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  • Writer's pictureRyan Allen

Should you fast?

Fasting has become an interesting component of the health toolkit of providers and patients in recent years, and yet many of the specifics surrounding the exact doses and effects still remain unclear. Nevertheless, it clearly holds great promise, with potential widespread benefits in disease prevention and longevity. Is it for everyone, though?


Let’s first clarify what we mean by “fasting.” While there’s no set definition, I would consider a fast to be any period of time over 24 hours in which one does not consume any calories (and no, trivial calories like 10 from a piece of gum would not be enough to “break” a fast). Anything less than 24 hours is what I and others would call “time-restricted eating,” or TRE. Now, I regularly utilize TRE for my daily nutritional habits, typically doing at least a 20:4 schedule (20-hour fasting window, 4-hour eating window). However, the benefits of this are just about entirely different from those of prolonged fasting.


TRE may be applicable to a broader group of people than prolonged fasting, as it generally has been shown to have benefits for caloric restriction and circadian rhythm regulation, both of which rather unambiguously carry health benefits. Additionally, a lot of the predominant concerns over prolonged fasting center on possible loss of lean mass and deterioration of body composition, although this proteolysis (muscle protein breakdown) does not appear to occur until at least 30 hours of fasting. In fact, it actually seems that reasonably fit people taking in sufficient daily protein can build muscle just about as effectively on TRE as they can with meals spread across the day. This paper from 6 months ago even showed that subjects were able to efficiently utilize protein even when consumed in quantities up to 100 g in one sitting, calling into question the old advice of splitting protein into doses of ~30 g at a time for maximal usage.


It is also likely that TRE carries little to none of the unique benefits of longer fasts. For example, it does not appear that autophagy, one of the strongest self-renewal mechanisms in our bodies, is upregulated in any metabolically significant way within 24 hours of fasting. It is also completely clear that one does not enter a state of fasting ketosis in as short a period, either, inasmuch as one believes there are benefits to short periods of ketosis for cognition or efficiency of fat metabolism, which remain subject to debate.


While many have questioned the use of longer-term fasting because of body composition concerns, I think it is important to recognize the individuality of this, as with any nutritional or broader health intervention. It will depend on the goals of the individual, as well as the individual response to the stimulus. Let’s take my situation as an example. I engage in a 72-hour fast once a month because, while the longevity benefits remain unconfirmed, I am very intrigued by the potential potency of fasting as a tool for disease prevention. In myself, I am currently fairly satisfied with my level of lean mass and body composition, and at this time I would potentially even be willing to sacrifice just a bit of it for the theorized benefits of prolonged fasting.


Coming back to the variability in individual response, I have not yet noticed any significant changes in my body composition since I have started this fasting, which has been about 8 months now. Note that I am especially diligent while fasting to purposefully strength train every day, in an effort to preserve every bit of lean muscle tissue through a mechanical stimulus. I may be yet to see a change thus far, but body composition is definitely something I will be monitoring closely, as I would not tolerate any major deviations in it. At the end of the day, while autophagy could hold the key to recurrent cellular restoration and “delay aging,” good body composition, lean mass, and strength are already proven contributors to quality lifespan and healthspan. We want to be careful that we’re never stepping over these $100 bills to pick up what could be pennies in terms of benefit.


So, what are your goals? If you are not adequately muscled (and you’ll know from previous posts that we hold a much higher standard for body composition than many current recommendations), or if you are otherwise looking to build muscle, I would say there is virtually no reason to be engaging in multi-day fasting now. Focus on getting sufficient protein every day, as well as enough strength training stimulus to provide growth. If you are like me, where you are presently comfortable with your level of muscle and, frankly, at a stage of life where you could course-correct for a bit of lean tissue loss, then by all means give it a try. In this case, be very mindful to strength train frequently while fasting to avoid muscle wasting, and be quick to make a change if you do notice deterioration in body composition or strength, as these should be your higher-yield priorities.





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