kuan

Does your body turn fat into glycogen and store it in your liver and muscles?

Enthalpic

No. Small amounts of protein turn to glycogen, but no fat.

ericgu

Enthalpic is right. Your body tears down your muscle to renew the glycogen.

Probably not the result that you were looking for...

DannoXYZ 

Fat cannot be converted into glucose then glycogen. Fats are only broken apart in the fatty-acid spiral into Acetyl-CoA which is used to feed the Krebs Cycle to generate ATP.

The glucose that's used to replenish glycogen then comes from ingested carbs or from existing muscle proteins through gluconeogenesis.

Some articles about nutrition and muscle-building. Note these are bodybuilders and notice the ratios of their nutrition:

Bodybuilding.com - Anabolic Ammunition Arsenal (look about 3/4 down page for calorie-mix calculator)
ABCbodybuilding - A Scientific Investigation into the Rationality of Post Workout Carbohydrate Consumption
ABCbodybuilding - Analysis of Nutrient use during Low, Moderate, and High Intensity Exercise
SparkNotes - Functions of Carbohydrates
Peak Performance - How much protein do athletes need? note "up to 150gm a day" for performance athletes.

kuan

Danno, yes or no and SIMPLE explanation would work. OK actually that was just about right for me.

OK thanks everyone.

branman1986

I love learning esoteric information like that Keep it up Danno/Eric!

We've got such a large forum base here, that we've got experts in a lot of different fields...might as well learn something from 'em

DannoXYZ 

figured you got the simple answer already, so I gave you a little more detail on why. Have fun and keep up the good work!

Here's how the two suppliers of raw-materials for glycogen-replenishment works:

carbs (glucose,fructose) -> glucose-6-phosphate -> glycogen

tissue-protein -> nitrogen-pool -> pyruvic-acid -> glucose-6-phosphate -> glycogen

Notice that the protein pathway is more complicated and slower. So during recovery with available glucose, that's used preferentially to proteins, thus sparing your muscles from catabolism.

The ingested proteins in the typically 4:1 carb-protein mix actually serves a different purpose. It actually causes a higher insulin-response than carbs alone and speeds transport of glucose into the muscles for quicker glycogen-replenishment.

Richard Cranium

Yeah, and what's missing is the fact that some glucose is necessary for direct metabolic support of the central nervous system. Therefore, if and when some one severely depletes their glycogen levels, the body will start sending "shut down" signals warning the brain that blood-glucose levels are not being maintained.

I've still never found out what it is about nerve cells, that require glucose. Nor have I ever discovered what percentage of the basal metabolic requirements have to be met by glucose.

If you could calculate the caloric value of remaining liver glycogen and knew the basal metabolic glucose requirements of a given person, you could predict the longest period they could safely fast. (take that you pro-anne nut jobs)

NoRacer

Not entirely true about shutdown messages, because of ketone bodies:

!!Comatoa$ted

Initially your body will tear down muscle to supply glucose to the body. It does this by the use of diabetogenic hormones like cortisol, epenephrine and glucagon. In a state of starvation the CNS will switch over to using ketones as its main source of fuel when the liver can no longer supply glucose via gluconeogenesis. The CNS will demand that it receives a certain amount of glucose, but when it does not it will use ketones to operate, as will muscles. The small intestine is not able to run on ketones, it can only use glutamate and glucose. If you are starving yourself there is probably no need for your small intestine to function as it normally does.

The interesting thing about nerve cells is that they do not need insulin to absorb glucose as does most of the bodies other cells, and this is why prolonged hyperglycemia can cause so many problems, especially retinopathy, as well as other neuropathies. The retina has the most biologically active nervous tissue by weight than almost any other part of the body.

In the average 70Kg male the liver is able to store about 300 calories worth of glycogen, and about 600 in the muscles. The muscles can export only a very minute amount of glucose for the rest of the body because the enzyme glucose 6 phosphatase is not present or present in only very small amounts in the muscle, but is present within hepatocytes.

Enthalpic

I’ve heard it’s because the axon can be quite long and reach large distances from the cell body. Other fuel substrates cannot diffuse sufficiently to feed the terminal ends.

!!Comatoa$ted

Fat is composed of fatty acids ester bonded to a glycerol. If you claim that fat can't be made into glucose, how come glycerol is used in gluconeogenis?

If you wish I can try to post the whole article.


And how about

Glycerol Metabolism

DannoXYZ 

Check this out: IndState.edu - Gluconeogenesis. The pathway is the important part, not the components. Just because something is made up of similar parts, doesn't mean it goes through the same pathway. Like fructose vs. glucose, they're all made up of CHO, but the pathway is vastly different on how they're metabolized. The 3-carbon glycerol backbone is tiny compared to the 48 or 54-carbon fatty-acid chains.

