T Nation

Understanding Carbs

I’m confused about carbs. In the ‘Milk: Good Nutrition?’ thread Joel Marion mentioned that galactose restores liver glycogen…not muscle glycogen. I also remember Dan Duchaine saying something similar about fructose [don’t quote me]. Only 50g of fructose can be absorbed per day.

Can anyone give me a ‘quick and dirty’ of where the different sugars are stored in your body?

Which carbs can you only eat in limited amount? [like Dan D recomendation for fructose]

Also I’m curious to know if the ‘complex carbohydrate’ and ‘simple carbohydrate’ categories are any indication of where the energy is stored?

All your help is much appreciated.

Somebody correct me if I’m wrong, but this is how I think it works…
There are 3 monosaccharides, glucose, fructose, and galactose. Of these only glucose can be stored as muscle glycogen, the rest go to refilling liver glycogen. Of the three common disaccharides there’s Sucrose, Lactose, and Maltose. Sucrose is fructose+glucose, so half refills liver glycogen, and half muscle glycogen. Lactose is galactose+glucose, so it acts much the same way as sucrose, half refills liver, and half muscle glycogen. And maltose is glucose+glucose, so it would fill muscle glycogen. Something to keep in mind though is that if the liver glycogen is already full, even if muscle glycogen isnt, incoming carbs will be converted to fat. (The liver glycogen trap). That’s why bodybuilders tend to avoid too much fructose and galactose. This is just my understanding anyway…

KingProtein has it almost nailed. The only
thing I think he might have incorrect is that
I don’t think that fructose or galactose
can be stored directly as fat. I think they
must be processed by the liver before they
can be stored or used at all (including being
stored as fat). Although I’m not 100% sure of
that. If someone else knows for certain about
that, that would be helpful. Glucose OTOH, can
be stored directly as fat, after muscle and
liver glycogen stores have been replenished.

Thanks guys, that was really helpful. Unfortunately its brought up another question. How does one deplete liver glycogen?

Sweet…now we’re getting somewhere, this is the stuff I was trying to get at in the milk thread. One thing I’m wondering is: being that lactose is a disaccharide made of galactose + glucose (here comes the hard core chemistry) are those two sugars chemically combined to form a new molecule or are the two sugars just ‘mixed’ together so the glucose molecules and the galactose molecules are separate? If they are combined into a new molecule, does the body break it back down into glucose and galactose during digestion? Just one more question…does fruit only contain fructose or are there other carbs (hopefully one’s that restore mucle glycogen) in fruit also. Thanks dudes…

Yes, the bulk of carbohydrates in such fruits as apples, oranges, pears, etc. is not fructose. Typically, the juice is mostly fructose, while the fruit itself is comprised of more “starchy” carbs.

so um…the carbs in the apple im eating dont come from fructose but ‘starchy carbs?’ like bread? what do you mean, and how does that translate into physiological effects? i eat massive amounts of fruits and vegetables all the time without any noticeable fat gain…

they are joined together via hydrolosis. This site ( http://www.chem.wsu.edu/ Chem102/102-PolySacch.html (remove spaces)) explains this rather well, but contains some “hardcore chemistry”

Some fruits supposed to be good carbs/ i.e containing dextrose is bananas and melons. I just dont know the % of fructose vs dextrose but maybe someone else knows?

liver glycogen is the reserves depot for glycogen which can be called upon when muscle glycogen runs out. example:

i always try to eat just enough carbs to keep all my glycogen stores full and no more, but sometimes i fall short over several days causing the primary muscle glycogen to deplete. this usually happens during continued bouts of long duration cardio which i often do. when your muscle that is being worked taps out of it’s primary stored glycogen your body starts to draw from the liver since one muscle can’t borrow glycogen from another muscle. i can easily tell when this is happening. as i am peddaling i feel my legs tire, i get dizzy and my legs start to burn a bit. this is when my primary glycogen has runout and my legs temporarily utilize blood glucose (which is reserved for the brain, cns etc.) this light-headedness is the result of the dip in blood glucose levels. after two to three minutes i come out of the dizziness my legs cool down as i begin running on the liver glycogen stores. after this transition i am good to go. when this happens i know i have to refuel big time right after and cut back on the cardio until i am fully recharged. kevo

The juice in the fruit is mostly fructose…the rest of the fruit (ya know, that crunchy part that you bite into) does help to replenish muscle glycogen.

Actually, I thought the “crunchy” part was soluble and insoluble fiber and the sweet portions in juice is both fructose and glucose.

Some of you guys are saying that fructose would be stored as fat after liver glycogen stores are full. couldn’t the body convert the fructose into glucose via an isomerase–> hexokinase enzymatic conversion? or does predominantly in the other direction (ie Glycolysis)