With the popularity and staying power of intermittent fasting, new research brings to light what is the best intermittent fasting schedule to get rid of stubborn belly fat.

As two engineers who have studied, written about, debunked myths, and also successfully followed intermittent fasting for years, we’re here to explain what the science says about this. (And not what the marketing wants you to think!)

Because let me tell you: one downside of IF is that with its popularity, there are a lot of bogus products and twisted headlines that make their way onto the airwaves.

We’re here to help clear the air and give recommendations based on the data on how to lose that stubborn belly fat (or visceral fat) with intermittent fasting.

The Best Intermittent Fasting Window to Lose Belly Fat

What is the best intermittent fasting window for Metabolism and Fat Loss?

To get started, let’s define intermittent fasting as an eating schedule that dictates when you have your meals instead of what you actually eat.

With such a loose definition, there’s a lot of wiggle room here up for interpretation, even when it comes to scientific studies. In reality, all of us fast whether we realize it or not because when we sleep, we’re not eating. That’s where the whole term of breakfast (breaking the fast) comes about anyway.

So what actually “counts” as intermittent fasting?

We’ve written about this extensively in several articles, like “Intermittent Fasting Meal Plan Printable PDF” and also our go-to “Intermittent Fasting How-to Guide,” but here’s a quick recap.

Common Intermittent Fasting Schedules

Intermittent fasting schedules typically vary from 12 hours of fasting a day and going up from there to one meal a day (OMAD) or alternate-day fasting. Alternate-day fasting, as it sounds, means eating every other day normally with days of fasting (or very low-calorie consumption, around 500 calories) in between.

The most popular IF plan that people follow is the 16/8 IF plan where you fast for 16 hours and eat your meals within 8 hours. This is mostly because 16/8 fits in relatively easily with folks’ current lifestyles, with something like a delayed breakfast at 11 am and finishing dinner before 7 pm.

So is the 16/8 plan the best one of all? Yes… and no. And with one huge caveat when it comes to losing belly fat (more on that below).

Since it’s incredibly hard to research people’s diets, the consensus amongst doctors and scientists is that the best IF plan is the one that you can most easily stick to.

However, when specifically talking about losing belly fat, a new study shows that there’s more to the puzzle. But first, let’s take a look at how intermittent fasting burns fat.

How Intermittent Fasting Burns Fat

Intermittent fasting health benefits are extensive: from insulin regulation to improved metabolic health to cancer reduction and improved longevity.

But, let’s face it: most of our readers and clients use intermittent fasting for weight loss. So let’s examine how this works, specifically when considering the most popular 16/8 plan (16-hours fasted with 8-hours of eating).

By the way, we don’t advocate using intermittent fasting for calorie restriction. This is sometimes a natural byproduct but, ideally, everyone is eating their recommended caloric intake and going about it healthily. (We share the calculation for your daily calorie target in our article, ‘30-Day Weight Loss Challenge.’)

So, great news if it helps you from overeating, but try not to undereat too.

How your body burns fat while intermittent fasting:

1) Insulin levels drop.

Your body finishes digesting the carbs you consumed at your last meal, which provide easy energy in the form of glucose. Once digested, this drops insulin levels.

2) Fat cells release stored sugar.

When insulin levels are low enough, the body signals that there is no more glucose readily available for energy. Fat cells then release their stored sugar for energy in two ways: lipolysis in adipose tissues and fat oxidation in mitochondria.

3) Your body gets a change to restabilize.

This break in digestion gives your body a chance to restabilize hormone levels, increase insulin sensitivity, and work on reducing inflammation. All of this works towards benefitting your metabolic health, which continues to promote weight loss.

The goal of having your body switch from burning carbs to burning fat is really what we’re interested in for this article. But how do you know when you’ve fasted long enough to burn fat, let alone burn belly fat?

There are two ways that Ryan and I personally go about this: measuring our metabolism in real-time or, if we can’t do that, making an educated guess based on the stages of fasting by hour.

So, it’s time to learn what is the best intermittent fasting window (for you) to lose belly fat. 

Intermittent and extended fasting bundle - printable pdf guide

The Proven Fasting Bundle (Printable PDF Guide)

Read by 143,364 fasters in 2022 alone, the Fasting Bundle is the structure and tools you need to succeed.

