Sunday, June 28, 2009

Glycogen Stores Energy


Adipose or fat tissue stores most of the body's energy reserves in the form of triglycerides. The body is also able to store a limited amount of energy as carbohydrates, and it does it in the form of glycogen.

Glycogen is a large, complex molecule made up of branched chains of glucose molecules. The illustration above, found at Wikipedia, shows a cross section through the middle of a spherical glycogen molecule. At the center is a glycosyltransferase enzyme. The enzyme takes glucose-6-phosphate (the form of glucose found inside a cell) and strings it together as long, branched chains. In the picture above, each tiny circle represents a glucose molecule. The glycogen molecules are therefore large polymers of glucose which are then packed together and stored in granules in the cytosol of liver and muscle cells.

Glycogen makes up as much as 10% of the weight of the liver and represents about 100 grams of glucose in the adult human. Glycogen in the liver can be broken down first into glucose-6-phosphate and then into glucose. In the form of glucose it can be released back into the circulation. In a previous post we have seen that release of glucose from liver glycogen is the body's chief means of maintaining a normal blood sugar between meals.

Glycogen can also be stored in skeletal muscle, as illustrated in the figure below.


When glucose is present in the blood (and in a living person, it always is), a muscle cell is able to take up the glucose both actively and passively. Once the glucose is inside the muscle cell, the glucose molecule is phosphorylated. This adds a large ionic group which makes it impossible for the glucose to diffuse back out of the muscle cell. The phosphorylated glucose then has two possible fates.

  1. It can proceed directly into glycolysis and be turned into pyruvate. If there is enough oxygen available, the pyruvate will enter the mitochondria and be turned into lots of ATP, the energy currency of the cell. If there is not enough oxygen available, the pyruvate will be turned into lactic acid plus a little ATP. The buildup of lactic acid produces a sensation of pain, and the pain will continue until the lactic acid diffuses back out of the muscle cell, a process which takes about an hour.
  2. Alternatively, the phosphorylated glucose may instead be stored in the muscle in the form of glycogen. Muscle glycogen makes up only 1-2% of the weight of skeletal muscle, but because the body contains so much skeletal muscle, the total quantity of muscle glycogen in an adult is about 200 grams.

What makes muscle glycogen different from liver glycogen is that when muscle glycogen is broken down, it cannot leave the cell. Muscle cells lack the enzyme that removes the large ionic phosphate group from the glucose, and the glucose cannot be returned to the blood. For that reason, the phosphorylated glucose must be used inside the muscle cell. What then?

No problem. The phosphorylated glucose feeds right into the glycolytic pathway inside the muscle cell, where it is turned into pyruvate and lots of ATP or into lactic acid and a little ATP, depending on the amount of oxygen available to it.

When we hear about carb loading for athletic events, it is tempting to think that most of the energy in our muscles comes from carbohydrates. It does not. There is only a little glycogen stored in each muscle cell, and it is easily exhausted. Compare the 200 grams of total muscle glycogen with the pounds of fat available in a healthy individual, and it becomes obvious that muscle cells must use free fatty acids for most of their energy. This is illustrated on the right side of the illustration above. As seen previously (How Are Fats Metabolized?), once the free fatty acids are inside the cell, they are broken down very efficiently to produce much more ATP than could be obtained from an equal number of glucose molecules. However, when an extra burst of energy is needed, muscle cells are able to use the glucose they have stored in glycogen granules to supply a little more ATP than they would normally receive from using fatty acids alone.

8 comments:

anne h said...

Very good information. Nursing school all over again!

dollface said...

Great read Stargazey, I've become quite interested in this, I've also just started weight training so it's a timely post. Keep up the great posts :)

Stargazey said...

JayCee Botha--I approved your comment about fat mobilization and it disappeared. Don't know where it went, so please try again if you'd like to. :(

Although I've signed up for Facebook, I don't really do it--sorry--like Twitter, it would be too much of a temptation to me to waste more time than I already do on the Internet!

Everybody, I am amazed that people are interested in posts on fat and glycogen. It's basic biochemistry, but maybe having it explained helps low-carbers understand that what they are doing does have a scientific basis. If there is a topic you would like me to cover, please let me know and I'll see what I can come up with. Thanks!

OnPoint said...

As the blogosphere turns . . .

Stargazey, this past week has been outright hilarious. I know you reached many of the same conclusions as I have.

Deflection. Excuses. 83%. 260 lbs. The attempted marginalization of sweet-free. No weight this month.

Oh, and did you catch JM's menu for July 2? W/ the exception of a few tomatoes, it's a ZC menu.

I have a lot of compassion for JM. After his pro-products stance, it's got to be hard to concede that something wasn't working about his WOE.

Something that's been on my mind - a theory - and I know you will blow it to smithereens. Two examples:

Mr. LowBodyFat achieved goal in the past few years and has maintained. He says to exercise, count calories, and keep carbs under 100/day.

Charles Washington achieved goal in the past few years and has maintained. He says to eat ZC (which I have no plans to do).

