jay wrote:

When we eat excess protein, how is it stored?

Cell growth uses protein. Otherwise there's comparatively little
ability to store protein. It's easy to find claims that the body can
store zero protein other than as lean mass but that seems to be a
circular argument - Protein equals lean so it's a definition. But if
you think of lean in terms of muscle mass then it stops being a circular
argument and also stops being true.

Cells can grow and shrink and that is some sort of storage mechanism for
protein. The body can store a very small amount of carbs as glycogen,
a couple of days of less. The body can store vast amounts of fat. How of
protein can be stored is obscured by claims that the amount is zero but
it isn't large. Muscle loss starts happening fast in stavation diets.
Since muscle loss is lower on low carb systems that's yet another reason
to understand that ketosis is the result of a success predator diet not
the result of a starvation diet. The fact that ketosis happens during
starvation does not mean it equals starvation. Sleep happens during
starvation so anyone who sleeps is starving, right?

I imagine some of it converted to glycogen and glucose by the liver.
Some enters cell to be burned.
Does the excess mostly stay in the blood?
How would we even know we have excess protein?
Is there a common blood measurement for this?

Protein not used by the cells (including some very difficult to
determine amount of storage that is limited and slow to build) is burned
as fuel into glucose. It's why people on high protein diets see a rise
in blood sugar.

Amino acids have nitrogen - amonia, urea, uric acid. Any test for these
nitrogen products in the urine will show it. If there's any amonia at
all something is wrong. It's supposed to get converted long before it
makes it to the kidneys.