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Fasting is the practice of restricting food or drink intake for a period and is often used for dieting, religious beliefs, or medical testing. It can cause a radical change in cellular physiology and metabolism, with blood glucose levels initially maintained by glycogen stores in the liver and skeletal muscle. Glycogen is a form of stored sugar or glucose, occupying 1%–2% of the volume of skeletal muscle cells. During fasting, glycogen stores are depleted, causing the body to use energy from adipose tissue and protein stores, which can lead to a reduction in muscle mass. While fasting, the body can enter a state of ketosis, where it burns fat for energy instead of glucose. This can be achieved through intermittent fasting, which involves restricting caloric intake during a set period.
| Characteristics | Values |
|---|---|
| What is glycogen? | A form of stored sugar or glucose formed by many linked glucose molecules. |
| Where is glycogen stored in the body? | Liver and muscle cells. |
| What is the role of glycogen? | It is the primary source of energy for the body. |
| What happens to glycogen during fasting? | Glycogen depletion occurs when the body needs energy during fasting. |
| What happens to muscle glycogen during fasting? | Muscle glycogen is broken down, and the associated water molecules are excreted in urine, leading to rapid weight loss. However, neither short-term fasting nor prolonged sedentary behavior affects muscle glycogen stores. |
| How does fasting affect metabolism? | During fasting, the body enters a fasting state and switches to a fat-burning mode, known as ketosis, where it uses fat and ketones for energy instead of glucose. |
| How does exercise impact glycogen levels during fasting? | Exercise can expedite the progression of a fast and deplete glycogen levels faster, especially high-intensity exercises. |
| What are the risks of fasting? | Fasting can lead to protein catabolism, where the body breaks down muscle tissue for energy, resulting in a reduction of muscle mass. |
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What You'll Learn
Fasting and muscle glycogen
Fasting involves a significant change in metabolism and cellular physiology. During fasting, the body's maintenance of blood glucose levels relies on glycogen stores in the liver and skeletal muscle. Glycogen is a form of stored sugar or glucose, with each gram stored with at least 3 grams of water. It is the primary source of energy, which the body stores in the liver and muscle cells. The liver has the greatest role in maintaining blood glucose during the first 24 hours of a fast.
After around 24 hours of fasting, glycogen stores in the liver are depleted, causing the body to utilise energy from adipose tissue and protein stores. The body's metabolic transition into the fat-burning zone and subsequently, ketosis, is triggered by the depletion of liver glycogen and reduced glucose availability. This metabolic shift is characterised by the breakdown of fat and the production of ketone bodies, which become the primary energy source for the brain and body.
During fasting, the body can also undergo protein catabolism, where amino acids are broken down from various tissues, including muscle. In more extreme fasting practices, where fat sources have been expended, the body may break down skeletal muscle for energy, leading to a reduction in muscle mass. However, it is important to note that neither short-term fasting nor prolonged sedentary behaviour significantly affects muscle glycogen stores.
Prolonged fasting and very low-carbohydrate diets can result in ketosis, which spares liver and muscle glycogen. Additionally, the fasting that occurs between meals or during sleep has a minimal effect on muscle glycogen concentration in resting individuals, as muscle glycogen is not a major fuel substrate at rest.
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Liver glycogen and fasting
Fasting involves a significant change in metabolism and cellular physiology. Blood glucose, which usually provides the body with energy, is no longer available during fasting, and the body must adapt to maintain its energy balance. This is where liver glycogen comes in.
Liver glycogen is a glucose-storage polymer that is used to replenish blood glucose levels during fasting. The liver plays the most significant role in maintaining blood glucose during the first 24 hours of a fast. After this period, glycogen stores in the liver are typically depleted, causing the body to turn to other sources for energy, such as adipose tissue and protein stores.
During a fast, the body experiences a decrease in blood glucose levels, which it detects at sites like the pancreatic islets and brain. This drop in glucose triggers a response in the body, which reduces insulin secretion from islet β cells and increases glucagon secretion from islet α cells. This response is part of a liver-brain-adipose neural axis that signals a switch in fuel source from liver glycogen to triglycerides in fat cells.
The activation of this neural axis is independent of blood glucose and insulin/glucagon levels, indicating that the shortage of glycogen itself is the key trigger for this switch in fuel source. This is supported by studies showing that blocking glycogenolysis, and thus preventing the depletion of glycogen, also prevents the activation of this neural axis and the switch to triglycerides as an energy source.
In summary, liver glycogen plays a crucial role in maintaining the body's energy balance during the initial stages of fasting. When liver glycogen stores are depleted, the body undergoes significant metabolic changes to utilise other energy sources, such as adipose tissue and protein.
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Metabolic changes during fasting
Fasting involves a significant change in cellular physiology and metabolism. Metabolic changes during fasting can be understood through the following points:
- During fasting, the body finds alternative ways to create energy. Typically, the body breaks down carbohydrates into glucose. However, during a fast, the body resorts to non-carbohydrate sources to produce glucose through a process called gluconeogenesis.
- Gluconeogenesis utilises glycogen reserves in the liver and skeletal muscle, along with water stored in the tissues, to synthesise glucose for energy. This process ensures that the body's blood glucose levels remain within the normal range of 70-120 mg/dL.
- After approximately 24 hours of fasting, glycogen stores become depleted, prompting the body to seek alternative energy sources. At this stage, the body primarily turns to fat cells, breaking down triglycerides in the adipose tissue to release free fatty acids and glycerol.
