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I'm trying to piece together a simplified model on how the energy flows in the human body.

From what I understand:

  1. We store enough ATP for around 2 seconds of maximum exertion
  2. We store enough kreatinine for around 8 seconds of heavy exertion
  3. Blood glucose amounts to around 60kcal worth, enough for 30 seconds of intense activity
  4. Our glycogen stores are what we mostly run off in the day to day, the amount seems to be anywhere between 1-3x of the daily resting metabolic requirement, depending on fitness, so 1500 - 4500kcal worth for most people.
  5. Fat is where excess goes but it takes time to build, looking at various studies it seems that the most extreme fat creation/destruction that our bodies are capable of is around 1000kcal worth per day or ~0.29kg/day

Based on this, I'm treating glycogen as our main "battery". Fat is for long term storage and everything else is a chain for increasingly more intense and shorter exertions.

Now to top off our glycogen stores, I see two typical ways - either digest food or burn fat. Leaving fat out for the moment, there seems to be a discrepancy between how quickly we can digest some foods like refined carbs and how fast our bodies can possibly restore our glycogen stores.

It seems that we can absorb some carbs like pure glucose for example almost immediately whereas glycogen stores take at least 20h to restore, even if you're a super fit athlete.

That leaves me with the question - where does this energy go meanwhile?

Of course, I imagine that just eating 1000kcal worth of pure glucose will probably result in a diabetic coma but it doesn't have to be this extreme. Even just eating 1000kcal worth of white bread seems to overwhelm our bodies ability to store it all in glycogen by the time we are done digesting it.

So again, where does this energy go in the meanwhile?

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    $\begingroup$ I love that you've presented what looks like research in your question, so +1. :) But where, may I ask, did you get this information? $\endgroup$ Nov 18, 2021 at 15:17
  • $\begingroup$ @anongoodnurse I didn't keep a list of sources but its what I noted down after reading a bunch of related articles and conclusions of studies. The numbers aren't nearly exact but I just need a general idea of how it works to create an approximation for a game. I did continue looking into this meanwhile and found a few errors - for one, blood glucose is never really supposed to be used up and is there just to shuttle the calories around and second, only the glycogen in the liver is available to the whole body (~400kcal worth) but it seems that's enough for a day or so as muscles have their own. $\endgroup$
    – user81993
    Nov 18, 2021 at 16:00
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    $\begingroup$ Thanks, that makes sense. The numbers are just way, way off. Serum glucose levels are kept between ~70 and ~110 mg/dl between meals. Above that, we're making glycogen, below, we use it. It's a constant balance. All of it, though, is homeostatic in mechanism. (2 seconds of max exertion? What's kreatinine? Do you mean creatine? 8 seconds?) It seems we'd all be dead after 10 seconds of hard exertion. If it's for a game, use any numbers you want. $\endgroup$ Nov 18, 2021 at 21:10
  • $\begingroup$ Your focus is on glycogen and athletics. Can you clarify whether you are concerned specifically with skeletal muscle glycogen by editing your question. At the same time, after reading my answer, please reconsider your use of the word energy. $\endgroup$
    – David
    Nov 19, 2021 at 14:14
  • $\begingroup$ @anongoodnurse Yeah, it seems that blood normally has just 4g of glucose in it in total and yeah, I do mean creatinine. The 2 and 8 seconds I fished out of some research done on weight lifters, apparently they hit similar "wall"s as marathon runners do after exhausting a particular store of energy and were unable to perform on the same level until recovery, all though these "wall"'s are easier to miss as it takes just 10 seconds to rebuild the ATP stores and around a minute for creatinine. $\endgroup$
    – user81993
    Nov 19, 2021 at 14:27

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It is hard to be sure what the poster is asking exactly, but my best effort is the following:

It takes 24 hours to replenish muscle (?) glycogen after carbohydrate intake, therefore the “energy” from the digestion of the latter must exist in some form in the intervening period. What and where?

There are several problems with this question which need to be clarified so that it can be recast in a form suitable for answering.

  • There is a failure to distinguish between energy and molecules.

Certain molecules have the potential to release biologically utilizable energy when subject to particular chemical transformations, or have the potential to be converted into compounds capable of such transformations and which can be stored in the body. This distinction must be made in a scientific discussion of the topic.

  • It is not clear whether the question relates to muscle or liver glycogen. From the reference to the (atypical) human beings termed athletes, I assume that skeletal muscle glycogen is intended.

This is important, because the functions (and regulation) of liver and muscle glycogen are quite different.

  • The question seems to assume that muscle glycogen is major end-product of ingested carbohydrate in mammals.
  • The question seems to assume that mammalian metabolism prioritizes the replenishment of muscle glycogen over other metabolic fates for ingested carbohydrate.

Neither of these assumptions are true. The skeletal muscle has a limited capacity for glycogen storage. Despite the plethora of irrelevant and undocumented figures in the question, this is not mentioned.

Likewise, the question does not consider whether there is any difference in replenishment of glycogen in the normal recovery after muscle exercise compared with that combined with a large intake of carbohydrate. This is relevant to the second point. The mammalian liver is the organ most involved in distributing glucose and fats to the body, and as far as glucose is concerned its first priority is the brain and nervous tissue, not skeletal muscle. The way the liver and its target tissues handle glucose depends on the hormones of feeding and starvation — insulin and glucagon, the latter of which does not affect skeletal muscle.

  • Finally, the actual question seems to have been obscured by scientifically meaningless assertions such as “I'm treating glycogen as our main "battery"”, and recitation of figures that are largely irrelevant to the actual question — when you unearth it.

Rephrasing the original question, removing the use of the term energy and its implicit, but erroneous, assumptions, actually gives us two questions.

  1. What, quantitatively, is the metabolic fate of glucose following a high-carbohydrate meal in an average (e.g.) Western adult male.
  2. Why does it take 24 hr to replenish muscle glycogen?

The quantitative answer to the first question can, I imagine, be found in the literature, although more thoroughly in laboratory animals. There must be radioactive tracer studies on rats exhausted in treadmills. In general depending on the physiological state of the organism and the tissue it can be oxidized via glycolysis and the tricarboxylic acid cycle, generating the ATP currently required for various anabolic reactions, converted to glycogen, fat or creatine (depending on the tissue) or its metabolites used in synthetic processes. I presume an excess can even be excreted in the urine.

If it really takes 24 hr to replenish muscle glycogen, I assume that is because the integrated regulation of cellular metabolism does not treat it as a priority. It may be that, even in the fed state, the combination of blood glucose and insulin concentration only drives a slow replenishment. It would be interesting to know the actual rate of replenishment.

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    $\begingroup$ I think the question was perhaps not asked with the specificity or precision that a biochemist would expect, but I think it's a reasonable question if you interpret it more from an exercise physiology standpoint. In that case, what OP describes as "energy" might ultimately come down to "(potential for) ATP". My interpretation is that they are not asking so much about "what happens" in terms of biochemical pathways of digestion but rather in resolving what they have found as some inconsistencies with the estimates they have learned about for different reservoirs of "potential ATP". $\endgroup$
    – Bryan Krause
    Nov 18, 2021 at 22:56
  • $\begingroup$ +1 one for the expanded answer, but one word of caution on the comment: "questions are written in English which has a scientific vocabulary of precise terms which may be understood unequivocally" : this website attracts interdisciplinary audience (of which I am). Most physicists would feel more at ease with the usage of the word "energy" in the OP than in your answer. This said, again, I am of the opinion that your answer is also very good for a non-biologist reader and that's the real point. $\endgroup$ Nov 19, 2021 at 16:12

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