Confused, need help

[The_KM]

Hey all...there's a question here that I just can't find the answer to.

Why does fructose only fill liver glycogen stores and not muscle glycogen as well?

Fructose is digested/absorbed in the small intestine, then it goes to the portal vein (just a vein that goes from the small intestine to liver). Then when the liver glycogen is full the excess is just converted to fat but there are several aspects I don't get.

But fructose is broken down into glucose, so why can't that glucose be used to restore muscle glycogen?

It might have something to do with GLUT2 and 5 but I'm not sure.

Can anyone help me nail this one? I think there's something simple I'm missing here.

Thanks!

Kev

 

[Ironslave]

It's not so much that it can't, it's more so that it doesn't get the chance to. Three factors really contribute to this:

(1) enzymes involved in fructose metabolism, fructokinase and triokinase, are highly expressed in the liver, while they are not in other sites.
(2) the liver is exposed to higher levels of ingested fructose than other tissues
(3) There is a high "first pass metabolism" effect, which causes extraction of fructose by the liver, which thus means there is less available for other tissues (ie, muscle).

 

[The_KM]

It's not so much that it can't, it's more so that it doesn't get the chance to. Three factors really contribute to this:

(1) enzymes involved in fructose metabolism, fructokinase and triokinase, are highly expressed in the liver, while they are not in other sites.

So because those enzymes are not expessed in other sites, the transportation can not be done? I think i'm making connections..

(2) the liver is exposed to higher levels of ingested fructose than other tissues

So the muscles can not take in that fructose ratio because its less exposed?

(3) There is a high "first pass metabolism" effect, which causes extraction of fructose by the liver, which thus means there is less available for other tissues (ie, muscle).

But if the liver is at its maximum storage why can't the body utilize the excess ass muscle glycogen instead of it being converted into triglycerides or BF? That's where I'm stuck; how come the body can't utilize the breakdown product of fructose (glucose)?

It takes me a while to get things, but when I get them -- I get them! Thanks for your help IS.

 

[Ironslave]

So because those enzymes are not expessed in other sites, the transportation can not be done? I think i'm making connections..

Not so much the transportation, but more so the metabolism of them. If you want to get in it, any reaction can really occur so long as there is a negative value for Gibb's free energy, (or have a favorable reaction provide energy for an unfavorable one), but it would happen MUCH slower. Enzymes speed up reactions by trillions of times.

So the muscles can not take in that fructose ratio because its less exposed?

yis, the liver has metabolized most of it.

But if the liver is at its maximum storage why can't the body utilize the excess ass muscle glycogen instead of it being converted into triglycerides or BF? That's where I'm stuck; how come the body can't utilize the breakdown product of fructose (glucose)?

It can.

 

[The_KM]

Not so much the transportation, but more so the metabolism of them. If you want to get in it, any reaction can really occur so long as there is a negative value for Gibb's free energy, (or have a favorable reaction provide energy for an unfavorable one), but it would happen MUCH slower. Enzymes speed up reactions by trillions of times.

So, more fructose would be converted to triglycerides, but more glucose, for example would go ahead and fill muscle glycogen? I read that phosphofructokinase signals that all glycogen storage is full...or does it just signal that liver glycogen is full?

Any thoughts?

Again, thanks for your help.

 

[Big_Guns_Lance]

^^PFK-1 signals that all glycogen storage is full (liver and muscles)