Gluconeogenesis

Gluconeogenesis is a metabolic pathway where the body synthesizes glucose using non-carbohydrate sources.

This article is part of the Complete Guide to Proper Nutrition — a key guide from the expert team at Sport Zona.

What it is

Gluconeogenesis (from Greek: "glykys" – sweet, "neos" – new, "genesis" – creation) is a biochemical process through which the organism synthesizes glucose from non-carbohydrate precursors. This complex metabolic pathway is essential for maintaining normal blood sugar levels, especially during periods of fasting, prolonged physical exertion, low-carbohydrate diets, or even under normal conditions when carbohydrate stores (glycogen) are low. It primarily occurs in the liver, but to some extent also in the kidneys.

Through gluconeogenesis, the body ensures a constant supply of glucose to cells and tissues that are highly dependent on it for energy, such as the brain, red blood cells, and renal medullary cells. This process is the reverse of glycolysis but uses different enzymes at several stages to bypass irreversible reactions. It is tightly regulated by hormones such as insulin, glucagon, and cortisol, which signal the body's glucose needs.

How it works

The process of gluconeogenesis involves the conversion of specific non-carbohydrate substrates into glucose. The main precursors are lactate (produced by anaerobic glycolysis in muscles and red blood cells), amino acids (especially those classified as glucogenic, derived from protein breakdown), and glycerol (released from the breakdown of triglycerides in adipose tissue). Fatty acids, in general, cannot be directly converted into glucose in mammals, except for the glycerol portion of triglycerides.

Glucose synthesis proceeds through a series of enzymatic reactions that effectively reverse the pathway of glycolysis, but with key differences. For example, pyruvate is converted to oxaloacetate and then to phosphoenolpyruvate, bypassing the irreversible step of pyruvate kinase. Glucose-6-phosphate is dephosphorylated to glucose by an enzyme called glucose-6-phosphatase, found primarily in the liver and kidneys. This mechanism allows the body to maintain a vital blood sugar reserve even when carbohydrate intake is limited and is crucial for long-term energy supply.

Why it's important for athletes

For athletes, especially those involved in endurance events or training with high intensity and volume, gluconeogenesis is a critical mechanism for maintaining energy balance. During prolonged physical exertion, muscle and liver glycogen stores are gradually depleted. When these primary carbohydrate reserves fall, gluconeogenesis is activated to provide additional glucose for muscles and the brain, preventing hypoglycemia and sustaining performance. In this way, it helps delay fatigue and maintain concentration.

Intense training also leads to increased lactate production in muscles, which can be used by the liver as a precursor for gluconeogenesis (known as the Cori cycle), converting a potentially harmful metabolite back into a useful energy source. Furthermore, in athletes on low-carbohydrate diets, gluconeogenesis plays an even more significant role in maintaining energy levels. Although the primary energy sources in such diets are fats and ketone bodies, the brain and some other tissues still require a minimal amount of glucose, which is provided precisely through this process. Recommendations from the American College of Sports Medicine (ACSM) and the International Society of Sports Nutrition (ISSN) emphasize the importance of adequate carbohydrate intake (typically 3-10 g/kg body weight, depending on the load) to minimize protein breakdown for gluconeogenesis needs and optimize muscle recovery.

Related concepts

• Glycogen — Glycogen is the stored form of glucose, serving as the primary and rapid energy reserve, while gluconeogenesis creates new glucose when glycogen is depleted.
• Ketosis — Ketosis is a metabolic state where the body uses ketone bodies for energy, and gluconeogenesis is an actively functioning process that provides glucose for cells that cannot use ketones.
• Anabolism — Anabolism is the process of building complex molecules from simpler ones, while gluconeogenesis is an example of a synthetic anabolic pathway producing glucose.
• Autophagy — Autophagy is a cellular detoxification and recycling process that can provide amino acids as a substrate for gluconeogenesis during nutrient deficiency.