  • Intermittent and alternate-day fasting meal plans
  • Checklist for before, during, and after a prolonged fast
  • Benefits, challenges, and a beginners how-to guide

Plus, we're just an email away for support.

Finding your best intermittent fasting window to Burn Fat

Historically, if you weren’t at a clinic, you couldn’t measure your metabolism or fat burn easily or cheaply in real-time. The best you could do was by guessing if your IF window was big enough based on your diet.

A ‘good’ diet might have you in fat burn in as few as 4 hours. A sugar-heavy diet… longer. Perhaps 16 hours, perhaps more.

We share a general timeline of what to expect in our article ‘Stages of Fasting By Hour,’ but let me warn you: Ryan and I measured our metabolism in real-time and our data didn’t match perfectly with predictions. So, at best, it can be used as a rule of thumb.

Instead, if you’re serious about burning fat and, specifically, belly fat with intermittent fasting, we highly recommend buying the Lumen.

How to Measure Fat Burn while Intermittent Fasting

Lumen is the only at-home, real-time metabolism tracking device available today. And it does far more than just that.

While Lumen has been making waves on the news and social media, Ryan and I have been using it and experimenting with it daily since May 2023. We’ve learned so much about how our bodies and metabolism respond to specific meals, workouts, stress, poor sleep, extended 3-day fasts through 5-day fasting-mimicking diets, and even pregnancy.

So, for people who want unparalleled insight into demystifying weight loss and our health, rejoice!

(Don’t miss our full Lumen review here. If you can’t guess, we think it’s groundbreaking and Lumen has helped so many of our readers too.)

Essentially, by blowing into the Lumen device for a few seconds, it will analyze your CO2 to tell you what percentage of carbs vs. fat your body is burning at that exact moment.

the best intermittent fasting window to lose belly fat and boost metabolism

Not only will this tell you, unequivocally, when you’re in fat burn mode, you can use Lumen to figure out what time exactly to break your fast. Why unnecessarily fast for 16 hours when your body is telling you to replenish with carbs at Hour 13?

Finally, the Lumen will analyze your scores over the weeks and make diet recommendations to improve your metabolic flexibility. This reminds us that burning carbs isn’t bad, it’s actually very important and necessary after a meal.

The goal is overall improved metabolic health and flexibility, so your body can make the best choices on how to process the food you eat. And, yes, that includes burning fat.

Which Intermittent Fasting Schedule Is Best for Losing Belly Fat?

Now let’s get to the crux of it all: what is the best intermittent fasting window to lose belly fat?

When we say belly fat, we’re referring to visceral adipose tissue stores in the abdomen, aka visceral fat. Excess visceral fat is considered the most dangerous type of fat as it surrounds your vital organs and increases the overall risk of mortality.

The good news is that a systematic review of intermittent fasting studies found that 79% of weight loss owed to fat specifically. But we don’t get to choose where that fat comes from — arms, hips, back, belly fat, or otherwise.

But when we start looking at visceral fat specifically, this is where the news headlines come in:

“Intermittent Fasting Makes It Harder to Get Rid of Belly Fat, Animal Study Finds.” — Forbes, March 2021

This article cites a study that made waves a few years ago about how repetitive fasting can make it harder to lose visceral fat because of a body’s response to preserve it for later energy use. Albeit in mice, at least.

What many interpretations of the study miss, however, is the keyword of repetitive fasting — as in fasting the same way, over and over again.

The good news here is, we can control that.

So if you’re trying to lose fat — belly fat, specifically — no matter what your current plan is, you need to change up your fasting plan from time to time and add variety.

Here’s how to do it.

Avoid Repetitive Fasting To Burn Belly Fat

We spoke so much about the Lumen metabolism tracker earlier because it is a great and clear-cut way to avoid repetitive fasting.

Since your metabolism depends so much on what you ate that day, your stress levels, how well you slept, and how well your workout went, your fasting window should be fluid and change from day to day. But without using the Lumen to measure it, there’s no obvious way of knowing.

[Box advertising Lumen discount and/or perhaps Lumen screenshot updating post-workout]

Another surefire way to avoid repetitive fasting is to do quarterly extended fasts alongside your preferred intermittent fasting schedule.

This also allows you to reap more fasting benefits that come with 24-72-hour fasts (ketosis, autophagy, and reduced inflammation) up to 120-hour fasts (immune system regeneration and stem cell production).

