a year, in this one trial, so that evidence is early.
In the 4:3 trial, both groups had a full weight-loss support programme. Both were also asked to build up to 300 minutes of moderate activity a week. The authors called the 4:3 advantage “modestly greater”. Only 125 of the 165 people finished.
Taken together, the evidence is moderate. Several human trials agree that fast-day methods match daily calorie cutting for weight loss. One year-long trial and a review found a small edge for 4:3 or alternate-day fasting.
Is 16:8 better than 5:2 for weight loss?
None of the studies checked for this guide shows one beating the other. In a 12-month trial, an 8-hour eating window plus a calorie target gave weight loss close to the calorie target alone. A 12-week trial of 16:8 eating from noon to 8 pm found no extra loss over three meals a day. In the 2025 review, alternate-day fasting was the only method to beat daily calorie cutting, and only by about 1.3 kg.
The trade-offs are practical:
- 16:8 is a rule every day, with a full day’s food in an 8-hour window.
- 5:2 gives two hard days and five days with no window or target.
- Alternate-day gives the most fast days, and in the year-long trial more people dropped out.
Our intermittent fasting for weight loss guide explains why the weekly calorie total does most of the work.
A worked example of the weekly numbers
This example assumes a person who needs 2,000 kcal a day to keep their weight steady. They eat 500 kcal on fast days and exactly 2,000 kcal on the other days.
| Method | Weekly intake | Weekly deficit (vs 14,000 kcal) | Daily average |
|---|---|---|---|
| 5:2 | 11,000 kcal | 3,000 kcal | about 1,570 kcal |
| 4:3 | 9,500 kcal | 4,500 kcal | about 1,360 kcal |
| Alternate-day (3.5 fast days on average) | 8,750 kcal | 5,250 kcal | 1,250 kcal |
Real weeks can differ from the table. Eat 3,000 kcal on three of the five normal days and the 5:2 deficit is gone. If your weight is not moving, our guide to not losing weight on intermittent fasting goes through the usual causes.
Planning your fast days
The planner does the calendar work. Choose 5:2, 4:3 or alternate-day, pick your fast days and a start date, and it shows the next four weeks with each fast day marked. It warns you if the two 5:2 fast days you pick sit next to each other. One click adds the fast days to your calendar.
For the 500-calorie day itself:
- Plan protein and vegetables first. Protein is linked to feeling full.
- Count the small things. A medium apple is about 95 kcal. A tablespoon (15 ml) of olive oil comes to about 120, nearly a quarter of the day.
- Drink freely. Water, plain tea and black coffee (about 2 kcal a cup) add almost nothing.
- Eat normally the next day. Large make-up meals after a fast day undo the weekly deficit.
Who should not do fast days
Speak to a doctor before your first fast day if you take any medicine for diabetes or blood pressure. Insulin and sulfonylureas can cause low blood sugar, and a low-calorie day adds to that risk. Your doctor may need to adjust them. Never change diabetes medicines yourself.
Do not start fast days without your doctor’s advice if you are trying to conceive, pregnant, breastfeeding, under 18 or frail. The same goes for type 1 diabetes. Ask your doctor first, too, if you take a medicine that has to go with food. If you are underweight or an eating disorder is part of your history, we advise against fast days. Speak to a doctor instead. If fast days lead to binges or worry about food, stop and talk to a doctor. Free helplines in your country are listed at findahelpline.com.
To see how 5:2, 4:3 or alternate-day fasting would sit in your next month, open the 5:2 and alternate-day planner and pick your days.
Frequently asked
Is 16:8 better than 5:2 for weight loss?
Neither has been shown to be better. Trials of daily eating windows and of fast days both found weight loss similar to cutting calories every day. Pick the pattern you can keep for months: a daily rule, or two hard days and five free ones.
What can I eat on a 500-calorie day?
Build the day around protein and vegetables, because protein is linked to feeling full. Water, black coffee and plain tea add almost nothing. A medium apple is about 95 kcal and a tablespoon of olive oil about 120, so check labels and portions.
Can my two fast days be back to back?
They can, but the planner flags back-to-back fast days, and the year-long 4:3 trial used non-consecutive days. A normal day in between also lets you eat properly before the next fast day.
Is intermittent fasting twice a week beneficial?
In a 6-month trial of 107 women, two low-calorie days a week lost as much weight as cutting calories every day. A 2025 review of 99 trials found fasting diets and ordinary dieting gave similar benefits for weight and heart-health markers. So the benefit is real but no greater than daily calorie cutting.
Related guides
Intermittent Fasting vs OMAD: 16:8, 18:6 and 20:4 Compared
Intermittent fasting vs OMAD: how 16:8, 18:6, 20:4 and one meal a day compare on weight loss, hunger and protein, and how to pick one.
Intermittent Fasting for Weight Loss: What the Trials Show
Does intermittent fasting work for weight loss? What the trials found, how much to expect, and how to set your window, calories and protein.
Intermittent Fasting Not Losing Weight? What to Check First
Intermittent fasting and not losing weight? Check water weight, your real window, drinks and portions against your maintenance calories, in that order.
This article is general information, not medical advice. See our medical disclaimer for who should not fast.
Talk to a doctor before fasting longer than 24 hours. If you feel faint, confused or unwell, or have chest pain, stop the fast and get help: call your local emergency number (for example 999 in the UK, 911 in the US and Canada, 112 in the EU). If food, weight or eating is causing you distress, the support services in the footer can help.