Glucose and glycogen

Geschatte leestijd: 11 minutenGlucose is the main fuel for the body. It can be stored in the form of glycogen. Among other things, insulin plays a determining role in the extent to which glucose is stored or released from previously stored reserves.

Carbohydrates, Glucose, and Insulin

In this article, we’re moving towards a more practical application of knowledge about carbohydrates.

However, don’t expect meal plans. Under the motto of “Don’t give a man omega-3 capsules, but teach him to fish,” it’s much more important to understand the insights (or lack thereof) on which meal plans are based.

This piece is about the role carbohydrates play in metabolism and provides insight into how the body deals with resources. When, how, and why do carbohydrates turn into body fat? What is the relationship between carbohydrate intake and muscle mass? Why is the timing of carbohydrates so important, not only around athletic performance but throughout the day?

You can follow one hype after another. Replace the poster of one diet guru with the new one. Try new methods every time to find out after months that they don’t work for you. However, maybe it’s useful to take the time once to inform yourself about the basics on which they base their claims.

You could be the type of person who trusts the car mechanic on the oil stains on his blue overall and always believes that a certain part needed replacement. After going broke every year after the MOT test, you might find it time to look under the hood yourself.

Glucose, the Main Sugar

In another article, we discussed various types of carbohydrates. One of the subdivisions was between simple and complex sugars.

Complex sugars cannot enter the bloodstream directly. They must first be broken down into simple sugars. This happens partly by enzymes in the mouth (amylase), then in the stomach, and finally in the small intestine.

Only when they are broken down into simple sugars (glucose, fructose, and maltose) are carbohydrates small enough to pass through the wall of the small intestine and move towards the liver.

Complex sugars -> simple sugars -> liver -> glucose

As you may have read in part one, there are several types of simple carbohydrates. Glucose distinguishes itself from these other simple carbohydrates. It is the only sugar that ultimately moves as fuel in the body’s bloodstream.

That’s why the various simple sugars, as the first stop after passing through the small intestine, visit the liver. There, they are converted into glucose.

… it is evident that glucose is a universal fuel for probably all cells in the body and carbohydrate is the cheapest source of calories and the major source of dietary fibre. These observations, together with the fact that glucose is the preferred metabolic fuel for the brain, permit us to recommend appreciable quantities of carbohydrate in all prudent diets.

J.T. Brosnan, Memorial University of Newfoundland

Preferred Fuel

So, glucose is the preferred fuel of your body.

The amount of glucose in the bloodstream and its availability for transport to various cells is also known as blood sugar levels. A shortage of glucose (so-called “hypo”) can therefore cause serious problems. Your body is capable of producing glucose from other substances.

At the same time, an excess of glucose (“hyper”) is undesirable. Therefore, the body has methods to temporarily store this surplus.

Carbohydrates can contribute to your training and nutrition goals or work against them. This largely depends on how successfully you manage to use these processes.

Glycemic Index and Glycemic Load

We’ll look at what happens to glucose after it enters the bloodstream. First, however, I want to address the different speeds at which various carbohydrates cause this. There are significant differences with practical consequences.

When glucose is consumed, it increases the amount of glucose in the blood (“blood sugar level”) at a certain speed. The Glycemic Index is an indication of the speed at which a particular type of carbohydrate raises blood sugar levels compared to glucose. Glucose has a benchmark score of 100.

This index was developed in Canada in the early 1980s so that diabetics can take this into account more [2]. However, the glycemic index is only an indication. The exact speed at which blood sugar levels are raised can vary [3]:

• Depending on the preparation method [4].
• Varies per person.
• Depends on the carbohydrates eaten before.

Another limitation of the glycemic index is that it only looks at two hours after ingestion.

Classification:

• High: > 70
• Medium: 56-69
• Low: < 55

So, the glycemic index is the score for a particular type of carbohydrate. There is no single overview of the various carbohydrates and their scores. An overview can be arbitrarily compiled. Namely, by showing the glycemic index of carbohydrates that are interesting to the compiler at that time or in that context.

Therefore, you can find many different overviews of carbohydrates and their Glycemic Index.

Glycemic Load

However, the biggest limitation of the glycemic index is that it does not take into account the quantity of carbohydrates eaten. The glycemic load does this. It is: The number of grams of digestible carbohydrates in the diet multiplied by the glycemic index of that type of carbohydrate, divided by 100.

