About Vitamin D
Vitamin D functions more like a hormone than a vitamin — your skin synthesizes it from UVB sunlight, and it regulates immune function, mood, bone density, calcium absorption, insulin sensitivity, and inflammation throughout the body. Deficiency is extremely common: an estimated 1 billion people globally have insufficient levels, largely because modern indoor lifestyles limit sun exposure. A blood level below 20 ng/mL is clinically deficient; most functional medicine guidelines suggest 40–60 ng/mL for optimal health.