The fatty-acid spiral cleaves off pieces of the chain and makes AcetylCoA. At no point is the fatty-acid converted to glucose. However, the small glycerol backbone can be converted to glucose, but it's a tiny fraction of the total energy generated from fatty-acids.

!!Comatoa$ted

Thank you for your wonderful diagrams.

I see what you are saying that the fats themselves are not made into glucose, but lipids can be used to make glucose, the lipid supplies the glycerol, which is a very important part of lipids. But using your logic one could say that lipids do not make acetyl co-a because the whole lipid is not used only the fatty acid minus the glycerol, and bodily fat, adipose, is made up of more than just fatty acids. The fact is part of the fat (aka triglycerides) is used to make glucose, so saying glucose cannot be made from stored fat does not seem accurate, since a portion of the lipid can be used to make glucose. Just like a portion of the lipid is used to make ketones, and acetyl co-a.

And as your nice diagram shows it is a fatty acid spiral, in order for this to happen it seems that you need to separate fatty acids from the glycerol in able to utilise the fatty acids in the spiral. What happens to the glycerol. As a result of the breakdown of lipids glycerol is used to make glucose via gluconeogenesis can be made now how can it be that lipids are not able to increase glucose in the blood, just as glucose in the blood can increase fat. In the gluconeogenic pathway glycerol is used in the pathway to make glucose.

If you have some nice diagrams of gluconeogenesis, take a look at them, what part of lipids do you see being used to make it?

Enthalpic

Even with short chain fatty acids the molecular weight of a triglyceride will be >800g/mol. The molar mass of glycerol is 92g/mol and yields 4.3kcal/g as food energy.

You can see that one gram of dietary fat will contain equal to or less than 0.1g of glycerol (1 x 92/800+), working out to be about 0.4kcal/g of “carb energy” per gram of fat. With fat providing about 9kcal/g, a whopping 4.5% of that could come from the glycerol. So to reload the muscles with carbs the body would have to burn a prohibitive amount of fatty acids to liberate enough glycerol. Furthermore, your body doesn’t like to burn glycerol at all unless starving; it prefers to keep the glycerol inside the cells for the next incoming free fatty acid to attach to.

sfrider 

One triglyceride consisting of 18-carbon fatty acids produces 139 molecules of ATP from the fatty acids and 9 molecules of ATP from the glycerol. Only the glycerol pathway, via the peruvic acid precursor or oxaloacetic acid can synthesize glycogen/glucose. (For those who aren't familiar with this, human fatty acids are 14-22 carbon, with 16 and 18 being most common. They're always even carbon counts because of how they're made.) Not sure what determines where the glycerol ends up (glucose or ATP), perhaps the presence/absence of CO2? Or NH3 blocking some enzymatic activity (like creation of peruvic acid)? Just because there happens to be a pathway for glycogen synthesis from glycerol, it's not a foregone conclusion that just because the CNS needs glucose that fat catabolism will produce it...

sfrider 

To answer the original question, fat synthesis is one-way; we can turn fatty acids to ATP, but not back to glycogen. Which accounts for why it's so bloody difficult to lose fat; if our bodies had any significant ability to turn it back into glycogen we'd lose it as quickly as we gained it. Our bodies are also extremely efficient at turning glycogen into fat, basically our adipose tissues can absorb it as fast as we can digest it.

!!Comatoa$ted

Thank you for the explanation.

I understand that the body does not usually obtain glucose from lipids, and when it does it is a very small amount, and pyruvate can be used to make glucose. To make glycogen from glycerol seems like the body would use a tremendous amount of energy, and in context to the OP I am sure that fat stores will not prevent proteins from being disassembled. I only wished to point out that lipids can be used to form glucose, and not that lipids are a major source of glucose. Even some of the people that have said it is in no way shape or form possible to make glucose from lipids have also indicated that there is a possibility for them to be made from glycerol, which is an integral part of lipids. This is all I wished to point out. I also believe that fatty acids do not form glucose, but to liberate the tremendous energy stored in lipids the glycerol is freed from the fatty acids, and then the glycerol may be used to form glucose. As well, the contribution of glycerol to form glucose is very limited but still exists.

This stuff interests me a great deal, and I have learned a lot from Danno et al, and I am not trying to be-little any one. I only wish to challenge views that are contradictory to what I have learned, and take part in a discussion in a subject that I am interested in, nothing more.

Again, thank you all I have learned a great deal from many forum members and have had my passion for biology fueled by reading many of the intelligent posts here, even if mine are not always so.

Richard Cranium

Screw it, reformat your posts - you've ff-ed up the thread too much to bother reading.

!!Comatoa$ted

I find reding through mozilla it is screwd up, but IE it is just fine. Or maybe it is a plot against you?