Larry...



M

Martin W. Smith

Guest
You were going to take a few days to find out if muscles recharge with glycogen by converting stored
fat, when there are no carbs present.

I think the answer is no, but I have read that the liver starts creating glycogen for this purpose
when there are no carbs present, but it take slonger to recharge the muscles this way. The question
then becomes, where does the liver get the material to make glycogen, from stored fat or from
ingested fat?

martin

--
Martin Smith email: [email protected] Vollsveien 9 tel. : +47 6783 1188
P.O. Box 482 mob. : +47 932 48 303 1327 Lysaker, Norway
 
>>You were going to take a few days to find out if muscles recharge with
glycogen by converting stored fat, when there are no carbs present.

>>I think the answer is no, but I have read that the liver starts creating
glycogen for this purpose when there are no carbs present, but it take slonger to recharge the
muscles this way. The question then becomes, where does the liver get the material to make glycogen,
from stored fat or from ingested fat?<<

The body _can't_ convert fat to glycogen! Glycogen can be made only from sugars and from protein
(amino acids) -- the latter through a process called gluconeogenesis. So, when you don't take in
sufficient carbohydrate to make glycogen, the only way that you can make it is to break down muscle
to release amino acids, which can then be converted to glucose, and from there to glycogen.

The reason that mammals can't convert fat to carbohydrate is that the only possible precursor to
sugar/gycogen from fatty acids is a two carbon molecule called acetyl CoA, formed from the breakdown
of fatty acids. Acetyl CoA is also formed from the breakdown of pyruvate, which, in turn, comes from
the breakdown of glucose. What happens is that glucose goes to pyruvate which goes to Acetyl CoA +
C02. So, in theory, fats could make glucose (and then glycogen) by being broken down to Acetyl CoA,
which could then combine with C02 to make pyruvate, which could then go back up the glyconeogenic
pathway to make glucose. In practice, however, this doesn't happen because the reaction pyruvate ->
Acetyl CoA + CO2 is essentially irreversible. So animals (including humans) can't make glycose (or
glycogen) from fat (plants can do this, but animals cannot).

I could have answered this question right away. The question that I said I'd look up (still haven't
had the time to research it, but I shall do so) is whether or not people on very low carbohydrate
diets do, in fact, have reduced glycogen stores.

Will get back when I've got an answer.

Larry Weisenthal

Certitude is poison; curiosity is life
 
My naturopathic doctor told me drinking a quart of OJ is a good way to quickly build up depleted
glycogen. The excess amount of juice is more than the body needs for energy and the liver converts
this surplus into glycogen. (This is probably a simplification of what happens. However, the times
I've tried this, I seem to be able to sustain harder workouts the following day)

I eat a low carb (no pasta, potato, banana, grains-a little no wheat bread) diet of mostly raw
fruits and veggies, nuts, seeds. After experimenting for a while, I find having a single orange
about an hour before my afternoon Masters' WO seems to give best energy for me. However, AM WOs I do
fine with no food.

This same doctor claims if glycogen storage is functioning properly, one should be able to go
without food for 24 hours without energy problems.
 
larry burns <[email protected]> wrote:
>My naturopathic doctor told me drinking a quart of OJ is a good way to quickly build up depleted
>glycogen. The excess amount of juice is more than the body needs for energy and the liver converts
>this surplus into glycogen. (This is probably a simplification of what happens.

It's close to 500Cal of fructose. It's a too strong for me, though, right after a hard session.

>This same doctor claims if glycogen storage is functioning properly, one should be able to go
>without food for 24 hours without energy problems.

I've seen estimates of typical glycogen reserves to be in the 2000-3000 range, which is a normal
day's diet.
--
Jason O'Rourke www.jor.com
 
> The question that I said I'd look up (still haven't had the time to research it, but I shall do
> so) is whether or not people on very low carbohydrate diets do, in fact, have reduced glycogen
> stores.<<

This was surprisingly hard to track down. I still don't have the answer with regard to a study in
humans (perhaps not surprising; would require multiple muscle biopsies).