JM, Laura Dolson, Dr. Vernon, and Dana Carpenter are all overweight and at least one has been gaining. They all fall (I presume) in the middle ground between Mr. LBF and CW. It seems to me that more dieters than not fail on LC well short of goal. And more, that if a person intends to lose weight on LC and realize the promise of not having to count calories, he or she may end up at ZC.

I read somewhere (can't recall where) that Dr. Atkins actually favored a ZC diet, but that he thought most dieters lost well despite his allowing them to "cheat" with Induction's 20 carbs/day.

My conclusion is that dieters looking to succeed either follow someone like Mr. LBF or CW, but that the middle ground may not be so productive a place to be.

I think the LC wave of the early naughts (the first decade of a century, as it was called a century ago) was doomed, Frankenfoods or not. ZC is very much a fringe movement, and I can't think of anyone I know (even of those who've done Atkins) who would even consider it. And how many people do you know who would want to stick to Atkins indefinitely w/o any products - diet sodas, LC wraps, the occasional sweet treat, and so on?

Yet and still, LC continues to cast a long shadow. Dieters across all plans are now aware of the perils of excessive carb intake.

Okay, it was a long and rambling post, but I hope I said something worth saying here. :-)

Stargazey said...

OnPoint, I always appreciate your thoughtful and insightful comments. Truly.

Regarding Jimmy Moore: He seems to have problems with sweet addiction. I don't expect to find him in the gutter with an empty can of Splenda Coke in his hand, but the necessity for something sweet does seem to take over his menu planning.

I'm addicted to sweet, too, which I learned when I went without it for 18 days. For me, however, the presence or absence of sweet has no effect on my weight, so I can still drink up to three servings a day, which is the guideline Dr. Atkins recommended.

In October and December of 2008 we learned (and this was a complete surprise to me) that the taste of sweet has a huge effect on JM's weight. For some people it's fruit, for some people it's wheat, for some people it's nuts, for some people it's dairy, and for JM it's the taste of sweet, but as we do the low-carb lifestyle each individual learns what works and what doesn't. And if we're serious about it, we will ditch the thing or things that trip us up. It may take a while to be successful, but after a certain number of stumbles, we can eventually learn that we really can live without [fill in the blank] and we're actually much happier without it.

Stargazey said...

Regarding your theories:

From what I have seen, if people like Mr. LowBodyFat want to eat as much as 100 carbs a day, they do have to count calories. The studies done at that carb level show that they will probably still get the health benefits of low-carb. From personal experience, they probably won't have the appetite-suppressing aspect of eating 20 or so carbs. If they have the willpower to count calories and maintain their weight that way, good on them.

As far as zero carb, it's not as straightforward as you might expect. Charles Washington is able to maintain his weight and energy level on fatty meat and water. So are some others. But for some people (including me), eating too much fat can cause an increase in weight, especially a roll of fat around the middle. For others (also including me), eating too much protein can lead to blood glucose levels that never dip below 100.

Right now I'm doing zero carb because I am struggling with satiety. I absolutely must get back to waiting 5-6 hours between meals and for me this seems to be the only way to do it. Eventually I want to add a few carbs back in so I can get my blood glucose back into a more normal range, but for now, one thing at a time.

But I guess I must disagree with you that the middle ground doesn't work. I think the question is, what is the middle ground for each individual? It seems to take constant observation and occasional modifications, but as long as we are honest with ourselves and don't make excuses, the middle ground may be the best place to stay.

OnPoint said...

You're right - if you eat under 100 carbs/day, you will indeed get many of the benefits of LC. When I keep my carbs in the 75-100 range, everything is good relative to my size - meaning I can control my bp and cholesterol w/ medication. Both improve, however, when I lose weight. When I overeat carbs, my chemistries go out of kilter.

I don't struggle with satiety at all on 80-100 carbs/day, so long as I do not restrict my bfast and lunch meals. I eat bfast on weekdays on my way to work about 8 a.m. I eat lunch at 1:30. Depending on whether or not I'm eating dinner that day, dinner is usually between 6-8 p.m.

Perhaps 100 carbs/day produces different levels of satiety for men and women. I'm sure I eat a lot more than you. No, a lot! 100 carbs is a much smaller percentage of my total calories than yours.

Like you, I was shocked to find out that the taste of sweet had some impact on Jimmy. Who'da thunk it?

I just saw that Jimmy just started his experiment using the testing strips you sent him. It was very kind of you to do that. I was surprised he got them so quickly. Did you drive them to his house personally? /kidding

Looks like JM is still contending w/ hypoglycemia. He possibly does not have symptoms b/c he's gotten used to it.

Stargazey said...

100 carbs is a much smaller percentage of my total calories than yours.

You know, you're absolutely right. I never thought of it in terms of percentages. Maybe that's why people find they have to eat fewer carbs as they lose weight.


I just saw that Jimmy just started his experiment using the testing strips you sent him.

No, to be honest, I won't be able to send them out until Monday because of the holiday. He's just using test strips he already has. Which is fine, because they go out of date.

It does look like he has hypoglycemia. He probably does gluconeogenesis in addition to any carbs he takes in, his insulin way overreacts, and his glucose comes right down. Until his beta cells start dying off. But since I'm not a doctor, that could be completely wrong. I think I'll try to post some ideas about it tomorrow.