Does Extended Fasting Lead to Long Term Fat Loss?

Dive our in-depth analysis that debunks the common misconception surrounding extended fasting and weight loss. Uncover the truth as we delve into the science and real-world outcomes of fasting for prolonged periods, shedding light on a balanced approach to achieving your health and weight loss goals. This piece aims to steer away from the hype and present a clear perspective on how to effectively incorporate fasting into your routine for lasting benefits.


If you’ve read the previous chapters of Big Fat Keto Lies, it may not be a surprise that I also got into the fasting craze when it was booming in Ketoland.  I was such a super fan that both Jason Fung and Jimmy Moore asked me to write a review of their book, The Complete Guide to Fasting.

The chart below shows my blood glucose and ketones from a seven-day fast where my blood sugars dropped, and my ketones rose to levels that my meter couldn’t read. 

But even though I was regularly doing multi-day fasts, I still wasn’t getting the results I wanted.  It seemed I was gaining weight over time rather than losing.  It seems I wasn’t alone. 

Many people fast for days at a time but are unable to lose the weight they want.  It seems that losing and gaining the same few pounds is a common side effect of extended fasting.

Mark, one of our Data-Driven Fasting Challenge participants, observed that, although he had been doing extended fasts, his weight had simply been yo-yo-ing up and down over time. 

What is Fasting? 

Simply put, ‘fasting’ is intentionally refraining from eating. 

Jesus fasted for 40 days and 40 nights (Matthew 4:2), and Muslims do not eat during daylight hours for the month of Ramadan. However, the duration of ‘fasting’ can be much shorter than these commonly cited examples. 

Many of us are accustomed to eating almost continually from soon after we wake up until just before going to bed, so an hour or two without food could be considered a fast (i.e. intentionally not eating). 

Energy toxicity (not insulin toxicity) is the root cause of the majority of our modern diseases. Hence, fasting has many benefits. But rather than thinking longer is necessarily better, you need to consider the minimum effective dose of fasting while still getting the nutrients you need when you eat.

How Long is Too Long?

While you can go for weeks without food, it may not be optimal for your long-term progress towards your health goals.  For the many people who eat all day, a couple of hours may feel ‘extended’. 

There is nothing wrong with going 24 or 36 hours without food so long as you are making good choices when you refeed to ensure you are getting the nutrients you need over the longer term and don’t have a history of binge eating. 

However, if you are not able to control food quality or quantity when you refeed, you are unlikely to be making sustainable long-term improvements, and it might be better to try shorter but more regular fasts in the future.

Do I Need Extended Fasting or Autophagy?

Autophagy is your body’s process for cleaning house and purging all the damaged cells and old protein.  Autophagy literally means ‘to self-eat’. 

For the most part, this is a desirable thing that happens when we have less energy available from our diet.  Your body starts to feed on itself, cleaning out the bits it doesn’t need.  Periods of low-energy availability allow our bodies to repair and purge.  It’s in our DNA to keep what we need, which allows us to survive and procreate another day when more food is available. 

It’s easy to fall into the trap of thinking that because a little is good, more is better.  We like to fit in and tend to compare ourselves with other people.

  • My ketones are higher than yours. 
  • My blood sugar is lower than yours. 
  • I eat less carbs than you.  
  • My fast was longer than yours.

But extremes are not necessarily better, including when it comes to fasting.  While there may be benefits of extended fasting, there are also downsides. 

You may have seen images like the one below that suggest that there is something magical about fasting for a certain length of time.  However, the reality is that there is no data on autophagy in humans. 

Even the smartest length of fasting is required to achieve benefits or the minimum effective dose of fasting to achieve meaningful results.  Do you need 24 hours, 36 hours, three days, seven days or maybe 14 days to get the full benefits of autophagy? 

Unfortunately, studies on yeast and worms in a Petri dish aren’t particularly relevant to humans living in the real world who also need to be robust and resilient to survive into old age.  Italian biologist Valter Longo has shown that cycles of 48-hour fasting produce benefits in mice.  But mice aren’t tiny humans.  One day for a mouse is equivalent to 40 human days.  So, the equivalent of a 48-hour fast in a mouse (i.e. 80 days) could kill many humans. 

While people often refer to Angus Barbieri, who fasted for 382 days and came out thin and healthy, they don’t talk about the ten young men in their 20s who died after not eating for between 46 and 71 days in the 1981 Irish Hunger Strike.

Autophagy occurs to some extent all the time, particularly when we are not overfed.  However, too much autophagy can be harmful if you lose precious lean muscle mass when you fast and don’t get enough protein when you eat. 

So, rather than trusting in long stints of extended fasting to ensure you achieve autophagy, it’s likely more ideal to get regular small episodes of autophagy as your critical markers (i.e. body fat, blood sugars, insulin, etc.) move towards optimal levels. 

The Problem with Extended Fasting

Sadly, extended fasting does little to teach you to eat well and may lead to reduced diet quality and subsequently fat gain and muscle loss. 

