In healthy adults, ingestion of amino acids signals the body to start muscle protein synthesis, the building of muscle mass.
However, this response is diminished in elderly persons, a phenomenon termed anabolic resistance.
The exact mechanisms behind anabolic resistance are not fully understood.
Elderly persons have a reduced capacity to produce nitric oxide, a molecule that stimulates vasodilation; the widening of blood vessels.
Nitric oxide increases the muscle blood flow and increases the area for amino acids to leave the blood through the vessel wall to enter the muscle cells.
Thus, the researchers speculate that the delivery of amino acids to muscle cells might be impaired in elderly and this contributes to anabolic resistance.
Therefore, they wanted to investigate the effects of amino acids on muscle when they increased nitric oxide. They infused amino acids and the nitric oxide booster sodium nitroprusside in both younger and older subjects.
To their surprise, they found that muscle protein synthesis was similar between the young and the old group as seen in the figure below. In other words, they did not find any anabolic resistance in the elderly.
Other recent investigations suggests that the delivery of amino acids into muscle cells is not impaired in the elderly. Therefore, the researchers speculate that nitric oxide does not influence muscle protein synthesis through an amino acid delivery effect, but rather an amino acid utilization effect. For example, the increased blood flow may deliver other nutrients or hormones that help the cell to use its amino acids to build muscle mass.
Whatever the mechanism might be, nitric oxide appears to be beneficial for elderly muscles. Find out more about nitric oxide in the upcoming days!
Source: Am J Physiol Regul Integr Comp Physiol. 2011 Aug 31.
Excerpt image by Andrew Mason