The downregulation of these modulators that are (a) fast fiber type specific, (b) dependent on vitamin D signaling, and (c) that impinge on the Stat3-Myostatin axis could render fast muscles more susceptible to atrophy than slow muscles, a phenomenon that is observed in vitamin D deficiencies in humans [20]. The gene discussed is MSTN; the disease is hyperinsulinemic hypoglycemia, familial, 4.