Calories not used for digestion while fasting

nomadreid

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Summary
I wish to counter my friend's assertion that benefits from fasting come partly from the availability of calories that would have otherwise been used for digestion. I know enough biology to know that this is too simplistic, but not enough to formulate a good argument.
A friend, whose knowledge of biology comes primarily from pseudoscientific works, considers occasional fasting beneficial -- on this I will not take a stand, because as far as I know, the jury is still out on this. However, his justifications are annoying, (To paraphrase a notorious quotation, when I hear the word "cleansing", I reach for my gun.) His reasoning is that, since one is not using the calories from either food or stored fat to execute all the processes connected to digestion, then these calories -- coming mainly from stored body fat during fasting -- would then allow the body to repair problems in the various mechanisms such as autophagy and the functions of organs (liver, kidney, lungs, skin, etc.). To my mind, the body's management of energy is not just a quantity that can be applied where ever it may be needed, but rather must be called up by specific pathways, and the absence of one does not imply the increase in another. However, I don't have sufficient grounding in the details to be able to formulate a cogent reply. Any input would be appreciated.
 
Recent research I read within the last year showed that energy usage is not constant. If you do not eat for 6-8 hours then the brain sends signals to the body to slow down it's energy consumption to preserve energy in anticipation of a prolonged period of no food intake. When you then eat after that point the body acknowledges that energy is going to come into the system and so sends the signal that energy can then be consumed again at normal level for bodily functions. It is believed this is ancient biological mechanism of the body from the days when we were hunter/gatherers and could not guarantee when the next meal would come. It's relatively new research and I canot recall where I read this to link to it.

Regardless of any latest research data, the body needs nutrients to survive. To think that depriving it of nutrients would somehow promote the body is heal itself better than if it had the nutrients required is just plain silly.

Fat stores are "emergency energy" to be used when no food is avialable, this is again an ancient biological mechanism to store unused energy for later use when food is not available. This mechanism works brilliantly well, just look at all the overweight people in the world (myself included.) If fat conversion to energy could provide all that is required to survive then it would not be unhealthy to starve yourself to lose weight, we know this method is very counter-productive and though you will lose weight, you would not be healthy loosing weight in this manner.
 

nomadreid

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MikeeMiracle, thanks, that is well stated, and I can easily find the relevant sources.
Contrary to
Then using logic with him is likely to be useless.
I can use it by not presenting the whole argument all at once, but get him to admit one point at a time. Sometimes that works, even if he is "schooled" by pseudoscience. The summary by MikeeMiracle is about the right level, because of course the best would be for my friend to attend a good biology course (as well as to understand what the scientific method is), but I have to work with what I have. I will look up some of the details, following MikeeMiracle's outline, so that I myself know what I am talking about, not to give my friend all the details (since there is a point when most persons not used to science rebel against detail).
 

jim mcnamara

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Calories are required to break down fat, with a higher return of energy. Anyway, fasting for the short short term there is no fat involved.

Your body has a short term reserve for glucose: glycogen. It breaks down on hydrolysis, which requires less energy to start the reaction than converting fat into glucose.


The underlying assumption of your friend's argument needs a tune up:

The REE (resting energy expenditure) is in part made up of calories burned by the brain in the form of glucose. People in the sports exercise field use 20% of REE minimum as the requirement for the brain. And they assume 6% of REE is for resting skeletal muscle. The rest goes for heat generation (we are homeothermic), energy for organs like the heart and the alimentary canal. Peristalsis goes on even during fasting. So does extraction of nitrogenous waste in the kidney. The gut transit time for humans averages between 24 and 48 hours -- in one end, out the other. This means your gut is still processing food from yesterday, as we speak. ... I know, too much detail.

No glucose = dead brain.

Glycogen is the first defense against that. What distance runners refer to as carbohydrate loading.
Eat lots of pasta and for a while after that, glycogen stores are high. Therefore their bodies can call on them during a prolonged run to get glucose pretty efficiently. Fat "burning" is a slower, somewhat less metabolically efficient process than the hydrolysis of glycogen into glucose. Consider reading the link above.
 