The following rat study was very interesting, however.

Basically, they looked at both red ("aerobic") and white ("anaerobic") muscles in animals on high
fat diets vs high carb diets, pre and post exercise, for glycogen content.

The only differences were in the case of the white muscle. In the high fat group, muscle glycogen
was lower than in the high carb group. In red muscle, there were no differences in glycogen, pre-or
post exercise. White muscle can burn only carbs (they are "white" because they lack mitochondria,
which are required to burn fat and lactate, which, along with glucose [from glycogen], are the main
fuels for muscle fiber contraction). It seems that with a high fat diet, there is a selective
dis-use (lack of use) of white muscle and more use of red muscle (capable of burning fat + lactate).
So the athlete burns more fat (but also takes in more fat, so the fact that the athlete burns more
fat does not necessarily mean that the net "fat balance" (intake vs burning) is skewed to promote a
greater net loss of fat).

Glycogen is probably not depleted in red muscle on high fat diets anymore than on high carb diets
because less glycogen is being burned in the red muscles on the high fat diet (red muscle can burn
either glycogen, fat, or lactate, while white muscle can burn only fat).

What is the potential disadvantage in not burning glycogen for fuel in exercising muscle? What is
the potential disadvantage in not using white muscle fibers?

Well, glycogen metabolism generates ATP faster than does fat metabolism. So the rate of ATP
production will be theoretically diminished, when glycogen isn't burned. And white muscle fibers are
the so-called "fast twitch" fibers which power explosive performance (as in sprinting). So not using
these fibers is a potential disadvantage in competition.

At the level of the ultra-endurance athlete or at the leve of the fitness swimmer, I doubt that
there would be a reproducible, measurable difference in performance.

Maybe the reason Martin's swimming performance has been just a little uneven since changing diets
has to do with the necessity of getting his muscle fibers to adjust to the changing energy sources,
which may take a little time. As I wrote some months ago (relating to my own experience), I do think
that when a swimmer loses weight that the swimmer's buoyancy changes, balance changes, and
performance may change (at least temporarily) as well.

The caveat to all of this is that this was a RAT study. Not sure that a similar study ever has been
(or will ever be) done in humans.

1: Int J Sports Med. 1998 Aug;19(6):419-24.

Glycogen depletion patterns in trained rats adapted to a high-fat or high-carbohydrate diet.

Nakamura M, Brown J, Miller WC.

Department of Kinesiology, Indiana University, Bloomington, USA.

Male Sprague-Dawley rats (n = 48, > 200 g) were progressively treadmill trained over 5 wk where they
were running 60 min/d, 5 d/wk. One-half of the group consumed a high-fat diet (HF, 78.7% of energy),
while one-half consumed a high-carbohydrate diet (HC, 68.7% of energy). On the day of the
experiment, 6 rats per diet were run at 29 m/min, 8% grade for 0, 10, 20, or 60 min. Immediately
post-exercise rats were anesthetized, and soleus (S), red vastus lateralis (RV), and white vastus
lateralis (WV) muscles were removed. There were no significant differences between diets for S
glycogen pre-, during, or post-exercise. RV glycogen (micromol x g(-1) wet wt) was lower (p < 0.05)
at rest for the HF (27.5 +/- 3.9, Mean +/- SEM) vs the HC (37.6 +/- 3.5), but similar to HC at 60
min (11.0 +/- 1.9, HF; 8.6 +/- 1.3, HC). RV glycogen use rates (nmol x g(-1) x min(-1)) were lower
for the HF (985 +/- 295, 356 +/- 61) than the HC (1593 +/- 144, 1055 +/-
2) for 0-10 and 11-20 min, respectively. Resting WV glycogen was lower for the HF (25.3 +/-
1.6) vs the HC
(3.7 +/- 5.8), while post exercise values were similar
(4. +/- 4.4, HF; 15.7 +/- 2.0, HC). WV glycogen use was negligible from 0-10 and 11-20 min in the HF
compared to the HC (280 +/- 169 and 1601 +/- 177 nmol x g(-1) x min(-1), respectively). These
data indicate that muscle glycogen is spared during the early stages of prolonged exercise in HF
adapted rats and that the sparing occurs according to expected muscle recruitment patterns.


