What is the First Source of Energy for the Body When We Fast?

According to physiological studies, the body’s first metabolic adaptation when we fast is to primarily utilize stored glycogen as its initial energy source. This process ensures a continuous supply of glucose to maintain normal bodily functions while food intake is absent.

Quick Summary

The body initially relies on glucose and stored glycogen for fuel. As fasting continues and glycogen is depleted, it transitions to breaking down fat stores, eventually producing ketones for energy.

Key Points

• Initial Fuel: The body's first energy source during a fast is stored glycogen in the liver and muscles.

• Glycogen Depletion: These reserves provide glucose for approximately 12-24 hours before becoming significantly depleted.

• Hormonal Trigger: Falling insulin and rising glucagon levels signal the liver to release stored glucose.

• Metabolic Shift: After glycogen is gone, the body begins breaking down fat stores for energy, a process known as lipolysis.

• Ketone Production: The liver converts fatty acids from fat stores into ketones, which become the brain's primary fuel source during prolonged fasting.

• Gluconeogenesis: A continuous process where the body creates new glucose from amino acids and glycerol to fuel specific cells.

• Fat Adaptation: During extended fasting, the body becomes more efficient at using fat and ketones, thereby preserving muscle protein.

The Body's Immediate Energy Source: Stored Glycogen

When you stop eating, your body uses its stored energy reserves, with glycogen being the first fuel source. Glycogen is stored glucose primarily found in the liver and muscles. This initial phase typically lasts 12 to 24 hours, varying based on factors like activity and metabolic health. The liver's glycogen is crucial, as glucagon prompts its breakdown into glucose (glycogenolysis) for blood circulation, providing energy to the brain and other cells. Muscle glycogen is mainly used by muscle cells themselves.

The Metabolic Shift to Fat Burning and Ketosis

After glycogen depletion, the body shifts from glucose dependence to fat dependence. Lipolysis, the breakdown of fat, increases, releasing fatty acids and glycerol. While tissues like the heart use fatty acids, the brain cannot. To fuel the brain, the liver produces ketones from fatty acids through ketogenesis. Elevated blood ketones result in ketosis, where the brain uses ketones for energy, a key adaptation during prolonged fasting.

The Continuous Backup: Gluconeogenesis

Even in a fat-burning state, some glucose is needed for cells like red blood cells. Gluconeogenesis produces new glucose from non-carbohydrate sources like amino acids, glycerol, and lactate, mainly in the liver and kidneys. This process helps maintain stable blood sugar levels during fasting. Learn more about these processes: Physiology, Fasting - StatPearls - NCBI Bookshelf.

Metabolic Timelines: A Comparative View

The body's energy source transitions during fasting are a display of metabolic flexibility. The timing is approximate and varies by individual.

Feature	Glycogen Phase	Fat-Burning/Ketosis Phase	Prolonged Fasting Phase
Timeframe	Approx. 0-24 hours	Approx. 12-48 hours	After 48+ hours
Primary Fuel	Glucose	Fatty Acids & Ketones	Ketones & Fatty Acids
Main Organ	Liver	Adipose Tissue & Liver	Adipose Tissue & Liver
Associated Process	Glycogenolysis	Lipolysis & Ketogenesis	Gluconeogenesis & Ketosis
Brain Fuel	Glucose	Transition to Ketones	Primarily Ketones
Key Hormonal Change	Decrease in insulin, increase in glucagon	Increase in glucagon, epinephrine, and cortisol	Continued low insulin, high glucagon

Conclusion: The Body's Adaptable Energy System

The body's first energy source when fasting is stored glycogen, used for the initial 12 to 24 hours. Once depleted, the body shifts to burning fat through lipolysis and ketogenesis, with ketones becoming a primary fuel for the brain. Gluconeogenesis simultaneously produces small amounts of glucose for essential functions. This metabolic adaptability is crucial for survival and is observed in practices like intermittent fasting.

Frequently Asked Questions

How long does it take for the body to burn through its initial energy source when fasting?

The body typically uses up its stored glycogen, the first source of energy, within the first 12 to 24 hours of fasting. The exact time can vary depending on factors like your activity level and last meal.

What is the second source of energy the body uses after the first source is gone?

Once glycogen is depleted, the body switches to burning fat for energy. This involves breaking down stored triglycerides into fatty acids and producing ketone bodies in the liver.

What is the difference between glycogen and glucose?

Glucose is a simple sugar used for immediate energy, while glycogen is a complex carbohydrate that serves as the body's stored form of glucose. Glycogen is broken down into glucose when energy is needed.

Does the brain use ketones for energy during a fast?

Yes, during prolonged fasting, the brain can adapt to use ketones as its primary fuel source. While it prefers glucose, this shift is a key survival mechanism once glycogen stores are gone.

What role does the liver play in fasting energy production?

The liver plays a central role in all phases of fasting. It stores and releases glycogen, produces ketones from fatty acids, and performs gluconeogenesis to create new glucose.

Is gluconeogenesis bad for you because it uses protein?

Gluconeogenesis is a necessary survival function. While it can use amino acids from muscle protein, the body becomes highly efficient at conserving protein during prolonged fasting by increasing its use of fat and ketones.

Do different types of fasting change the energy source timeline?

Different fasting regimens, like intermittent fasting versus prolonged fasting, can affect how often your body enters and exits the various metabolic stages. However, the order of energy sources (glycogen first, then fat) remains the same.