The glycemic index therefore looks at the actual amount of carbohydrates ingested. Some products contain carbohydrates with a high glycemic index. However, if such a product contains very few of these carbohydrates, the glycemic load can still be low.

For example, watermelon has a higher glycemic index than a banana. However, because a relatively larger amount of carbohydrates is present in a banana, the value of the glycemic load of a banana is higher.

Classification:

• High: > 20
• Medium: 11-19
• Low: < 10

Glycogen

For example, when consuming food with a high glycemic load, this causes the amount of glucose in the blood to rise. Excess glucose is prevented by converting some of the glucose into glycogen. Glycogen is a so-called “polymer of glucose monomers”. This means that it is a compound chain of multiple single glucose chains (see image to the right).

Osmosis

Glucose itself cannot be stored in cells because its osmotic value is too high. Without going too deep into osmosis, I can give a brief explanation here.

In osmosis (in biology), water moves in and out of cells through the cell membrane. However, the dissolved substances in the water cannot pass through the membrane.

Depending on (the difference in) the concentration of substances inside and outside the cell, water will move towards or away from the cell (diffusion). The high osmotic value of glucose would cause too much water to enter the cell.

In plants, cells have a cell membrane and a cell wall. In plants, cells can become so large that the membrane presses against the cell wall. If it continues to grow, the cell wall may burst. However, humans and animals do not have a cell wall, so the membrane can rupture in that situation.

Glycogen, the form in which glucose can be stored by muscles, has a lower osmotic value, thus preventing this danger.

Primary Storage and Secondary Storage

Glycogen is an important form of energy storage. This storage takes place in the liver (about 100 grams), but especially in the muscles.

Glycogen provides fast “cheap” energy. By “cheap,” I mean that it takes little energy to release energy from glycogen. However, glycogen can only be stored in limited amounts. Some researchers mention 30 to 40 minutes of intensive activity as the maximum duration within which glycogen is depleted.

Body fat is therefore called the primary storage. It costs more energy (in the form of oxygen) to make energy from fats available, but much more can be stored.

I have once made the comparison between the heaviest bodybuilder ever, who, when dry (with very little body fat), weighs just over 140 kilograms, and people with enormous excess weight weighing more than 400 kilograms. Moreover, glycogen is responsible for only a small part of the weight of muscles. Per 100 grams of “wet muscle,” a maximum of 3-4 grams of glycogen is stored. And that’s when the stock is first depleted and then replenished in about three days. However, this is already according to a special protocol where glycogen is first depleted by eating little to no carbohydrates and being intensely active, and then replenished (carb depletion and loading). Normally, the amount of glycogen is about 1.5 to 2 grams per 100 grams of “wet muscle.”

The maximum storage of glycogen can be improved by increasing muscle mass, but this is limited. For the average person, there is 200 times more energy stored in body fat than in glycogen.

When the glycogen in the muscles (“muscle glycogen”) is depleted, glycogen in the liver (“liver glycogen”) can be used. As mentioned, glycogen cannot be transported in the bloodstream, so it must first be converted back into glucose. Once it reaches the muscles, glycogen is reformed there. One difference between glycogen in the muscles and the liver is that glycogen once in the muscles cannot be converted back into glucose. The glycogen is the same. However, the enzyme that converts glycogen into glucose in the liver (glycogen-6-phosphatase) is absent in the muscles.

Glycogen in Consumed Meat

Before I continue with the conversion of carbohydrates into glycogen and vice versa, I first want to briefly describe what happens to the glycogen in the meat you eat (if you eat it).

After all, meat mainly consists of the muscles of animals and the fat that is in and around them. So, glycogen is also present in those muscles. As mentioned, only simple carbohydrates can be absorbed into the bloodstream. Glycogen is a complex carbohydrate since it can consist of up to 10,000 glucose chains.

Before glycogen in consumed meat in your own body is stored as glycogen, it must first be broken down into glucose. Then, glycogen can be newly formed in the liver and also in the muscles after the glucose has been transported there via the blood.

Regulatory Hormones: Insulin

The amount of glucose in the blood must be maintained. It must not become too low or too high. When this happens, we see the symptoms we see in diabetics (in severe cases).

To regulate the amount of glucose in the blood, your body uses hormones such as insulin, glucagon, and adrenaline. First, I will discuss insulin, and then briefly on glucagon and adrenaline.

