Gluconeogenesis: What is a protein converted to glucose called?

November 21, 2025 •

3 min read

The human brain alone requires approximately 100 grams of glucose per day for energy. This demand for glucose persists even during periods without carbohydrate intake, revealing the body's sophisticated backup plan known as gluconeogenesis, which explains what a protein is converted to glucose called. This vital metabolic pathway ensures a continuous supply of glucose for essential organs and bodily functions.

Quick Summary

The conversion of protein into glucose is called gluconeogenesis, a metabolic process performed predominantly by the liver. It utilizes glucogenic amino acids to synthesize new glucose, a critical function during fasting to maintain stable blood sugar levels.

Key Points

Name of the Process: The conversion of protein to glucose is called gluconeogenesis, which literally means 'creation of new sugar'.
Primary Location: This metabolic process occurs mainly in the liver, with a smaller contribution from the kidneys, especially during prolonged fasting.
Amino Acid Sources: The building blocks of protein, known as glucogenic amino acids (most amino acids), are the key precursors for glucose synthesis.
Hormonal Control: Gluconeogenesis is promoted by hormones like glucagon and cortisol, which signal the body's need for more glucose, while insulin works to inhibit it.
Purpose: The main function of gluconeogenesis is to ensure a continuous supply of glucose for organs like the brain, which depend on it for energy, when dietary carbohydrates are unavailable.
Fasting Role: During prolonged fasting or low-carb diets, gluconeogenesis becomes the body's primary method for producing glucose after glycogen stores are depleted.

The Process of Gluconeogenesis Explained

Gluconeogenesis (GNG), from the Greek meaning 'creation of new sugar', is the metabolic pathway responsible for synthesizing glucose from non-carbohydrate precursors. This process is crucial when dietary carbohydrates are scarce, such as during fasting, prolonged exercise, or following a low-carb diet. The primary organs involved are the liver (responsible for about 90% of the process) and, to a lesser extent, the kidneys. Instead of simply reversing glycolysis (the breakdown of glucose), gluconeogenesis cleverly bypasses three irreversible steps of that pathway using a unique set of enzymes. This energy-intensive process is powered by the breakdown of fatty acids, which provides the necessary ATP.

The Role of Amino Acids

Protein, broken down into its amino acid building blocks, serves as a major source of precursors for gluconeogenesis. These amino acids are classified into two groups based on their metabolic fate:

Glucogenic amino acids: These are converted into intermediates of the citric acid cycle or pyruvate, which can then be converted into glucose. The majority of amino acids fall into this category, including alanine, glutamine, glycine, serine, and many others. For example, during fasting, muscles release alanine, which is transported to the liver and converted into glucose via gluconeogenesis in a process known as the glucose-alanine cycle.
Ketogenic amino acids: Leucine and lysine are the only two purely ketogenic amino acids. Their carbon skeletons are converted into acetyl-CoA or acetoacetate, which can be used to produce ketone bodies or fatty acids, but cannot be used for the net synthesis of glucose in humans.

The Hormonal Regulators

The activation and inhibition of gluconeogenesis are tightly controlled by hormones to maintain glucose homeostasis.

Glucagon: Secreted by the pancreas in response to low blood sugar, glucagon is a major stimulator of gluconeogenesis. It upregulates key enzymes in the gluconeogenic pathway.
Cortisol: Known as the stress hormone, cortisol increases during times of stress and fasting. It enhances gluconeogenesis by promoting the release of glucogenic amino acids from muscles and increasing the expression of gluconeogenic enzymes in the liver.
Epinephrine (Adrenaline): Released during stress, epinephrine also stimulates gluconeogenesis, working alongside glucagon.
Insulin: Conversely, insulin is released when blood sugar levels are high and acts as a powerful inhibitor of gluconeogenesis, directing the body to store glucose instead.

Gluconeogenesis vs. Other Metabolic Pathways

Gluconeogenesis works in concert with other metabolic processes to balance the body's energy needs. It is particularly contrasted with glycogenolysis, another method for glucose production.

Comparing Gluconeogenesis and Glycogenolysis


Feature	Gluconeogenesis	Glycogenolysis
Source	Non-carbohydrate precursors like glucogenic amino acids, lactate, and glycerol.	Glycogen (stored glucose).
Timing	Activated during prolonged fasting (after about 8-12 hours when glycogen stores are depleted).	Active during shorter fasting periods (e.g., between meals or overnight).
Process	De novo synthesis of new glucose molecules from scratch.	Breaking down existing glucose stores.
Location	Primarily liver, with some contribution from the kidneys.	Liver (to release glucose systemically) and muscle (for local use).
Duration	Can sustain glucose production for extended periods of fasting.	Supplies glucose for a shorter duration (around 18-24 hours).

Conclusion

Gluconeogenesis is a cornerstone of human metabolism, allowing the body to produce new glucose from protein, fats, and other non-carbohydrate sources when dietary intake is insufficient. This complex pathway, orchestrated by hormones like glucagon and cortisol, ensures a steady supply of energy for the brain and other glucose-dependent tissues during periods of fasting or low-carb intake. While proteins serve as a critical source of glucogenic amino acids for this process, it is important to remember that most of the body's energy needs during starvation are covered by fatty acid breakdown, which also fuels the energetically expensive gluconeogenic reactions. Understanding this process provides deeper insight into how our bodies adapt to changing nutritional demands, and how metabolic disorders, like type 2 diabetes, can disrupt this delicate balance.

Further reading on metabolic processes can be found at the National Institutes of Health.

Frequently Asked Questions

When you don't consume enough carbohydrates, your body can break down protein from sources like muscle tissue into glucogenic amino acids. These amino acids are then used in the process of gluconeogenesis to synthesize glucose, ensuring vital organs like the brain have a fuel source.

In healthy individuals, eating protein does not cause a significant rise in blood glucose levels. The amino acids from protein can stimulate both insulin and glucagon, and their conversion to glucose via gluconeogenesis is a slower, more controlled process than carbohydrate metabolism.

The majority of amino acids are considered glucogenic and can be converted to glucose. Important examples include alanine, glutamine, serine, and glycine. Only two amino acids, leucine and lysine, are purely ketogenic and cannot be used for net glucose production.

Gluconeogenesis takes place mainly in the liver. During prolonged fasting, the kidneys also contribute significantly to the process.

Cortisol promotes gluconeogenesis by encouraging the breakdown of muscle protein into amino acids and stimulating key enzyme activity in the liver. This helps to increase blood glucose levels, particularly during times of stress.

Glycogenolysis is the breakdown of stored glycogen into glucose, typically for short-term energy needs. Gluconeogenesis, on the other hand, is the synthesis of new glucose from non-carbohydrate sources, primarily for long-term glucose supply during extended fasting.

After about 8-12 hours of fasting, the body's stored glycogen is depleted. Gluconeogenesis then becomes the main mechanism for producing glucose, providing necessary fuel for the brain and other glucose-dependent tissues to function properly.