What happens when we exercise or go on a restrictive diet? What is glycogen depletion and should we avoid it? Does glycogen depletion have any benefits?

What happens to the body is that we go into fasting mode (not starving mode). It is a state that is completely different from starvation.

In the first stage, the body will burn readily available calories in the form of stored sugar glycogen. Same as fat our bodies have storage of ready-to-use sugar (glycogen) that is easily metabolized. It is quick energy that is available in an instant and unlike fat does not require any metabolic pathway. Our cells use glycogen directly.

Most of the sugar is stored primarily in the cells of the liver and the muscles, hydrated with water. When we start to burn more energy than we have available in the bloodstream our blood sugar level is going to drop. At the first stage as a countermeasure, our body is going to use an easily available source of stored energy or in other words stored glycogen. Because glycogen is hydrated with water inside the cells when we burn sugar water molecule is an excess or byproduct of metabolism. Because of that, we will lose some water weight in the first stage.

 If you do not know this, you can be surprised when you go on a diet and lose water and be happy, to gain it all back after. Water loss released from glycogen and sodium is usually the culprit for dramatic first-week weight loss.

In the liver, glycogen reserves can build up to 5–6% of the organ’s flesh weight (100–120 grams in an adult). Muscles have a much lower concentration of glycogen, in the range of one to two percent of the total muscle mass.

The untrained individual holds typically about 400 grams of glycogen that is stored in the entire body, in both muscles and the liver. A trained professional athlete can hold double that amount. This amount of glycogen is enough to last for several hours of intensive exercise without replenishment.

When we train our body adapts, and the condition improves. Professional athletes also do something called carbohydrate loading after the exercises to force their bodies to adapt by increasing the storage capacity of intramuscular glycogen stores. There are some studies done on this. If we take caffeine or drink coffee glycogen stores tend to be replenished more rapidly.

Long-distance athletes often experience glycogen depletion. It is called “hitting the wall”. In professional sports, it has a powerful influence because after depleting sugar reserves, fatigue follows and sometimes to the point that it is difficult to move. This is the reason why you see athletes bonking. Bonking (Glycogen depletion) is not the state in which you are just feeling tired. Bonking is when your glycogen reserve stores get so low that your brain starts to run out of energy and then shuts your body down.

After the initial stage and metabolization of easily available reserves of glycogen, the liver will begin to break down fat and protein to form energy immediately. The problem is that this process takes time and until gluconeogenesis kicks in, an athlete may experience symptoms of hypoglycemia. If this happens, it will not be uncommon to see professional athletes collapse from extreme fatigue.

Hypoglycemia comes with dizziness, blurred vision, and hallucinations. Loss of consciousness may also occur under these conditions. The combined use of several different energy sources that allow extended high muscular power outputs that can be maintained for an extended period is a big deal in professional sports and research. Also, no we cannot go on a diet before the marathon to tap into gluconeogenesis and then run. Running a marathon using fat alone as a fuel source is not plausible. If we are able to metabolize fat at a higher rate then we would never become tired and could run indefinitely. The energy efficiency of fat metabolization, unfortunately, is not at that level.

Moreover, our body can only process a limited amount of carbohydrates per hour also, around 30-60 grams depending on individual efficiency. You probably won’t race a marathon, but it is essential to understand how the body works if you want to exercise, you can potentially do yourself damage, or don’t get the desired results.

In bodybuilding, it is a big deal also because catabolizing muscle mass for energy is not the desired course. After glycogen depletion from 16 hours to 72 hours, the body will lean heavily on amino acids and protein catabolism for energy creation. Amino acids will be used, and some of the muscle mass will be lost when going on a fast with or without exercise. We can try to minimize it, but some amino acids will be used for energy.

The loss of some of the tissue is not as bad as it sounds. It has an evolutional purpose and health benefits if done moderately. It is normal for all animals including humans to go into fasting mode periodically. Our bodies have adapted to fasting during evolution in the same way that we have adapted to physical activity. Moderate periodical fasting will help our body to clean its mutated and damaged precancerous cells in a process known as autophagy (self-eating). When there are two cells and one is damaged and one has to be destroyed for energy, the body will catabolize damaged cells first and thus clean itself in a sense. We will have a negative impact on our health if we do not incorporate exercise and fasting into our regular life.

After this initial period, our metabolism will shift to ketosis where it gets almost all of its energy from ketone bodies from fat metabolism. The basal metabolic rate will drop also, or in other words, the use of energy will become more efficient. If you want to diet this is a state you would want to be in. A permanent caloric deficit in the amount that you can handle. Running a marathon or doing extensive exercise in a fast state might not be so beneficial because our brain will be left without food and will be forced to use our own muscle tissue for energy. This is not the desired outcome.

1. Feeding – Insulin levels rise during meals. This allows glucose to be absorbed into tissues such as the muscle or brain and used directly for energy. Excess glucose is stored in the liver as glycogen.
2. The post-absorptive phase occurs 6-24 hours after fasting begins. Insulin levels begin to drop. Glycogen breakdown produces glucose for energy. Glycogen stores last about 24 hours.
3. Gluconeogenesis can take anywhere from 24 hours to 2 days. In a process known as “gluconeogenesis,” the liver creates new glucose from amino acids. This literally translates as “creating new glucose.” Glucose levels in non-diabetics fall but remain within the normal range.
4. Ketosis – 2-3 days after fasting begins – The low insulin levels reached during fasting promote lipolysis or the breakdown of fat for energy. Triglycerides, the fat storage form, are composed of a glycerol backbone and three fatty acid chains. Glycerol is used in the process of gluconeogenesis. Many tissues in the body can use fatty acids directly for energy, but not the brain. Ketone bodies, which can cross the blood-brain barrier, are synthesized from fatty acids and used by the brain. Ketones provide approximately 75% of the energy used by the brain after four days of fasting. The two major types of ketones produced are beta-hydroxybutyrate and acetoacetate, both of which can increase by more than 70 percent during fasting.
5. Protein conservation phase. High levels of growth hormone help to keep muscle mass and lean tissues in check. The energy required to maintain basal metabolism is almost entirely supplied by free fatty acids and ketones. Increased norepinephrine (adrenaline) levels prevent the metabolic rate from decreasing.

The human body has well-developed mechanisms for dealing with hunger. In essence, what we are describing here is the process of transitioning from burning glucose to burning fat. That is it. Fat is simply stored food energy in the body. When there is a scarcity of food, stored food is naturally released to fill the void. So, no, the body does not ‘burn muscle’ to feed itself until all fat stores are depleted. There will be some catabolism at acceptable levels if you don’t have diabetes. In situations where there is hypoglycemia for survival, our body will burn everything to preserve brain cells from dying. If you don’t have hypoglycemia then no the body will not burn muscle but fat.

We can last for a period of 2.5 to 3 months just drinking water depending on how much fat we have to begin with. If we have large amounts of fat, we can last much longer, but nutrient deficiencies will occur. Because of this reason low-calorie nutrient-dense, whole fiber-rich food is a base for every diet.  

A good example of this is a man named Angus Barbieri, who water fasted for an entire year under medical supervision study (3). He was given vitamin supplements only. No calories, no protein.

Sources:

Passages selected from a book: “Go Vegan? Review of Science: Part 1” [Milos Pokimica]

1. Fundamentals of glycogen metabolism for coaches and athletes doi: 10.1093/nutrit/nuy001
2. Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis DOI:10.1007/s40279-018-0867-7
3. Features of a successful therapeutic fast of 382 days’ duration, Postgrad Med J. 1973 Mar; 49(569): 203–209

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