Larry Weisenthal

Certitude is poison; curiosity is life
 
Larry Weisenthal wrote:
> The only differences were in the case of the white muscle. In the high fat group, muscle glycogen
> was lower than in the high carb group. In red muscle, there were no differences in glycogen,
> pre-or post exercise. White muscle can burn only carbs (they are "white" because they lack
> mitochondria, which are required to burn fat and lactate, which, along with glucose [from
> glycogen], are the main fuels for muscle fiber contraction). It seems that with a high fat diet,
> there is a selective dis-use (lack of use) of white muscle and more use of red muscle (capable of
> burning fat + lactate). So the athlete burns more fat (but also takes in more fat, so the fact
> that the athlete burns more fat does not necessarily mean that the net "fat balance" (intake vs
> burning) is skewed to promote a greater net loss of fat).

This might well explain the results I saw. I swam slower because my white muscle fibers were not
being used as much or not at all. I assume by "high fat" we mean high in percentage terms. My diet
is not particularly high in fat in grams, I think. It might not be much higher than it was prior to
the diet at all, given that I have eliminated cookies and brownies. I eat more eggs, more cheese,
and I eat bacon, but I am not eating a lot more. I am eating a lot more nuts, which are high in fat.

> Maybe the reason Martin's swimming performance has been just a little uneven since changing diets
> has to do with the necessity of getting his muscle fibers to adjust to the changing energy
> sources, which may take a little time. As I wrote some months ago (relating to my own experience),
> I do think that when a swimmer loses weight that the swimmer's buoyancy changes, balance changes,
> and performance may change (at least temporarily) as well.

I think this is the case, but does a white fiber actually change to a red fiber, or does the white
fiber simply not contribute in the absense of carbs?

martin

--
Martin Smith email: [email protected] Vollsveien 9 tel. : +47 6783 1188
P.O. Box 482 mob. : +47 932 48 303 1327 Lysaker, Norway
 
"Martin W. Smith" <[email protected]> ha scritto nel messaggio news:[email protected]...
> > use of red muscle (capable of burning fat + lactate). So the athlete
burns
> > more fat (but also takes in more fat, so the fact that the athlete burns
more
> > fat does not necessarily mean that the net "fat balance" (intake vs
burning) is
> > skewed to promote a greater net loss of fat).
>
> This might well explain the results I saw. I swam slower because my white muscle fibers were not
> being used as much or not at all. I assume

It seems much more likely, to me, that you swim slower due to the necessity to adapt your tecnique
to your different buoyancy.

O.
 
P.S. Just a follow-up to the discussion on the RAT study, in which a high-fat/low-carb diet produced
a depletion in glycogen content in white muscle fibers (which was compensated for by increased
use of red muscle fibers, which can adapt to low carbohydrate intake by *burning more fat).

The other thing to consider is that white muscle fibers are more prominent in upper body ("turkey
wing") muscles than in lower body ("duck wing") muscles in humans.

If the swimmer disproportionately relies on white muscle fibers (relative to runners and cyclists),
which I believe is true, then glycogen depletion would be expected to have a greater deleterious
effect on swimmers than on runners and cyclists. A swimmer (or kayaker) might have more trouble with
the Atkins Diet than a runner or cyclist, performance-wise.

Again, just based on data from a single rat study; still haven't found any comparable studies in
humans (looking at muscle fiber glycogen as a function of high fat/low carb vs high carb/low fat).

