When the diet is deficient in carbohydrates, the body will make a complex metabolic shift

January 9, 2026 •

2 min read

Within hours of carbohydrate restriction, the body depletes its stored glucose (glycogen) and initiates a profound metabolic switch. This shift forces the body to seek alternative fuel sources, fundamentally altering how it generates and uses energy to survive.

Quick Summary

If deprived of carbohydrates, the body burns stored fat for energy via ketosis and manufactures glucose from protein through gluconeogenesis to fuel essential functions.

Key Points

Glycogen Depletion: The body first burns through its stored glucose, or glycogen, in the liver and muscles, leading to initial water weight loss.
Ketosis Initiation: After glycogen is depleted, the body enters ketosis, a state where fat is broken down into ketones to be used as the primary fuel source.
Ketone Production: The liver produces ketones from fat, which can cross the blood-brain barrier and serve as the main energy source for the brain.
Gluconeogenesis Activation: The liver begins converting non-carbohydrate sources, like amino acids from protein and glycerol from fat, into glucose to power essential bodily functions.
"Keto Flu" Symptoms: The metabolic transition can cause temporary side effects such as headaches, fatigue, and digestive issues, collectively known as the "keto flu".
Muscle Sparing: Adequate protein intake is crucial during carbohydrate deficiency to prevent muscle tissue from being broken down for gluconeogenesis.

The Initial Response: Glycogen Depletion

When your dietary intake of carbohydrates drops significantly, the body's first response is to tap into its readily available glucose stores, known as glycogen. Glycogen is stored primarily in the liver and muscles and acts as a short-term energy reserve. As liver glycogen stores are depleted, this initial phase ends, and the body seeks a new primary fuel source. Initial water loss from glycogen depletion can cause a noticeable drop in body weight.

The Shift to Fat and Ketosis

After glycogen depletion, the body shifts to ketosis, burning fat for fuel. The liver produces ketones from fat, which most tissues, including the brain, can use. Ketones provide the brain with energy when glucose is limited, demonstrating the body's metabolic flexibility.

Gluconeogenesis: Making New Glucose

Some cells still need glucose. The body uses gluconeogenesis, mainly in the liver, to create new glucose from non-carbohydrate sources like amino acids (from protein) and glycerol (from fat). Sufficient protein intake helps preserve muscle mass during this process.

Short-Term Side Effects: The 'Keto Flu'

The transition can cause a temporary "keto flu" with symptoms from electrolyte imbalances and adaptation to using ketones.

Symptoms may include:

Headaches and brain fog
Fatigue and irritability
Dizziness and nausea
Muscle cramps and weakness
Constipation or other digestive issues
Increased thirst and bad breath

Hydration and electrolyte replenishment can help.

Long-Term Considerations and Fuel Source Comparison

Long-term effects of severe carbohydrate restriction are still studied. Potential concerns include nutrient deficiencies and impacts on organ health. Diet quality is key.

Comparison of Fuel Sources: Carbohydrates vs. Ketones


Feature	Carbohydrate-Fueled State (Glycolysis)	Ketone-Fueled State (Ketosis)
Primary Fuel Source	Glucose (from carbs or stored glycogen)	Ketones (from fat)
Energy Delivery Speed	Rapid, quick bursts of energy	Slower, sustained energy supply
Mental Acuity	Can be affected by blood sugar spikes/crashes	Can result in enhanced focus and clarity after adaptation
Initial Adaptation	None (standard metabolic pathway)	"Keto flu" symptoms are common
Fuel for the Brain	Glucose is the primary fuel source	Ketones become the main fuel source
Protein Sparing	Spares protein from being used for energy	Spares protein from gluconeogenesis once fully adapted

Conclusion: A Remarkable Metabolic Flexibility

In summary, when the diet is deficient in carbohydrates, the body will first use stored glycogen, then enter ketosis to burn fat and produce ketones for the brain. Gluconeogenesis provides essential glucose from protein and fat. This metabolic flexibility aids survival during scarcity. Long-term restriction requires careful monitoring and is best undertaken with professional guidance. For more on metabolic effects, see the NIH resource: Metabolic Effects of the Very-Low-Carbohydrate Diets.

Frequently Asked Questions

The very first thing the body does is deplete its stores of glycogen, which is the stored form of glucose found in the liver and muscles. This process typically occurs within 12 to 24 hours of carbohydrate restriction.

The body primarily uses ketones for brain energy during carbohydrate deficiency. The liver produces ketones by breaking down fat, and these molecules are used as an alternative fuel source to glucose.

Yes, through a process called gluconeogenesis, the body can convert certain amino acids from protein into glucose. This is used to fuel cells, like red blood cells, that cannot run on ketones.

The 'keto flu' is caused by the body's adaptation to using fat instead of glucose for fuel, as well as electrolyte imbalances due to increased water excretion. Symptoms include headaches, fatigue, and irritability.

Some research suggests that long-term restriction of carbohydrates can potentially lead to nutrient deficiencies, digestive issues, and in some cases, affect kidney and heart health. The overall health impact depends heavily on the quality of the diet.

Ketosis is a normal metabolic state where the body burns fat for fuel, resulting in a safe level of ketones in the blood. Ketoacidosis is a dangerous, life-threatening condition, typically in uncontrolled diabetes, where a massive buildup of ketones makes the blood dangerously acidic.

One common sign of ketosis is a fruity or sweet odor to the breath, often referred to as 'keto breath,' which is caused by the release of acetone, a type of ketone body.