Branched-chain amino acids (BCAAs) consist of three amino acids—leucine, isoleucine, and valine—that belong to the group of essential amino acids and are characterized by their branched side chains. Since the human body cannot synthesize these amino acids, they must be obtained through dietary intake. BCAAs play a critical role in numerous physiological processes, particularly in muscle metabolism, energy production, protein synthesis, and the repair of exercise-induced muscle damage. In recent years, research on the importance of BCAAs in sports nutrition has increased, and their effects on performance enhancement, fatigue delay, and recovery processes have been extensively examined.

Chemical and Biological Properties of BCAAs

Molecular Structure and Classification

BCAAs are structurally distinct from other amino acids due to their aliphatic, branched side chains. Leucine (C₆H₁₃NO₂), isoleucine (C₆H₁₃NO₂), and valine (C₅H₁₁NO₂) are similar in both structure and function but are metabolized through different enzymatic pathways. While most amino acids are processed in the liver, BCAAs are predominantly metabolized in muscle tissue. This unique property allows them to be directly utilized for energy production within muscle cells.

Protein Synthesis and mTOR Activation

Leucine, in particular, plays a central role in cellular protein synthesis. By activating the mammalian target of rapamycin (mTOR) signaling pathway, leucine enhances muscle protein synthesis. Through this mechanism, BCAAs support anabolic processes and promote muscle hypertrophy. Experimental studies have shown that BCAA supplementation increases mTOR activation and positively affects muscle protein balance.

Nitrogen Balance and Catabolic Processes

BCAAs are also effective in maintaining nitrogen balance. In catabolic conditions such as illness, injury, or intense exercise, the body breaks down proteins to supply energy and structural components. During such processes, BCAA supplementation may reduce muscle tissue loss and enhance nitrogen retention. This effect is particularly important for athletes and individuals engaged in physically demanding professions.

Exercise Performance and Recovery

Energy Utilization During Exercise

BCAAs are known to serve as an energy source, especially during prolonged or high-intensity exercise. As muscle glycogen stores deplete, BCAAs become an alternative energy substrate. Moreover, the keto acids generated through BCAA metabolism enter the Krebs cycle, thereby contributing to energy production.

Fatigue Reduction and the Serotonin Theory

BCAAs also influence the central nervous system. During exercise, elevated levels of tryptophan in the bloodstream lead to increased serotonin levels in the brain, which is associated with enhanced perception of fatigue. Since BCAAs and tryptophan share the same transport system, BCAAs may limit the entry of tryptophan into the brain. Through this mechanism, BCAA supplementation may help delay the onset of fatigue during exercise.

Reduction of Muscle Damage and Lactate Levels

Intense exercise causes microscopic damage to muscle tissue. Clinical studies have demonstrated that BCAA supplementation results in lower levels of muscle damage markers such as creatine kinase (CK) and lactate dehydrogenase (LDH). Furthermore, reduced muscle soreness and shorter recovery periods are among the potential benefits of BCAA use.

Clinical and Therapeutic Applications

Use in Muscle-Wasting Conditions

BCAAs are not only important in sports nutrition but also have clinical relevance. Beneficial effects of BCAA supplementation have been reported in conditions characterized by muscle wasting, including sarcopenia, cancer cachexia, liver cirrhosis, and chronic kidney disease. In such cases, BCAAs may help prevent functional decline by reducing protein degradation.

Role in Liver Diseases

In chronic liver diseases such as cirrhosis, BCAAs are used to regulate nitrogen balance and alleviate hepatic encephalopathy. Randomized controlled trials have indicated that BCAA supplementation can improve cognitive function and enhance quality of life. Additionally, an increase in plasma albumin levels has also been observed with BCAA use.

Endocrine and Metabolic Disorders

Studies investigating insulin resistance and type 2 diabetes have also examined the effects of BCAA levels on glucose metabolism. Some findings suggest that BCAAs may stimulate insulin secretion, although chronically elevated BCAA levels might increase the risk of metabolic disorders. Therefore, BCAA supplementation should be tailored based on dose, duration, and individual metabolic status.