You may have superpowers of self-restraint when you refeed, but most people tend to gravitate to energy-dense, nutrient-poor foods.  This means you may not get the protein and other nutrients you require in the fullness of time. 

If you find yourself reaching for the peanut butter, nuts, cream or pizza after fasting, chances are you’ll do better if you are a little less ambitious with your fasting duration next time.

What We Do Know About Fasting

The good news is that most of the benefits of extended fasting can be achieved by dialling in your daily meal routine (with nutrient-dense food when you do eat) to ensure a long-term energy deficit that leads to more optimal body composition.  

Regular bursts of fasting, followed by nutrient-dense refeeding, are much more likely to be beneficial than a few multi-day fasts followed by energy-dense, nutrient-poor binging (which can be exacerbated by a belief that fat is a free food because it will keep your insulin levels low). 

While there is plenty of talk about the benefits of fasting for autophagy, cancer and Alzheimer’s, there is very little data in humans to validate the theoretical benefits.  What we do know is that all of these conditions are vastly improved by achieving and maintaining more optimal body composition. 

As shown in the chart below, the relative risk of a range of cancers increases with increasing BMI (according to the Quantitative association between body mass index and the risk of cancer: A global Meta-analysis of prospective cohort studies: Obesity and cancer risk). 


Our overall mortality risk increases with a higher BMI (chart from Waist-to-height Ratio Is More Predictive of Years of Life Lost Than Body Mass Index).

Many people are concerned about insulin, mTOR and IGF-1 and hence avoid protein and fast for days or weeks, hoping to maximise autophagy and minimise these hormones.  However, these things are all correlated with energy toxicity.  As you bring your body fat levels down to more optimal levels, all these other things will fall into line. 

To age well, as well as not be overfat, you also want to be as robust and resilient as you can be.  We cannot apply theories from worms and yeast to free-living humans who need to be as strong as they can be as they age to maximise immunity, cognitive function and reduce the risk they will fall and break a hip and then never make it out of the hospital system. 

Progressive Overload for Your Metabolism

When you work out in the gym, you get progressively stronger and can lift heavier weights through ‘progressive overload’.  If you go all out on the first day in the gym lifting the heaviest weights you possibly can, you will be overly sore and will not return to the gym for another session for days, if ever.  You are also likely to be super hungry and eat everything and anything you can to recover, thinking you’ve ‘earned it’. 

It’s much more effective to build up slowly, lifting weights that stress your system a little, but that don’t take you too long to recover from.  You can then return to the gym on a regular basis and make progress over the long term. 

Similarly, when you fast, it’s much better to push your metabolism a little, but not too much that you break, so you can make consistent progress over the long term. 

How to Optimise Your Fasting Routine to Achieve Your Goals

Many people wonder, ‘How many days should I fast for?’ However, what you really need to know is, ‘How can I optimise my normal eating to ensure I am moving towards a more optimal body composition and staying below my Personal Fat Threshold?’

The Data-Driven Fasting 30-Day Challenge 

The Data-Driven Fasting 30 Day-Challenge is a structured approach to optimise your intermittent fasting to align with your goals, preferences and routine.

Over the 30 days, you will learn to optimise your health, gain control of your blood sugars and ensure healthy body fat levels to give you the best chance for a long, healthy and vibrant life!

The Data-Driven Fasting 30-Day Challenge will guide you through the process of optimising your fasting routine as you work through the following phases of the program:

  • Baselining – Identify Your Current Trigger that you will use to identify genuine hunger and when you need to eat to refuel.
  • Your current foods/meals – Learn how the foods you currently eat affect your blood sugar – which ones leave you satisfied and which ones lead you to eat more than your body requires.
  • Hunger training – Learn to delay eating until your blood sugar is below Your Current Trigger to make sure that you eat only when you need to refuel. This will ensure that you are lowering your blood sugars and burning body fat.
  • Curb your late-night binging – Use your waking blood sugars to identify if you are eating too late.
  • Main Meal vs Discretionary Meals – Identify the Main Meal you will use to anchor your eating routine and ensure you get enough nutrients. Based on Your Current Trigger, you can then treat the others as Discretionary Meals to meet your energy needs as required.
  • Optimise your eating routine – Lock in your new habits to guarantee you are moving towards your goals.

What you will get:

  • 30-day program to guide you to optimise YOUR fasting routine to ensure you achieve your goals.
  • The Data-Driven Fasting app to track your progress and fine-tune your intermittent fasting routine.
  • 130-page manual complete with detailed instructions and answers to 99 frequently asked questions.
  • A workbook that you can use to reflect on your learning during the challenge.
  • Weekly Live Q&A sessions where you can ask all your questions and get extra support.

Best of all, we get to do this in an interactive community forum where you can share your journey and ask lots of questions.  You an join our next challenge here.


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2 thoughts on “Does Extended Fasting Lead to Long Term Fat Loss?”

  1. Marty, I’ll say it again–you need to put down the hardhat and put on a white coat! The field of “nutritional engineer” needs to be started, and I recommend YOU as the lead educator.

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