Vitamin D
The hormone your body makes from sunlight
Plain English
Vitamin D is technically a hormone, not a vitamin: your skin synthesizes it from sunlight, and it circulates through the body regulating immune function, muscle strength, bone density, and mood. Most adults in northern latitudes or office jobs are chronically deficient without realizing it, because deficiency develops silently over months and standard blood panels often miss it.
The Mechanism
When ultraviolet B (UVB) light hits the skin, a cholesterol precursor is converted into an inactive form of vitamin D. The liver then converts it into 25-hydroxyvitamin D (25(OH)D), which is what blood tests measure. The kidneys and various tissues then activate it into its hormonal form, 1,25-dihydroxyvitamin D (calcitriol), which binds to vitamin D receptors found in nearly every cell in the body.
Calcitriol regulates the expression of hundreds of genes. The most well-characterized roles include calcium absorption in the gut (essential for bone density), modulation of the immune system, and production of antimicrobial proteins that defend against respiratory infections. The immune connection is why vitamin D deficiency is strongly associated with increased infection susceptibility and autoimmune conditions.
Deficiency is defined as serum 25(OH)D below 20 ng/mL (50 nmol/L) in most clinical guidelines, but functional optimization for immune function and muscle performance appears to occur at levels above 40 ng/mL. The Endocrine Society and many longevity-focused researchers consider 40-60 ng/mL the functional target range.
Why It Matters
Reference range is not optimal range: most lab normal cutoffs are set to prevent bone disease, not optimize performance.
Vitamin D deficiency is among the most common modifiable nutrient deficiencies in adults, affecting an estimated 40% of US adults and a majority of people in northern climates during winter. Low levels are associated with fatigue, mood changes, more frequent illness, slower muscle recovery, and reduced bone mineral density. For athletes and people who train consistently, suboptimal vitamin D blunts the muscle protein synthesis response and slows adaptation to training stimulus.
Common Misconception
Most people assume they get enough vitamin D from brief outdoor exposure or from diet. In practice, diet provides very little: few foods contain meaningful amounts, and fortified milk provides only 100-400 IU per serving, well below the 1,000-5,000 IU daily many people need to maintain functional levels. Even sun exposure is unreliable in winter above 35 degrees latitude, where the UVB angle is too low for vitamin D synthesis for months at a time.
What a Healthy Range Looks Like
Deficient
<20 ng/mL
Clinical deficiency; bone disease risk, immune suppression, fatigue
Insufficient
20-29 ng/mL
Below functional threshold; common in office workers in northern latitudes
Adequate
30-39 ng/mL
Meets clinical guidelines but below performance-optimization range
Optimal
40-60 ng/mL
Target for immune function, muscle performance, and longevity; endorsed by Endocrine Society
The clinical cutoff of 20 ng/mL was set to prevent rickets, not optimize health. Most functional medicine researchers and the Endocrine Society target 40-60 ng/mL for broad health benefits. Levels above 100 ng/mL from supplementation can cause toxicity; this is only achievable through prolonged excessive supplementation, not sun exposure.
Signs It Is Disrupted
- Fatigue that persists despite adequate sleep and recovery
- Frequent upper respiratory infections or slow recovery from illness
- Muscle weakness and slower recovery after training
- Low mood or seasonal mood changes, particularly in winter months
- Bone pain or aching joints without a clear structural cause
- Elevated parathyroid hormone (PTH) on labs, often the earliest signal of chronic insufficiency
How to Improve It
3 Things to Remember
The clinical cutoff of 20 ng/mL prevents bone disease but does not reflect immune function or performance optimization; target 40-60 ng/mL.
Diet provides very little vitamin D: supplementation (1,000-4,000 IU D3 daily) is the most reliable strategy for most adults living above 35 degrees latitude.
Test serum 25(OH)D at least once per year; deficiency is silent until it is severe, and standard panels often miss it unless specifically requested.
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