Insulin is an abbreviation for the hormone named after the “islets of Langerhans”. These are clusters of cells in the pancreas that are clustered together like islands (Latin = “insula”). These were discovered by Paul Langerhans (1847 -1888). Insulin is one of the hormones made here.

Insulin is a polypeptide hormone* and as such has a messenger function. Insulin tells various cells in the body how to deal with the glucose in the blood. When the amount of glucose in the blood rises due to the intake of (digestible) carbohydrates, the “islets of Langerhans” produce insulin. This insulin, along with glucose, is transported to various cells in the body. Think especially of muscle cells and fat cells. Insulin can give these cells commands that lead to a reduction in the amount of glucose in the blood. These commands are:

• Increase protein synthesis in muscles: Produce more protein in the muscles from available amino acids in the blood. This step leads to muscle growth!
• Increase the synthesis of fatty acids in fat cells, “lipogenesis”. When glycogen stores are full and glucose levels are still high, glucose is converted into fatty acids in the so-called citric acid cycle (via Acetyl-CoA). This step leads to the production of body fat!
• Liver and muscles: Take up more glucose and convert it into glycogen (increase “glycogenesis”, the production of glycogen)
• Reduce “gluconeogenesis/glycogenolysis”: The opposite of the previous step: The production of new glucose (gluconeogenesis) from glycogen is reduced. Another way to put this is that the breakdown of glycogen (glycogenolysis) into glucose should be reduced.

*_{A polypeptide hormone consists of chains of amino acids and differs from protein only by the smaller number of amino acids.}

Regulatory hormones: Glucagon and adrenaline

When the amount of glucose in the blood becomes too low, the release of insulin by the pancreas will decrease. Instead, it releases glucagon into the bloodstream. Glucagon, like insulin, is a polypeptide hormone. Like insulin, it travels in the blood to give instructions to various cells. In this case, commands that lead to an increase in the amount of glucose by increasing its production (‘gluconeogenesis’).

Adrenaline also acts as a signaling molecule, but it is produced in the adrenal medulla and brain and works through the nerves. The commands issued by glucagon and adrenaline are:

• Increase glycogenolysis: Break down more glycogen into glucose. This occurs by separating individual glucose molecules from glycogen, which are then converted to glucose-1-phosphate and then glucose-6-phosphate.
• Increase proteolysis. (Also occurs under the influence of noradrenaline). Break down more protein into amino acids that can eventually be converted into glucose and ketone bodies. Causes muscle breakdown!
• Increase lipolysis. Break down more body fat into glycerol and free fatty acids. These can then be utilized in the citric acid cycle to provide energy again. Burns body fat!

Balance between muscle building and fat burning

As you can see, insulin promotes muscle building, but also body fat, while glucagon breaks down both. It is therefore often very difficult in practice to gain muscle mass and burn fat at the same time.

This usually only works when people are just starting to train or have a high percentage of body fat. Or when someone naturally has no predisposition for large fat storage (ectomorphs). For others, the challenge is to find the difficult balance between eating enough carbohydrates to grow muscles, but not gaining too much body fat.

This can be achieved, for example, by subsequently eating fewer carbohydrates and thus burning body fat. In this classic, so-called bulk and cut method, people try to ensure that there are enough amino acids available by, for example, replacing carbohydrates with protein. This limits the breakdown of protein in the muscles.

Timing of carbohydrates

Not only the amount, but also the timing and type of carbohydrates eaten can make a big difference. Eating fast carbohydrates, for example, before sleeping is often discouraged because there is little need for glycogen at that time. The chance of conversion into body fat is then greater. However, before heavy exertion, it would ensure that there is enough glucose available to prevent the need for muscle breakdown.

However, there are countless strategies to use carbohydrate intake to your advantage. Some are diametrically opposed and can nevertheless both work or not work due to personal differences. Think of the well-known Atkins diet, the Ketogenic diet, or any other low-carb diet. Also consider things like carb-loading and carb back-loading, which are all covered in separate articles.

Insights into the place of carbohydrates in a diet are constantly changing. Therefore, it is good to keep yourself informed, but above all to experiment and see what works best for you personally.

Glucose as an intermediary of ATP in the citric acid cycle / Krebs cycle

Until now, I have often spoken of glucose as fuel. However, glucose often also serves as an intermediary, a kind of “intermediate product”.