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russ_watters

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His reasoning is that, since one is not using the calories from either food or stored fat to execute all the processes connected to digestion, then these calories -- coming mainly from stored body fat during fasting -- would then allow the body to repair problems in the various mechanisms such as autophagy and the functions of organs (liver, kidney, lungs, skin, etc.).
This is not logically consistent, which is at least part of the reason it is wrong. The body does indeed manage its energy expenditure (though I doubt it can go above what is "normal" for certain functions), but the entire point of fasting is to have less energy available, so your body is forced to cannibalize itself. This means that your body almost immediately starts reducing the energy expenditure of all functions it can, including healing.
 
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In my experience, there are often germs of truth (no pun intended...) in folk medicine although the explanations do not make scientific sense and are almost certainly bogus.
In this case I direct you to all the recent fascinating research implicating the biome in our gut to a host of immune functionality. The effects of fasting may if fact be very complicated when seen through this lens. I think mainstream medicine would, until recently, have dismissed much of that with disdain. Maybe less-so now.
But I'm a physicist.....so what do I know
 

nomadreid

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Thank you, jim mcnamara, russ_watters and hutchphd. All this is very useful. Some comments and questions: starting with
research implicating the biome in our gut to a host of immune functionality.
I assume you are referring to things like
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5988561/
https://diabetes.diabetesjournals.org/content/67/9/1867
https://www.diabetes.co.uk/news/2017/oct/fasting-exerts-fat-burning-effects-through-changing-the-microbiome,-study-finds-94489935.html
indicating that intermittent fasting may have benefits (albeit via other mechanisms than the "energy-saving" argument), although the exact mechanism is not yet clear (so we are at the stage of correlation, not yet to causation). Another aspect of looking at the gut biome (causation) would seem to be that fasting deprives the gut biome of the nutrients which they require to survive So I am not sure what the balance would be here.

Regardless of any latest research data, the body needs nutrients to survive. To think that depriving it of nutrients would somehow promote the body is heal itself better than if it had the nutrients required is just plain silly.
The end result, whether beneficial or harmful or, more likely, both, of fasting will be due to a number of factors, and the lack of nutrients would be, as mentioned above, one factor in the harmful side, but the links above indicate that there may perhaps be (with emphasis on "perhaps") other factors that might weigh in on the beneficial side in terms of gut health. In other words, I am not against my friend's conclusion that intermittent fasting is beneficial -- I do not know, nor do I think anyone does -- but against his form of reasoning.

Next, a question about
20% of REE minimum as the requirement for the brain
How does that figure jive with
"The brain lacks fuel stores and hence requires a continuous supply of glucose. It consumes about 120 g daily, which corresponds to an energy input of about 420 kcal (1760 kJ), accounting for some 60% of the utilization of glucose by the whole body in the resting state. " (from https://www.ncbi.nlm.nih.gov/books/NBK22436/)
Anyway,
It is believed this is ancient biological mechanism of the body from the days when we were hunter/gatherers and could not guarantee when the next meal would come.
(I don't know why one always refers to hunter-gatherers, as it would be more accurate to refer to our pre-agricultural ancestors as hunters-gatherers-scavengers, but that is a side point.) Are you referring to https://en.wikipedia.org/wiki/Starvation_response ?

your body almost immediately starts reducing the energy expenditure of all functions it can
True, but according to https://www.ncbi.nlm.nih.gov/books/NBK22436/, this is not uniform across all functions -- I tend to think that there would have been an evolutionary advantage to make sure that the body gives more priority to those functions which are more necessary for survival. (Sort of like embassies who first evacuate all "non-essential personnel" in times of crisis.)
 

russ_watters

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True, but according to https://www.ncbi.nlm.nih.gov/books/NBK22436/, this is not uniform across all functions -- I tend to think that there would have been an evolutionary advantage to make sure that the body gives more priority to those functions which are more necessary for survival.
It does. But 'not reducing as much as other functions' is still reducing, not increasing.
 

nomadreid

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Thanks, every one, for your very helpful replies, which have enabled me to formulate an appropriate response to my friend. (I also followed the links and research suggestions -- oh, and I understood that the source I cited in Post #8 about the percentage of energy used by the brain was, after checking other sources, apparently mistaken.) Whether it will convince him remains to be seen, but I will have done all I can.
 

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