5:2 vs 4:3 Diet: Why Some People Are Adding a Third Low-Calorie Day
For years, the 5:2 Diet has been one of the best-known forms of intermittent fasting.
Its popularity is easy to understand.
Instead of trying to reduce calories every single day, the basic idea is to divide the week into two different types of days: five normal eating days and two days with substantially reduced calorie intake.
For many people, that feels more manageable than being “on a diet” seven days a week.
But there is another approach that follows a very similar principle: the 4:3 Diet.
Instead of five normal days and two lower-calorie days, 4:3 uses four normal eating days and three lower-calorie days.
It sounds like a small difference.
After all, we're talking about just one additional day per week.
But when weight loss depends heavily on the calorie deficit created over time, adding a third lower-calorie day can make a meaningful difference to the structure of the week.
So, if you already understand the appeal of 5:2 but aren't seeing the progress you hoped for, is 4:3 worth considering?
Let's look at the difference.
What Is the 5:2 Diet?
The 5:2 Diet is a form of intermittent energy restriction.
Rather than reducing calorie intake every day, you eat normally on five days of the week and substantially reduce your calorie intake on two days.
Different versions use slightly different calorie targets, but research into 5:2 has commonly used around a 75% energy deficit on the two restricted days.
One of the reasons the approach became so appealing is its simplicity.
You aren't necessarily counting calories and restricting yourself seven days a week. Instead, the more difficult part of the diet is concentrated into two specific days.
That distinction can be psychologically appealing.
Monday might be a lower-calorie day, for example, while Tuesday is normal again. Some people separate the restricted days across the week, while other versions allow them to be consecutive.
The important feature is the weekly structure:
5 normal eating days
2 lower-calorie days
And 5:2 can work.
A 50-week randomized controlled trial involving adults with overweight or obesity found that participants assigned to 5:2 intermittent calorie restriction lost weight. At the end of the study, average weight loss was approximately 5.2% in the 5:2 group.
So the argument for considering 4:3 isn't that the 5:2 Diet “doesn't work.”
The more interesting question is whether two lower-calorie days provide enough of a weekly calorie deficit for the results you're trying to achieve.
What Is the 4:3 Diet?
The 4:3 Diet takes the same general concept and changes the balance of the week:
4 normal eating days
3 lower-calorie days
In the 2025 randomized clinical trial of 4:3 intermittent fasting, participants were instructed to reduce their energy intake by 80% on three nonconsecutive days each week, while having no prescribed calorie restriction on the other four days.
That's important because 4:3 doesn't necessarily mean dieting moderately every day.
You still have four days of the week without a prescribed calorie restriction.
For someone attracted to the structure of 5:2, 4:3 therefore isn't a completely different philosophy.
It's essentially a different balance.
And that additional lower-calorie day can significantly change the mathematics of the week.
What Difference Can One Extra Day Really Make?
Potentially quite a lot.
Imagine, purely as an example, that someone normally consumes around 2,000 calories per day to maintain their weight.
Now imagine that on a lower-calorie day they consume 500 calories.
That's a difference of:
1,500 calories per lower-calorie day.
With two lower-calorie days:
1,500 × 2 = 3,000 calories
With three:
1,500 × 3 = 4,500 calories
So, in this simplified example, adding one additional lower-calorie day increases the planned weekly deficit generated by those restricted days from 3,000 to 4,500 calories.
That's 50% more.
This does not mean someone following 4:3 will automatically lose 50% more weight than someone following 5:2.
Human bodies and eating behavior aren't simple equations.
People may eat more on unrestricted days, actual energy needs differ substantially between individuals, activity levels change, adherence varies, and weight loss itself can affect energy requirements.
But the example illustrates an important point:
One additional lower-calorie day isn't necessarily a small change when you look at the entire week.
What Does the Research Say About 5:2?
The research on 5:2 is more nuanced than either “it works” or “it doesn't work.”
In one randomized trial involving 150 adults with overweight or obesity, participants following 5:2 lost 7.1% during the initial 12-week intervention period, compared with 5.2% for continuous calorie restriction. However, by the final 50-week assessment, average weight loss was 5.2% with 5:2 and 4.9% with continuous calorie restriction, and the difference between the two approaches was not statistically significant.
Another one-year randomized trial involving 332 adults compared continuous energy restriction, 5:2, and another intermittent approach. Among participants who completed the study, average weight loss at 12 months was 5.0 kg with 5:2 versus 6.6 kg with continuous restriction, with no statistically significant difference between the approaches.
There is also an interesting issue around adherence.
A 2021 randomized trial of 300 adults found that adherence to self-directed 5:2 was initially 74% at six weeks, but declined to 31% at six months and 22% at one year. The 5:2 group lost an average of 1.9 kg at one year, compared with 1.8 kg among participants given standard weight-management advice.
