After digesting food, where does the energy go before being stored by the body?

Question

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?

Answer 1

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.