It is long known that ingestion of protein raises the level of essential amino acids in the blood, which in turn stimulates muscle protein synthesis – the building of new muscle tissue.
However, after ~2 hours, muscle protein synthesis returns to its normal level even when amino acids are still elevated. This phenomenon has been termed the muscle full effect.
Researchers from the USA might have out the mechanism behind the muscle full effect and what can be done about it.
The muscle full effect can be seen in the graphs. The first graph shows that myofibrillar fractional synthetic rate (FSR), a measurement of muscle protein synthesis has dropped to its normal value after 120 minutes despite that essential amino acids where still elevated at this point as seen in the second graph.
The researchers found that the drop that muscle protein synthesis was inversely associated with the AMP/ATP ratio, which reflects the energy status of the cell.
A reduced energy status activates AMPK, which is an important regulator in metabolism. Its function is to improve energy status by increasing the energy producing catabolic processes (i.e. breakdown of fats for energy) and reducing energy expensive anabolic processes (i.e. building muscle mass).
Therefore, the researchers investigated the effects of supplements that could prevent the increase in AMPK activation that occurs ~2h after a meal.
They administrated carbohydrates and/or leucine, an essential amino acid that has previously shown effective to reduce the activity of AMPK. Indeed, either, or the combination of both supplements reduced AMPK activation and prolonged muscle protein synthesis after a meal.
Interestingly, the amount of the leucine supplement was much lower than the amount of carbohydrate supplement.
Thus, using a leucine supplement between meals might be an effective method to improve muscle mass, without adding as much potentially fattening calories as you would with carbohydrates.