Vitamin D Is a Hormone, Not Just a Vitamin, and Deficiency Is Widespread

Vitamin D is misleadingly named. It is not a vitamin in the traditional sense; it is a hormone that the kidneys produce to control blood calcium concentration and influence the immune system, bone metabolism, and metabolic function. The Endocrine Society's updated 2024 Clinical Practice Guideline, published in the Journal of Clinical Endocrinology & Metabolism, reflects a significant shift in how the field thinks about this critical hormone.

The guideline now recommends empiric vitamin D supplementation for several high-risk groups: children and adolescents aged 1 to 18, adults over 75, pregnant women, and those with high-risk prediabetes. Notably, the updated guideline moves away from previous target serum levels, no longer endorsing the prior threshold of 30 ng/mL as defining sufficiency or the specific boundaries between sufficiency, insufficiency, and deficiency. This reflects the complexity of vitamin D biology and the recognition that optimal levels may vary by individual, age, and clinical context.

The scope of vitamin D's influence extends well beyond bone health. Research has linked deficiency to cardiovascular disease, type 2 diabetes, autoimmune conditions, depression, and impaired immune response. A study during the COVID-19 pandemic found that over 80% of hospitalized patients had vitamin D deficiency, though causation versus correlation remains debated.

For the general population, the most reliable source of vitamin D remains sunlight exposure, which triggers synthesis in the skin. But modern indoor lifestyles, sunscreen use, and geographic latitude make dietary and supplemental sources essential for most people, particularly in midlife and beyond.

The Tracking Connection

Kairos™ includes vitamin D as a core tracked biomarker because of its role as a hormone precursor that interacts with nearly every other hormonal system. By monitoring 25(OH)D levels alongside calcium, bone density results, immune markers, and mood data over time, Kairos helps users maintain optimal levels and understand how vitamin D status influences their broader hormonal health.