Diet Physiology Questions

[Paul]

Its generally accepted that in order to build new muscle, a caloric surplus must exist.
But...

How does the body "know" its in a caloric surplus?

How long must a surplus be maintained before muscle starts to "build"?

While cutting, if you ate 1000 kcal over maintenance one day, would that add a few grams of muscle?
Could this explain people who claim to "recomp"? A few high days here and there adding up over time?

While bulking, if you completely fasted one day (say, Sunday on a MWF split), obviously energy will be pulled from fat stores. Would this slow fat accumulation without derailing your gains, weekly calories being equal?

Mostly just curious. I've seen similar questions before, but the answer has always been very bro science or "it doesn't matter."

[Wilhelm]

How does the body "know" its in a caloric surplus?

https://www.ihmc.us/stemtalk/episode-70/

[00:20:50] Ken asks about how nutrients and other inputs are sensed and integrated by the mTOR complexes, given how one of the most fascinating aspects about mTORC1 is its role as a nutrient sensor.
[00:23:46] Ken asks why both nutrients and growth factors are required to activate mTORC1.

[TimK]

Muscle protein synthesis and muscle protein breakdown are always taking place in the body; it’s the net balance over time that determines whether you gain or lose muscle.

[Paul]

I'm going to listen to that podcast a few times, but it seems to answer the "how?" a bit.

I realize MPS and MPB is always occurring, but obviously one outpaces the other depending on nutrition and training. And MPS relative to fat gain/loss is more to the point.

If I have good training and nutrition in a caloric surplus, some percentage of weight gain will be muscle.

If I am in a caloric deficit, with good nutrition and training, I will lose very little if any muscle. If there is a spike in calories, it seems that some portion might still go to muscle?

If someone is "recomping", could it not be that their big, flashy instagram "cheat days" are when muscle is added, while losing fat the rest of the week? Leading to a slow change in body composition?

[FredM]

If someone is "recomping", could it not be that their big, flashy instagram "cheat days" are when muscle is added, while losing fat the rest of the week? Leading to a slow change in body composition?

Not really. The body is a lot more complicated than that and timescales for these things are generally a lot longer than a day.