Larry Weisenthal

Certitude is poison; curiosity is life
 
In article <[email protected]>, [email protected] (Larry
Weisenthal) writes:

> (red muscle can burn either glycogen, fat, or lactate, while white muscle can burn only fat).

Geez...misspoke/flying fingers. White muscle can burn only carbohydrate (glucose, from stored
glycogen and from circulating glucose). This is because white muscle lacks mitochondria, as
previously explained.

So a Sprague-Dawley rat on the Atkins diet gets depleted of glycogen in his white (Fast
Twitch/"anaerobic") fibers and has to use his red (slow twitch/"aerobic") fibers, which become more
adapted to fat burning, but which can't contract as rapidly as the rats now unused white fibers. And
the rat doesn't decrease his net fat balance because, even though he's burning more fat, he's also
eating more fat.

Larry Weisenthal

Certitude is poison; curiosity is life
 
On Tue, 12 Aug 2003 14:50:46 +0200, "Martin W. Smith" <[email protected]> wrote:

>Larry Weisenthal wrote:
>> The only differences were in the case of the white muscle. In the high fat group, muscle glycogen
>> was lower than in the high carb group. In red muscle, there were no differences in glycogen,
>> pre-or post exercise. White muscle can burn only carbs (they are "white" because they lack
>> mitochondria, which are required to burn fat and lactate, which, along with glucose [from
>> glycogen], are the main fuels for muscle fiber contraction). It seems that with a high fat diet,
>> there is a selective dis-use (lack of use) of white muscle and more use of red muscle (capable of
>> burning fat + lactate). So the athlete burns more fat (but also takes in more fat, so the fact
>> that the athlete burns more fat does not necessarily mean that the net "fat balance" (intake vs
>> burning) is skewed to promote a greater net loss of fat).
>
>This might well explain the results I saw. I swam slower because my white muscle fibers were not
>being used as much or not at all. I assume by "high fat" we mean high in percentage terms. My diet
>is not particularly high in fat in grams, I think. It might not be much higher than it was prior
>to the diet at all, given that I have eliminated cookies and brownies. I eat more eggs, more
>cheese, and I eat bacon, but I am not eating a lot more. I am eating a lot more nuts, which are
>high in fat.
>
>> Maybe the reason Martin's swimming performance has been just a little uneven since changing diets
>> has to do with the necessity of getting his muscle fibers to adjust to the changing energy
>> sources, which may take a little time. As I wrote some months ago (relating to my own
>> experience), I do think that when a swimmer loses weight that the swimmer's buoyancy changes,
>> balance changes, and performance may change (at least temporarily) as well.
>
>I think this is the case, but does a white fiber actually change to a red fiber, or does the white
>fiber simply not contribute in the absense of carbs?

According to what I've read their are three muscles fibers, Can't think of there technical
names off teh type of my head, but.... One would be what you're calling white, the other
red. The third is a bit of a hybrid between the two. Present study is conflicted on whether
the white is turning into red or visa versa, or whether the hybrid fiber is simply acting
more like the other fibers. If you'd like I'll try and dig up more of the techy stuff.

>
>martin
>
>--
>Martin Smith email: [email protected] Vollsveien 9 tel. : +47 6783 1188
>P.O. Box 482 mob. : +47 932 48 303 1327 Lysaker, Norway

I've been following this conversation and although I don't have nearly teh grasp on the
chemistry as Larry apparently does I have a bit of a differing opinion. It seems to me that
a person on the Atkins diet would suffer from a low carb diet over a period of time and
under certain training conditions. I believe that it has been shown in a few studies that
muscle glycogen is replenished significantly quicker on a high carb diet. Certain levels of
activity rely on higher levels of glycogen metabolism due to the slower rate of fat
metabolism. So if one were to go out and do certain excercises, such as 60-120 sec intervals
at a higher intensity 80+ VO2max one would run into a glycogen depletion situation. If one
were on a low carb diet and attempt to do these excercises on a regular basis performance
would eventually suffer, in some cases fairly quickly. Considering that nearly all levels of
excercise depend on glycogen metabolism to one extent or another I can see this depletion
being a factor in many less extreme cases also. For instance a long run/swim/bike day
followed by a track/interval/hill workout etc.