Again, none of this means that 5:2 doesn't work.
It tells us something more useful:
The effectiveness of a diet isn't determined solely by how good the idea looks on paper. What you can actually follow over time matters enormously.
And What Does the Research Say About 4:3?
This is where things get particularly interesting.
In 2025, researchers published a 12-month randomized clinical trial specifically examining 4:3 intermittent fasting.
Adults aged 18 to 60 with a BMI between 27 and 46 were randomly assigned to either a 4:3 intermittent-fasting program or daily calorie restriction. Both groups also received comprehensive behavioral weight-loss support.
The 4:3 participants reduced energy intake by 80% on three nonconsecutive days per week and had no prescribed calorie restriction on the remaining four days.
The daily-restriction group was instructed to reduce calories every day, with the intended weekly calorie deficit designed to be comparable between the groups.
After 12 months, participants in the 4:3 group had lost an average of 7.6% of their starting body weight, compared with 5.0% in the daily calorie-restriction group.
That's an encouraging result.
But it is important not to overstate what the study proves.
It compared 4:3 with daily calorie restriction, not directly with 5:2.
We therefore cannot take this study and conclude that 4:3 produces more weight loss than 5:2.
A direct, well-controlled comparison between the two approaches would be needed to answer that question properly.
What the study does demonstrate is that a structured 4:3 intermittent-fasting approach can be an effective weight-loss strategy over a full year.
Why Might Someone Move From 5:2 to 4:3?
For someone who already likes the basic concept of 5:2, there is an obvious attraction to 4:3.
You don't have to abandon the weekly structure that attracted you to intermittent fasting in the first place.
Instead, you add one more lower-calorie day.
Imagine someone who enjoys having unrestricted days, finds two lower-calorie days manageable, but feels that their overall progress is slower than they'd like.
Their first instinct might be to reduce calories on all five of their normal days as well.
But at that point, one of the main attractions of intermittent fasting begins to disappear.
You're back to thinking about calorie restriction every day.
4:3 offers another possibility.
Rather than making every day a little harder, you make one additional day more restrictive while retaining four normal eating days.
For some people, that structure may simply fit better.
5:2 vs 4:3: Which Is Better?
There isn't one answer for everyone.
If 5:2 is producing the results you want and you find it easy to maintain, there may be no compelling reason to change.
Two lower-calorie days are obviously easier to fit into a week than three for many people.
But if you like the concept of 5:2 while feeling that two restricted days aren't creating enough progress, 4:3 may be worth exploring.
The biggest difference isn't complicated:
5:2 gives you five normal days and two lower-calorie days.
4:3 gives you four normal days and three lower-calorie days.
That third day creates an opportunity for a larger weekly calorie deficit without requiring calorie restriction on all seven days.
But ultimately, the better approach isn't simply the one that looks best mathematically.
It's the one you can actually follow.
Why I Chose 4:3
That's what attracted me to the 4:3 Diet personally.
I had tried losing weight before, and losing weight itself wasn't the only problem.
The bigger problem was finding something I could continue doing without feeling as though I was permanently on a diet.
With 4:3, I still had four normal eating days every week.
But the three lower-calorie days gave my week enough structure to produce the results I was looking for.
When I started, I weighed approximately 250 pounds.
About nine months later, I weighed around 200 pounds.
I had lost 50 pounds.
More importantly for me, I've managed to keep the weight off while continuing to follow the 4:3 approach.
Those are my personal results, not a promise of what someone else will experience. Weight-loss results vary considerably between individuals.
But my experience is ultimately why I became interested enough in 4:3 to spend so much time researching it — and eventually write an entire guide about it.
The Bottom Line
The 5:2 Diet helped popularize a simple idea: weight loss doesn't necessarily require approaching every day of the week in exactly the same way.
For some people, two lower-calorie days may be enough.
For others, adding a third may provide a weekly structure that better matches the results they're trying to achieve.
The difference between 5:2 and 4:3 is only one day.
But across weeks and months, that additional day can substantially change the planned calorie deficit.
Research has shown that 5:2 can produce weight loss, although longer-term trials have generally not demonstrated clear superiority over continuous calorie restriction. More recently, a 12-month randomized clinical trial found encouraging results for the 4:3 approach, with participants in that program losing more weight on average than participants assigned to daily calorie restriction.
That doesn't make 4:3 a magic formula.
It simply makes it another option — and, for someone who likes the idea behind 5:2 but wants to explore what difference a third lower-calorie day could make, a particularly interesting one.
Want to learn more about the 4:3 Diet?
The Complete 4:3 Diet Guide explains how the approach works, the research behind it, how to structure your week, how to determine your lower-calorie targets, and what I've learned from following 4:3 myself.
Sources:
https://pubmed.ncbi.nlm.nih.gov/30475957/
https://pubmed.ncbi.nlm.nih.gov/30470804/
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0258853