Vitamin D (Vit D) is not technically a vitamin or essentialdietary factor. Vit D is actually a pro-hormone produced phytochemically in the skin. And unlike traditional vitamins, vit D has its own hormone receptor (VDR). VDRs are found in at least 36 different organs in the body.
As for VDRs in muscle tissue, limitations of technology have left the question somewhat open, yet there is compelling reason to believe that they are there. For example, the enzyme required to convert inactive vit D (D2) into active vit D (D3) is found in muscle tissue. And many studies have indeed found DNA from VDRs in skeletal muscle cells. To further establish this fact and to see if vit D3 may actually have an anabolic effect, researchers carried out an in-vitro study using muscle cells, insulin, leucine, and vit D.
The focus of this study was on the mTOR anabolic signaling pathway in muscle cells. It is well known that insulin facilitates the activation of the mTOR pathway,particularly high insulin levels. Leucine is a direct activator of the mTOR pathway and in this way can “switch on” protein synthesis in muscle cells. These researchers took muscle cells and exposed them to leucine and insulin, or leucine, insulin, and vit D.
Results demonstrated that the addition of vit D to insulin and leucine significantly enhanced the activity of the mTOR pathway and protein synthesis. The authors conclude that vit D has the potential to directly alter protein synthesis in muscle cells. Previous studies have found an association between vit D levels and muscle strength in the elderly.
It was once thought that 15 minutes of sun exposure to the face and hands was enough to meet daily vit D requirements. Despite this, vit D levels in the general population have been found to be below “optimal.” From this most recent research we now know that vit D can play an important part in maximizing the anabolic effect of our diet so adding vit D to your supplement regimen is warranted. Try to get 2,000–5,000 IUs of vit D per day with meals containing fat. - FLEX