- The liver then converts these free fatty acids and glycerol into ketone bodies and glucose. This process is known as ketosis, which is a state where the body uses ketones as a primary energy source instead of glucose.
- Prolonged fasting and very low-carbohydrate diets can lead to ketosis or ketoacidosis, which spares liver and muscle glycogen. This metabolic state is often sought after by endurance athletes, as it can enhance their performance.
- Fasting also affects the pancreas, which releases glucagon during periods of low plasma glucose. Glucagon primarily targets the liver, which stores most of the body's glycogen, but it also influences skeletal muscle to a lesser extent due to the lower glycogen concentration in muscle tissue.
- When fasting continues for an extended period, the body may start breaking down skeletal muscle for energy, leading to a reduction in muscle mass. This process provides the body with amino acids that can be metabolised for energy.
- The metabolic changes during fasting are highly individualised, influenced by factors such as genetics, health, and lifestyle.
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Fasting and exercise
When it comes to exercise, fasting can be beneficial in some cases. For example, studies have shown that exercising during a fasting state increases lipolysis in adipose tissue, resulting in increased fat utilization and weight loss. Additionally, fasting and exercise training are non-pharmacological and inexpensive ways to help manage obesity- and overweight-related complications. However, it is important to note that exercising while fasting can also increase the risk of dehydration and heat illness, especially if the fast involves abstaining from all food and drink. Therefore, it is recommended to schedule workouts during non-fasting times, or to exercise before breaking the fast, so that the body can hydrate and fuel before, during, and after the workout.
The impact of fasting and exercise on muscle glycogen is a complex topic. On the one hand, skeletal muscle glycogen is used during exercise, and its depletion can contribute to fatigue. However, neither short-term fasting nor prolonged sedentary behavior affects muscle glycogen stores. In fact, cardiac muscle glycogen may even be increased by fasting. Additionally, prolonged fasting and very low-carbohydrate diets result in ketosis, which spares liver and muscle glycogen.
Exercising during a fasted state has been shown to have beneficial effects on the body's hormones, particularly growth hormones, which can help with weight loss and muscle gain. This is because the body's fuel source shifts from glucose to fat, optimizing hormones and making the body more insulin-sensitive. However, it is important to note that performance may drop initially when adapting to this new fuel source, and it can take up to six months for athletes to fully adjust their endurance.
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Fasting and muscle mass
Fasting involves restricting food or drink intake for a period and has been practised for various reasons, including dieting, religious beliefs, and medical testing. During a fast, the body undergoes a radical change in cellular physiology and metabolism, initially relying on glycogen stores in the liver and skeletal muscle for energy.
Glycogen is a form of stored sugar or glucose that the body stores in the liver and muscle cells. It is made up of chains of polymerized glucose monosaccharides that are broken down through the process of glycogenolysis to provide energy. While the liver stores most of the glycogen, skeletal muscle also contains a small amount, with glycogen particles in skeletal muscle cells being much smaller than those in liver cells.
During a fast, the body's immediate response is to release glucagon from the pancreas, which primarily affects the liver. After around 24 hours of fasting, the glycogen stores in the liver are typically depleted, causing the body to utilise energy from adipose tissue and protein stores. This leads to a drastic change in metabolism as the body begins to rely on triglyceride stores in adipose tissue.
Prolonged fasting and very low-carbohydrate diets can result in ketosis, where the body starts to burn fat for energy instead of glucose. This process can be accelerated by strenuous exercise, which facilitates glycogen depletion. However, it is important to note that while fasting can aid in weight loss and fat reduction, it may also lead to a decrease in muscle mass as the body breaks down skeletal muscle for energy. This process, known as catabolism of skeletal muscle, provides the body with amino acids that can be metabolised, but it results in a reduction in muscle mass, which is counterproductive for individuals seeking to build or maintain muscle.
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Frequently asked questions
Yes, fasting does drain muscle glycogen. Glycogen is a form of stored sugar or glucose, which is the primary source of energy for the body. It is stored in the liver and muscle cells. During fasting, the body uses glycogen to maintain blood glucose levels. However, the liver has a much higher concentration of glycogen than skeletal muscle. Therefore, while fasting does deplete muscle glycogen, it is to a lesser extent than liver glycogen.
The time it takes to deplete muscle glycogen through fasting varies depending on factors such as diet, muscle mass, and exercise intensity and duration. Generally, it takes around 24 hours of fasting to significantly deplete glycogen stores. However, strenuous exercise or high-intensity workouts can expedite the process, potentially achieving glycogen depletion in as little as 90-120 minutes.
Once muscle glycogen is depleted, the body enters a state of ketosis, where it begins to burn fat for energy instead of glucose. This is known as the Fasted State or Ketogenic State. The body breaks down adipose tissue and protein for energy, which can lead to a reduction in muscle mass if the fast is extended and fat sources are expended.
Yes, there are potential benefits associated with fasting and depleting muscle glycogen. Intermittent fasting, which involves periods of fasting and is often combined with a low-carb diet, can aid in weight loss and lead to beneficial physiological changes. Fasting has been shown to improve metabolic indicators, reduce body weight, and decrease stored fat. Additionally, reaching the ketogenic state through fasting can be advantageous for endurance athletes as it provides an alternative energy source.
