Active vitamin D (1,25(OH)2D) enhances skeletal muscle cell differentiation, insulin-mediated protein synthesis, and mitochondrial respiration in human myoblasts and myotubes.

Study: 1,25(OH)2-vitamin D3 enhances the stimulating effect of leucine and insulin on protein synthesis rate through Akt/PKB and mTOR mediated pathways in murine C2C12 skeletal myotubes.

The study found that the active form of vitamin D helps muscle cells build more protein when insulin and leucine are present, which matches part of the claim. Even though it was done in mouse cells, not human ones, the mechanism is very similar.

Study: 1,25‐Dihydroxyvitamin D3 Mediates L6 Myoblast Differentiation via Vitamin D Receptor (VDR)

This study found that the active form of vitamin D helps muscle precursor cells in rats mature into muscle cells, which supports part of the claim — but it didn’t test how vitamin D affects protein building or energy production in human muscle cells.

Study: Effects of vitamin D on primary human skeletal muscle cell proliferation, differentiation, protein synthesis and bioenergetics.

The study gave human muscle cells the active form of vitamin D and found that it helped them grow into mature muscle cells better, made them more responsive to insulin for building protein, and improved their energy production — exactly what the claim says.