Intermittent fasting is an eating pattern that involves regular fasting.
The 5:2 diet, also known as The Fast Diet, is currently the most popular intermittent fasting diet.
It was popularized by British doctor and journalist Michael Mosley.
It's called the 5:2 diet because five days of the week are normal eating days, while the other two restrict calories to 500–600 per day.
This diet is actually more of an eating pattern than a diet. There are no requirements about which foods to eat, but rather when you should eat them.
Many people find this way of eating to be easier to stick to than a traditional calorie-restricted diet (1).
This article explains everything you need to know about the 5:2 diet.
The 5:2 diet is actually very simple to explain.
For five days a week, you eat normally and don’t have to think about restricting calories.
Then, on the other two days, you reduce your calorie intake to a quarter of your daily needs. This is about 500 calories per day for women, and 600 for men.
You can choose whichever two days of the week you prefer, as long as there is at least 1 non-fasting day in between.
A common way of planning the week is to fast on Mondays and Thursdays, with 2 or 3 small meals, then eating normally for the rest of the week.
You should eat the same amount of food as if you hadn't been fasting at all.
Bottom Line: The 5:2 diet involves eating normally for five days a week, then restricting your calorie intake to 500–600 calories on the other two days.
There are very few studies that test the 5:2 diet specifically.
One study showed that the 5:2 diet caused weight loss similar to regular calorie restriction. Additionally, the diet was very effective at reducing insulin levels and improving insulin sensitivity (8).
One randomized controlled trial in both normal weight and overweight individuals showed major improvements in the group doing 4:3 fasting, compared to the control group that ate normally (12).
After 12 weeks, the fasting group had:
- Reduced body weight by more than 5 kg.
- Reduced fat mass by 3.5 kg, with no change in muscle mass.
- Reduced blood levels of triglycerides by 20%.
- Increased LDL particle size (which is a good thing).
- Reduced levels of CRP, an important marker of inflammation in the body.
- Decreased levels of leptin by up to 40%.
Bottom Line: The 5:2 diet may have several impressive health benefits. These include weight loss, reduced insulin resistance and decreased inflammation. Blood lipids may also be improved.
If you need to lose weight, the 5:2 diet can be very effective when done right.
This is mainly because the 5:2 eating pattern helps you consume fewer calories.
Therefore, it is very important not to compensate for the fasting days by eating much more on the non-fasting days.
That being said, fasting protocols similar to the 5:2 diet have shown a lot of promise in studies on weight loss:
- A recent review found that modified alternate day fasting caused weight loss of 3–8% over the course of 3–24 weeks (15).
- In the same study, participants lost 4–7% of their waist circumference, meaning that they lost a lot of harmful belly fat.
- Intermittent fasting causes a much smaller reduction in muscle mass than weight loss with conventional calorie restriction (15, 16).
Intermittent fasting is even more effective when combined with exercise, such as endurance or strength training (17).
Bottom Line: The 5:2 diet should be very effective for weight loss, if done correctly. It may help reduce belly fat, as well as help maintain muscle mass during weight loss.
There is no rule as to what or when you must eat on the fasting days.
Some people function best by beginning the day with a small breakfast, while others find it best to start eating as late as possible.
Generally, there are two meal patterns that people use:
- Three small meals: Usually breakfast, lunch and dinner.
- Two slightly bigger meals: Only lunch and dinner.
Since calorie intake is limited — 500 for women and 600 for men — it makes sense to use your calorie budget wisely.
Here are a few examples of foods that may be suitable for fast days:
- A generous portion of vegetables.
- Natural yogurt with berries.
- Boiled or baked eggs.
- Grilled fish or lean meat.
- Cauliflower rice.
- Soups (for example miso, tomato, cauliflower or vegetable).
- Low-calorie cup soups.
- Black coffee.
- Still or sparkling water.
There is no specific, correct way to eat on fasting days. You have to experiment and figure out what works best for you.
Delicious Low-Calorie Meals
There are plenty of websites with delicious meal plans and recipes for the 5:2 diet.
- Check out this site for plenty of low-calorie meal ideas.
- This site offers ideas for 10 fasting days that are worth checking out.
- Here are 27 meal plans for 500-calorie fast days.
- You can find all kinds of information and recipes on the chat forum of the official Fast Diet website.
- There are also several books and cookbooks available for the 5:2 diet, including the original best-selling The Fast Diet book.
Bottom Line: There are many meal plans and recipes available on the internet for 500–600 calorie fast days. Sticking to nutritious, high-fiber and high-protein foods is a good idea.
During the first few fast days, you can expect to have episodes of overwhelming hunger. It is also normal to feel a little weaker or slower than usual.
However, you'll be surprised about how quickly the hunger fades, especially if you try to keep busy with work or other errands.
Additionally, most people find that the fast days become easier after the first few fasts.
If you are not used to fasting, it may be a good idea to keep a small snack handy during your first few fasts, just in case you feel faint or ill.
But if you repeatedly find yourself feeling ill or faint during fast days, then have something to eat and talk with your doctor about whether you should continue.
Intermittent fasting is not for everyone, and some people are unable to tolerate it.
Bottom Line: It is normal to be hungry or feel a little weaker during the first few fasts. If you repeatedly feel faint or ill, then you should probably stop the diet.
Although intermittent fasting is very safe for healthy, well-nourished people, it does not suit everyone.
Some people should avoid dietary restrictions and fasting completely. These include:
- Individuals with a history of eating disorders.
- Individuals sensitive to drops in blood sugar levels.
- Pregnant women, nursing mothers, teenagers, children and individuals with type 1 diabetes.
- People who are malnourished, underweight or have known nutrient deficiencies.
- Women who are trying to conceive or have issues with fertility.
Some women have reported that their menstrual period stopped while they were following this type of eating pattern. However, things went back to normal when they returned to a regular diet.
Women should therefore be careful when starting any form of intermittent fasting, and stop doing it immediately if any adverse effects occur.