If you work indoors, sit at a desk all day, and train under fluorescent gym lights, there is a high probability that your body is starved of one of its most critical performance nutrients.

We casually call it a "vitamin," but medically speaking, Vitamin D is a steroid hormone. Every single cell in your body has a receptor for it. When your levels are optimized, it acts as a master regulator for your entire physiological system. When your levels drop, the fallout extends far beyond just a weak immune system—it directly impacts your psychology, your hormones, and your physical results in the gym.