Typically, a 3-day fast can be done without medical supervision (we’ve done one, they’re empowering and awesome but fairly disruptive to your life.) Here’s our full step-by-step and hour-by-hour experience consuming only water (and black coffee) for three days.

Instead, over a 3-day fast we now recommend a 5-day fasting-mimicking diet after trying both. Here’s our review of the most reputable one we’ve found with ProLon, which has some pretty impressive science and clinical trials behind it.

With a fasting-mimicking diet, you eat about 700-1000 calories a day while “tricking” your body that you’re in a fasted state. So, you get 2 more days of fasting benefits without any disruption to your sleep — that was a big problem for us during our 3-day water fast — and without hunger.

Seriously! For us, it was a far better and easier experience.

Closing Thoughts on Intermittent fasting and Losing belly fat

When researching “What is the best intermittent fasting window to lose belly fat?” the science tells us that it’s a two-part answer.

Part 1:

Start by selecting the best intermittent fasting schedule that suits your lifestyle. The most successful and “easiest” for you to do will allow you to enter fat-burn mode more frequently without the bad side effects of overeating and binging.

For most people, this will be the 16/8 eating schedule with 16 hours of fasting and an 8-hour eating window, but don’t be scared to experiment on your own.

Part 2:

Next, keep your body on its toes by adding variety to your fasting schedule. An often-cited study found that repetitive fasting makes visceral fat more resistant to fat loss.

Therefore, get more precise with your fasting windows by tracking your metabolism in real-time daily, like with the Lumen device, which will change your fasting window daily. Or, consider quarterly extended fasts.

Both of these options will only further support your fasting, fat-burn, and weight-loss efforts without derailing your diet or arbitrarily making you change it up for change’s sake.

And, as always, follow these best practices while fasting to see the best results:

  • Optimize your diet and cut out high-sugar and ultra-processed foods. This goes hand-in-hand with successful intermittent fasting and will give you the best results.
  • Do your best to reduce stress and increase sleep quality. Both of these affect your metabolic health.
  • Add more daily movement and exercise to your day. Focus on building muscle and total-body strength to burn belly fat. For a free and well-rounded workout program, we recommend our ‘12-Week Gym Workout Plan for Beginners
  • Stay hydrated!

As we mention in the article, you can’t necessarily target belly fat when losing weight. Fat will come off from different areas at different speeds. But what you can do is make sure your fasting doesn’t fall into a repetitive pattern in case it makes certain fats (like visceral fat) more resistant to fat loss.

It takes patience, but stay the course!

Feel free to download the Fasting Bundle below to help you plan, track, and succeed with both intermittent and prolonged fasting.

For more resources and guides about fasting, such as our guide to intermittent fasting and sleep, head over to our Fasting Page.

Intermittent and extended fasting bundle - printable pdf guide

The Proven Fasting Bundle (Printable PDF Guide)

Read by 143,364 fasters in 2022 alone, the Fasting Bundle is the structure and tools you need to succeed.

  • Intermittent and alternate-day fasting meal plans
  • Checklist for before, during, and after a prolonged fast
  • Benefits, challenges, and a beginners how-to guide

Plus, we're just an email away for support.

30-Day Weight Loss Challenge Tracker Form V2 Ryan and Alex Duo Life

30-Day Weight Loss Challenge

As engineers with a combined twelve years of health coaching experience, we needed to create a data-driven way for our clients to sustain weight loss.

Too many weight loss challenges involve a long list of what you can, cannot, and need to do every day. We’ve reduced the overwhelm and only require you to track five numbers a day — calories, steps, fiber, sleep, and waistline.

You’ll track five numbers daily to give you the highest return for sustainable weight loss. As a bonus, we’ll share our Weight Loss Bundle, which includes strategies, progress tracking tools, and additional weight loss plans.

Learn more about the program.

As Seen In Feature Bar Ryan and Alex Duo Life

Hey we're Ryan and Alex

The creators of Ryan and Alex Duo Life. We are a husband-wife duo and “lifestyle engineers.”

After eight years working in the corporate world as engineers, we left our high-powered jobs to tackle our true passion — helping couples engineer their best lives.

The synergy of our engineering minds and ten years of health coaching experience produced Ryan and Alex Duo Life. Our mission is to help you transform your bodies, minds, and relationship as a couple.

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