~Matt
 
Fast twitch fibers can be converted into slow twitch fibers. And slow twitch fibers can be converted
to fast twitch fibers. Experiments in animals have identified a few targets the drugs can be
developed :). One day you may be able to convert any one into a super sprinter. On the other hand,
low carbo diet may reduce the number of fast twitch fibers (Type II).

"Martin W. Smith" <[email protected]> wrote in message news:<[email protected]>...
> Larry Weisenthal wrote:
> > The only differences were in the case of the white muscle. In the high fat group, muscle
> > glycogen was lower than in the high carb group. In red muscle, there were no differences in
> > glycogen, pre-or post exercise. White muscle can burn only carbs (they are "white" because they
> > lack mitochondria, which are required to burn fat and lactate, which, along with glucose [from
> > glycogen], are the main fuels for muscle fiber contraction). It seems that with a high fat diet,
> > there is a selective dis-use (lack of use) of white muscle and more use of red muscle (capable
> > of burning fat + lactate). So the athlete burns more fat (but also takes in more fat, so the
> > fact that the athlete burns more fat does not necessarily mean that the net "fat balance"
> > (intake vs burning) is skewed to promote a greater net loss of fat).
>
> This might well explain the results I saw. I swam slower because my white muscle fibers were not
> being used as much or not at all. I assume by "high fat" we mean high in percentage terms. My diet
> is not particularly high in fat in grams, I think. It might not be much higher than it was prior
> to the diet at all, given that I have eliminated cookies and brownies. I eat more eggs, more
> cheese, and I eat bacon, but I am not eating a lot more. I am eating a lot more nuts, which are
> high in fat.
>
> > Maybe the reason Martin's swimming performance has been just a little uneven since changing
> > diets has to do with the necessity of getting his muscle fibers to adjust to the changing energy
> > sources, which may take a little time. As I wrote some months ago (relating to my own
> > experience), I do think that when a swimmer loses weight that the swimmer's buoyancy changes,
> > balance changes, and performance may change (at least temporarily) as well.
>
> I think this is the case, but does a white fiber actually change to a red fiber, or does the white
> fiber simply not contribute in the absense of carbs?
>
> martin
 
On Tue, 12 Aug 2003 18:07:59 GMT, MJuric wrote:

>On Tue, 12 Aug 2003 14:50:46 +0200, "Martin W. Smith" <[email protected]> wrote:
>
>>Larry Weisenthal wrote:
>>> The only differences were in the case of the white muscle. In the high fat group, muscle
>>> glycogen was lower than in the high carb group. In red muscle, there were no differences in
>>> glycogen, pre-or post exercise. White muscle can burn only carbs (they are "white" because they
>>> lack mitochondria, which are required to burn fat and lactate, which, along with glucose [from
>>> glycogen], are the main fuels for muscle fiber contraction). It seems that with a high fat diet,
>>> there is a selective dis-use (lack of use) of white muscle and more use of red muscle (capable
>>> of burning fat + lactate). So the athlete burns more fat (but also takes in more fat, so the
>>> fact that the athlete burns more fat does not necessarily mean that the net "fat balance"
>>> (intake vs burning) is skewed to promote a greater net loss of fat).
>>
>>This might well explain the results I saw. I swam slower because my white muscle fibers were not
>>being used as much or not at all. I assume by "high fat" we mean high in percentage terms. My diet
>>is not particularly high in fat in grams, I think. It might not be much higher than it was prior
>>to the diet at all, given that I have eliminated cookies and brownies. I eat more eggs, more
>>cheese, and I eat bacon, but I am not eating a lot more. I am eating a lot more nuts, which are
>>high in fat.
>>
>>> Maybe the reason Martin's swimming performance has been just a little uneven since changing
>>> diets has to do with the necessity of getting his muscle fibers to adjust to the changing energy
>>> sources, which may take a little time. As I wrote some months ago (relating to my own
>>> experience), I do think that when a swimmer loses weight that the swimmer's buoyancy changes,
>>> balance changes, and performance may change (at least temporarily) as well.
>>
>>I think this is the case, but does a white fiber actually change to a red fiber, or does the white
>>fiber simply not contribute in the absense of carbs?
>
> According to what I've read their are three muscles fibers, Can't think of there technical
> names off teh type of my head, but.... One would be what you're calling white, the other
> red. The third is a bit of a hybrid between the two. Present study is conflicted on whether
> the white is turning into red or visa versa, or whether the hybrid fiber is simply acting
> more like the other fibers. If you'd like I'll try and dig up more of the techy stuff.
>
>>
>>martin
>>
>>--
>>Martin Smith email: [email protected] Vollsveien 9 tel. : +47 6783 1188
>>P.O. Box 482 mob. : +47 932 48 303 1327 Lysaker, Norway
>
> I've been following this conversation and although I don't have nearly teh grasp on the
> chemistry as Larry apparently does I have a bit of a differing opinion. It seems to me that
> a person on the Atkins diet would suffer from a low carb diet over a period of time and
> under certain training conditions. I believe that it has been shown in a few studies that
> muscle glycogen is replenished significantly quicker on a high carb diet. Certain levels of
> activity rely on higher levels of glycogen metabolism due to the slower rate of fat
> metabolism. So if one were to go out and do certain excercises, such as 60-120 sec intervals
> at a higher intensity 80+ VO2max one would run into a glycogen depletion situation. If one
> were on a low carb diet and attempt to do these excercises on a regular basis performance
> would eventually suffer, in some cases fairly quickly. Considering that nearly all levels of
> excercise depend on glycogen metabolism to one extent or another I can see this depletion
> being a factor in many less extreme cases also. For instance a long run/swim/bike day
> followed by a track/interval/hill workout etc.
>
>~Matt
>

Another potential problem I meant to mention was the increased potential for "Bonking". It
is my understanding that even at low levels of activity, such as the levels present in Ultra
Marathons, IM's etc, that it takes a certain amount of glucose to use in the process of
metabolising fat. If one runs into a severe glucose depletion nearly all sources of energy
are unavailable and you "Bonk". I would consider this to be a very real potential problem if
one has not replinished the glycogen stores from a previous workout and is heading into a
longer workout. However probbaly not an issue during Martins 1500m swims.

~Matt
 
In article <[email protected]>, <MJuric> wrote:
> Another potential problem I meant to mention was the increased potential for "Bonking". It
> is my understanding that even at low levels of activity, such as the levels present in Ultra
> Marathons, IM's etc, that it takes a certain amount of glucose to use in the process of
> metabolising fat. If one runs into a severe glucose depletion nearly all sources of energy
> are unavailable and you "Bonk".

Yes, and it's a big problem for the elites in the ironman length races. You have to go at a rate
that burns enough fat that you can make it, since most people can't consume more than 200Cal/hr when
competing, on top of that 2 or 3k Cal in gylcogen. And once you've gone over the edge, there's not
much recovery, even if you're one of the top ranked triathletes in the world.
--
Jason O'Rourke www.jor.com
 
"Larry Weisenthal" <[email protected]> wrote in message
news:[email protected]...
> >>You were going to take a few days to find out if muscles recharge with
> glycogen by converting stored fat, when there are no carbs present.
>
> >>I think the answer is no, but I have read that the liver starts creating
> glycogen for this purpose when there are no carbs present, but it take slonger to recharge the
> muscles this way. The question then becomes, where does the liver get the material to make
> glycogen, from stored fat or from ingested fat?<<
>
> The body _can't_ convert fat to glycogen! Glycogen can be made only from sugars and from protein
> (amino acids) -- the latter through a process
called
> gluconeogenesis. So, when you don't take in sufficient carbohydrate to
make
> glycogen, the only way that you can make it is to break down muscle to
release
> amino acids, which can then be converted to glucose, and from there to glycogen.

okay, that now raises a question... Body can't convert fat into Protein... and now you say that body
can't convert fat to carbohydrate...

