Key takeaways
Three things to remember
- 01
The clinical cutoff of 20 ng/mL prevents bone disease but does not reflect immune function or performance optimization; many practitioners aim for 40-60 ng/mL, but the evidence for that higher range is limited.
- 02
Diet provides very little vitamin D: supplementation (commonly 1,000-4,000 IU D3 daily) is the most reliable way to raise low levels, especially for adults living above 35 degrees latitude in winter.
- 03
If you are at risk of low levels, ask for a serum 25(OH)D test; it is not usually included in standard panels unless requested, and low levels often cause no obvious symptoms.
How it works
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, while the Endocrine Society's 2011 guideline suggests staying above 30 ng/mL for fuller effects on calcium, bone, and muscle. Many longevity-focused clinicians aim for 40-60 ng/mL, though the evidence that this higher range improves outcomes is limited.
In practice
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, low vitamin D has been linked to weaker muscle function, though trials of supplementing people who are not deficient have mostly shown little benefit for training adaptation.
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 amounts many people take in supplements to keep their levels up. 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.
Reference ranges
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. The Endocrine Society's 2011 guideline suggests above 30 ng/mL, and many functional medicine practitioners aim for 40-60 ng/mL, though evidence for the higher range is limited. 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
What moves it
How to Improve It
Supplement daily. 1,000-4,000 IU of vitamin D3 daily (with K2 for calcium routing) is the most reliable way to maintain 40-60 ng/mL for most adults in northern latitudes, per Holick et al., 2011 Endocrine Society guidelines.
Get tested first. A serum 25(OH)D test (standard blood panel add-on) establishes your baseline before choosing a supplementation dose; starting blind at 2,000 IU is reasonable, but testing at 90 days confirms whether the dose is working.
Midday sun exposure. 15-30 minutes of midday sun (arms and legs exposed, no sunscreen) during summer at latitudes below 35 degrees can generate 10,000-20,000 IU; this is the most efficient natural source but unreliable for most of the year.
Eat fatty fish. Fatty fish (salmon, mackerel, sardines) provide 300-600 IU per serving, making them the best dietary source, though supplementation is still needed for most people to reach functional levels.
Pair with K2 and magnesium. Vitamin K2 (MK-7, 100-200 mcg) directs calcium from blood into bone and prevents soft tissue calcification; magnesium is required for vitamin D metabolism and is commonly depleted alongside deficiency.
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