Episode 54 – Thoughts on Intermittent Fasting

Key Topics Covered


  • There are a lot of big claims made by proponents of intermittent fasting (IF). Weight loss is definitely one of the more common ones but there are several more that get thrown around.  
  • From reductions in cancer risk to improving IBS, we are going to go through a handful of common claims made about IF and review some of the research   

Intermittent fasting definition 

  • Intermittent fasting basically means what it sounds like – it is alternating cycles of fasting and eating. 
  • The most common type is 16:8 – which involves a 16-hour fast each day, and an 8-hour eating window.  
  • Loose definitions have it so that it can extend as broadly as a short 12-hour fast OR as much as a 48-hour fast. 
  • Basically, we are just defining it to clarify that we are not talking about LONG fasts e.g. week-long or more. We are mostly focusing on shorter fasts.    
Scheme and concept. eating and fasting windows. Vector illustration. Infographic

Weight loss  

  • There is no difference between fasting and eating more frequently when calories are matched over longer periods of time. 
  • For example, if somebody doing 16:8 fasting and was consuming 1800kcal and 150g of protein in their eating window will get the same weight loss results as somebody having 6 meals/snacks across the day consuming the same calories and macros. 
  • Obviously, the nuanced way of looking at it though is that it may or may not be easier for somebody to consume fewer calories if they fast, since they won’t have as much room during their eating window to consume more calories. Personal preference comes into play here.  

Muscle growth 

  • Very briefly touching on this.  
  • Theoretically spreading protein throughout the day helps muscle growth. 
  • This is because muscle protein synthesis is often maximised by as little as 20-40g of protein in a sitting, depending on size. 
  • That being said, muscle retention during a calorie deficit is actually quite good with intermittent fasting if total calories and protein are matched to a comparison with a more even distribution.  
  • This is because total protein matters more than distribution.  
  • That being said, it is pretty clear to OPTIMISE muscle growth, consuming 4-6 decent servings of protein throughout the day is beneficial. It probably only takes a 10-12 hour eating window to maximise this though, we don’t need protein around the clock e.g. it doesn’t make sense to get up in the middle of the night to consume protein. 
  • There is minimal research on muscle growth in a calorie surplus with IF approaches since almost all IF studies end up involving a calorie deficit.  


  • There is a concept of “gut-rest” where because the gut is not processing food, it is resting and recovering. 
  • That concept by itself does not seem to have much merit. The gut does not need rest. 
  • But from a practical perspective, if you have minimal or no food coming in, of course symptoms will also be minimal during that phase. 
  • But once again, if total intake is matched, this now means you have a bunch more food coming in, during a shorter timeframe, likely leading to MORE symptoms during that time frame. 
  • Calore deficit from IF = possible reduction in symptom 
  • On that topic, most people with IBS actually notice significantly less symptoms when they are on lower calories. This is due to a combination of factors, including consuming less trigger foods. But obviously a calorie deficit is not a long term solution since you can’t just stay in a deficit forever.  

Cholesterol + BGLs + blood pressure + insulin resistance 

  • The majority of studies on IF involve improvements in these markers of health 
  • Obviously a calorie deficit is the driving explanation here though for the longer term stuff 
  • On an acute basis, you can clearly see how IF leads to improved markers.
    • One example could be BGLs. If you don’t have any food coming in, of course BGLs will lower (if they were previously elevated). 
  • The issue is that if we compare on a calorie matched basis, if you have a fasting window that leads to improvements on an acute basis, it means you are now putting more calories into the feeding window, which likely means BGLs go HIGHER during that window. These things balance out.  
  • Obviously you can criticise that logic by being like “well fasting does typically lead to lower calorie intake overall” – which is fair. But I still think it’s worth thinking about things from a calorie matched perspective, since it gives flexibility to choose an approach that works well for you.   


  • There is a proposed theory that because fasting promotes autophagy, it leads to a reduction in cancer risk. 
  • Autophagy is basically the process of clearing out old cells. 
  • Theoretically this is useful because cancer is literally due to cell mutations. Clearing away old/damaged cells could reduce the liklihood of this being an issue. 
  • This is probably the most difficult area of fasting to interpret. Because the thing is: 
    1. A calorie deficit also causes autophagy 
    2. In fasting studies, autophagy occurs at high amounts in the acute timeframe. 
    3. With a regular calorie deficit, it occurs in more moderate amounts over a longer timeframe. 
    4. Arguably, if you matched the same total calorie intake over the course of a month, this could all equal out. 
    5. We don’t really have studies measuring that, since it is a difficult thing to measure. 
    6. Having a lower body weight can reduce cancer risk – arguably because there are less cells available for mutation, amongst other things. 
    7. Because of that, studies that just measure fasting and cancer risk have quite a few in-built issues that need to be accounted for.   Fasting as a cancer treatment?  
  • We won’t be covering fasting as a cancer treatment option since it is a complex topic that really is a deep podcast by itself.  
  • The logic is that fasting would reduce glucose available to fuel the growth of cancer cells and there is some promising early research.  Most of that research is on prolonged fasting (>2 days) not IF though.  
  • But the counter to that argument is that malnutrition and cancer cachexia are also serious issues too. So we will just leave that point for now.   


  • Calorie restriction (not necessarily fasting) has long been associated with longevity 
  • We see this more clearly in easily controlled animal studies e.g. a study where monkeys were fed 30% less calories than the control group showed improved longevity. 
  • Theoretically these benefits of autophagy play a huge role here.  
  • But there are a lot of variables – for example – it’s hard to say that fasting provides these benefits more than having the same total calorie intake average over a longer time frame. 
  • Another variable is that calorie restriction obviously has carryover effects to your body composition.  
  • Assuming exercise is accounted for, the lower your average calorie intake, the lower your body weight will likely end up being.  
  • This has implications in the elderly population for things like preventing falls risk. But obviously there are a bunch of other implications. And the benefits of calorie restriction are also only seen if it does not also contribute to malnutrition, which is also a relevant point. 
  • After a certain age, higher levels of muscle mass appear to be linked with longevity too.  
  • Based on that logic, instead I’d say an appropriate calorie average over the long term is likely a better marker for longevity.   

Practical takeaway 

  • Fasting can be a useful tool for some. 
  • I wouldn’t 100% rule out the autophagy aspect, but my current stance is that when averaged out over time, it still comes down to average calorie intake.  
  • Because there are likely no inherent benefits to fasting, it comes down to personal preference.
  • If somebody is convinced that fasting provides these benefits, an option a lot of “bio-hackers” use is infrequent fasts of say 24 hours once every 3 months or so. 

Useful Links/ Resources

Studies Mentioned:

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