Just how *does* the fat get used up? I do know from experience that fat *can* get used up.
 
Larry Weisenthal wrote:
>
> In article <5my0b.6105$S_.2673@fed1read01>, "DaKitty" <[email protected]> writes:
>
> >okay, that now raises a question... Body can't convert fat into Protein... and now you say that
> >body can't convert fat to carbohydrate...
> >
> >Just how *does* the fat get used up? I do know from experience that fat *can* get used up.
>
> Fat (in the form of stored triglycerides) gets broken down to free fatty acids and free fatty
> acids get broken down to form acetyl CoA. Acetyl CoA enters the Krebs cycle (citric acid cycle) in
> the mitochrondria of the cell (red muscle/aerobic muscle has lots of mitochondria; white muscle
> has very few), where it generates ATP which is the "fuel" for cellular metabolism (in the case of
> a muscle cell, ATP fuels muscle contraction).

So, here is another, related question. Assuming the Atkins diet works because the calories going in
are less than the calories being burned, can I be adding muscle while losing weight. ie, can muscle
be added when the calorie intake is insufficient to maintain weight? It seems to me I am getting a
lot stronger, but I am losing weight. Someone in the low-carb-diet newsgroup says you can't add
muscle when you are losing weight. Is he correct?

martin

--
Martin Smith email: [email protected] Vollsveien 9 tel. : +47 6783 1188
P.O. Box 482 mob. : +47 932 48 303 1327 Lysaker, Norway
 
DaKitty <[email protected]> wrote:

: "Larry Weisenthal" <[email protected]> wrote in message
: news:[email protected]...
:> >>You were going to take a few days to find out if muscles recharge with
:> glycogen by converting stored fat, when there are no carbs present.
:>
:> >>I think the answer is no, but I have read that the liver starts creating
:> glycogen for this purpose when there are no carbs present, but it take slonger to recharge the
:> muscles this way. The question then becomes, where does the liver get the material to make
:> glycogen, from stored fat or from ingested fat?<<
:>
:> The body _can't_ convert fat to glycogen! Glycogen can be made only from sugars and from protein
:> (amino acids) -- the latter through a process
: called
:> gluconeogenesis. So, when you don't take in sufficient carbohydrate to
: make
:> glycogen, the only way that you can make it is to break down muscle to
: release
:> amino acids, which can then be converted to glucose, and from there to glycogen.

: okay, that now raises a question... Body can't convert fat into Protein... and now you say that
: body can't convert fat to carbohydrate...

: Just how *does* the fat get used up? I do know from experience that fat *can* get used up.

It's been a while since I did biochem at uni, so forgive me if I misremember. Stored fat exists in
the body as triacylglycerides. This is broken down via "fatty acid oxidation" or "beta-oxidation"
into Acetyl-CoA and perhaps some NADH and FADH2 is released here too. The Acetyl-CoA then enters the
Citric Acid Cycle to produce NADH, FADH2 and some ATP, as well as the waste product CO2. NADH and
FADH2 are then converted into ATP, which is the main energy carrying molecule of the body.

Essentially, this is a catabolic pathway and the fat is burned up and used as an energy source. It's
a very dense energy source for a number of reasons. Firstly, stored TAG is hydrophobic so it's quite
dense (no water to disolve into), secondly the carbon atoms I believe are more reduced than the
carbons from glycogen and thus yeild more ATP per molar equivalent.

Like I said, it's been a while, but if you search on "Fatty Acid Oxidation", "beta-oxidation" and
"the citric acid cycle", or even "respiration" you should be able to get a more complete and
possibly more accurate overview.
 
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