From the Guidelines
Alanine is the primary amino acid that transports nitrogen from muscle to liver. This process occurs through the glucose-alanine cycle, where muscles break down proteins and release amino acids, transferring nitrogen to pyruvate to form alanine, which then enters the bloodstream and undergoes deamination in the liver, releasing nitrogen for excretion as urea and converting the carbon skeleton to glucose through gluconeogenesis 1. The role of glutamine in nitrogen transport is also noted, particularly in certain conditions such as acidosis, but it is not the primary transporter of nitrogen from muscle to liver 1. Key points about this process include:
- The glucose-alanine cycle allows muscles to dispose of excess nitrogen while providing the liver with substrates for glucose production
- Alanine is deaminated in the liver, releasing nitrogen which enters the urea cycle for excretion as urea in urine
- The carbon skeleton of alanine is converted to glucose through gluconeogenesis, which can then return to the muscle for energy use
- This cycle is crucial during periods of increased energy demand or protein catabolism, such as fasting or exercise.
From the Research
Transport of Nitrogen from Muscle to Liver
- The transport of nitrogen from muscle to liver is a complex process involving various amino acid transporters and metabolic pathways.
- According to the study 2, the liver controls the systemic exposure of amino acids entering via the gastro-intestinal tract, and hepatic uptake is very efficient for most amino acids except branched chain amino acids.
- Branched-chain amino acids (BCAAs) play a crucial role in nitrogen transport and metabolism, as they are involved in the synthesis and degradation of various amino acids and proteins.
- The study 3 found that BCAA infusion in addition to lactulose treatment was associated with improved short-term outcomes in patients with hepatic encephalopathy, suggesting a potential role for BCAAs in nitrogen transport and metabolism.
- However, the exact mechanisms and transporters involved in the transport of nitrogen from muscle to liver are not fully understood and require further research.
- The study 4 suggests that non-absorbable disaccharides, rifaximin, and BCAAs may have beneficial effects on hepatic encephalopathy, but the evidence is not directly relevant to the transport of nitrogen from muscle to liver.