The various cells themselves do not use glucose or glycogen as fuel but ATP, adenosine triphosphate. For example, glycogen is used to quickly produce new ATP during high-intensity activity. In other cases, this happens via lactate.

In the series on energy systems, it is explained which source (glucose/glycogen, fats, or amino acids) is used depending on activity and duration.

ATP

ATP is the stock of readily available energy. The body has various sources to produce new ATP, depending on the situation. For example, the brain uses a large part of the daily glucose intake to generate ATP. As much as 120 grams. Or in other words: At rest, about 60% of all glucose. In the absence of sufficient carbohydrates, the brain uses ketones for this [15].

Also read the article: Body energy systems, the ATP system

Many of the conversions of glucose into other substances and vice versa, as well as those of fats and amino acids, covered in this article, take place in a complex process called the Krebs cycle or citric acid cycle.

For a good understanding of the exact functioning of the metabolism of carbohydrates, fats, and proteins, you should actually study the citric acid cycle. However, it is so complex that it is almost impossible for a layperson to understand. Unless you are very motivated and patient enough to consult multiple sources for a good understanding of the interaction for each new term, besides Wiki.

It would be too extensive to discuss this here extensively as it would create too much distance between theory and practice, especially if this is not done with practical examples. The various so-called pathways, the routes from starting material to end product, I will only address in subsequent parts where necessary in context.

References

1. Brosnan JT. Comments on metabolic needs for glucose and the role of gluconeogenesis. Eur J Clin Nutr. 1999 Apr;53 Suppl 1:S107-11.
2. Jenkins DJ, Wolever TM, Taylor RH, et al. (March 1981). “Glycemic index of foods: a physiological basis for carbohydrate exchange”. Am. J. Clin. Nutr. 34 (3): 362–6. PMID 6259925.
3. Freeman, Janine (September 2005). “The Glycemic Index debate: Does the type of carbohydrate really matter?”. Diabetes Forecast. Archived from the original on February 14, 2007.
4. “GI Database”. Web.archive.org. Retrieved 2012-08-01.
5. Jørgen Jensen, Per Inge Rustad, Anders Jensen Kolnes and Yu-Chiang Lai. The Role of Skeletal Muscle Glycogen Breakdown for Regulation of Insulin Sensitivity by Exercise. Front Physiol. 2011; 2: 112. doi: 10.3389/fphys.2011.00112PMCID: PMC3248697
6. Acheson K. J., Schutz Y., Bessard T., Anantharaman K., Flatt J. P., Jequier E. (1988). Glycogen storage capacity and de novo lipogenesis during massive carbohydrate overfeeding in man. Am. J. Clin. Nutr. 48, 240–247.
7. van Loon LJC. The effects of exercise and nutrition on muscle fuel selection. Maastricht: Universitaire Pers Maastricht, 2001.
8. Hansen BF1, Asp S, Kiens B, Richter EA.Glycogen concentration in human skeletal muscle: effect of prolonged insulin and glucose infusion.
9. Kochan RG, Lamb DR, Lutz SA, Perrill Cy Reimann EM, Schlender KK. Glycogen synthase activation in human skeletal muscle: effects of diet and exercise. Am J Physiol 1979: 236: E660-6.
10. Bergstrom J, Hultman E. Muscle glycogen synthesis after exercise: an enhancing factor localized to the muscle cells in man. Nature 1966: 210: 309-10.Scand J Med Sci Sports. 1999 Aug;9(4):209-13.
11. Saltin B, Hermansen L. Glycogen stores and prolonged severe exercise. In: Blix G, ed. Nutrition and physical activity. Uppsala: Almqvist & Wiksell, 1967: 32.
12. Hultman E, Bergstrom J, Roch-Norlund AE. Glycogen storage in human skeletal muscle. In: Pernov B, Saltin B, eds. Muscle metabolism during exercise. New York – London: Plenum Press, 1971: 273-88.
13. http://www.rcsb.org/pdb/101/motm.do?momID=24
14. Jensen J., Lai Y. C. (2009). Regulation of muscle glycogen synthase phosphorylation and kinetic properties by insulin, exercise, adrenaline and role in insulin resistance. Arch. Physiol. Biochem. 115, 13–21. doi: 10.1080/13813450902778171.
15. Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Section 30.2, Each Organ Has a Unique Metabolic Profile. Available from: http://www.ncbi.nlm.nih.gov/books/NBK22436/