Vitamin D: The Blood Level That Actually Matters

Vitamin D is one of the most commonly deficient nutrients in American adults. Studies consistently find 30-50% of the US population has vitamin D levels below 20 ng/mL (clearly deficient), and much higher fractions below 30 ng/mL (insufficient). The deficiency is particularly pronounced in northern latitudes, darker-skinned individuals, indoor workers, and people over 50.

Correcting deficiency is cheap and straightforward. Supplementation costs under $10 per month, has decades of safety data, and produces measurable improvements in bone health, immune function, and probably mood. Yet most deficient men either don't know they're deficient (never tested) or have been tested but not treated to adequacy.

What Vitamin D Actually Does

Vitamin D is technically a hormone rather than a traditional vitamin. It's produced in skin when UVB radiation hits cholesterol precursors, or consumed in a few foods (fatty fish, egg yolks, fortified dairy). The inactive form circulates as 25-hydroxyvitamin D (the form measured in blood), then converts to active 1,25-dihydroxyvitamin D in the kidneys and target tissues.

Active vitamin D regulates:

• Calcium and phosphorus absorption from the gut (primary established role)
• Bone mineralization and remodeling
• Immune cell function (both innate and adaptive immunity)
• Cell growth and differentiation
• Neuromuscular function
• Cardiovascular function (emerging understanding)
• Inflammation regulation

Vitamin D receptors are expressed in virtually every tissue — bone, gut, kidney, muscle, immune cells, brain, heart, and many more. This ubiquity is part of why deficiency affects so many systems.

The Target Blood Level

There's substantial debate about optimal vitamin D levels. Current consensus:

• Under 20 ng/mL: Deficiency. Universally considered inadequate. Risk of osteomalacia, bone fragility, immune dysfunction.
• 20-30 ng/mL: Insufficient. Most professional societies consider this below optimal.
• 30-50 ng/mL: Adequate to optimal. This is the generally accepted target range.
• 50-80 ng/mL: Higher end of optimal. Some evidence supports this range for additional benefits.
• Over 100 ng/mL: Potentially problematic. Risk of hypercalcemia and associated complications.

Most functional medicine practitioners target 40-60 ng/mL. Traditional medicine is often satisfied with 30+ ng/mL. The meaningful practical question is: are you above 30? If yes, probably fine. If no, supplement.

How to Test

The test is 25-hydroxyvitamin D (25(OH)D). Not to be confused with the 1,25-dihydroxy form, which is less informative for assessing status.

Standard test, widely available, covered by most insurance, especially with any clinical indication (fatigue, osteoporosis risk, autoimmune condition, etc.). Direct-to-consumer cost $30-60.

Season matters. Test late winter/early spring for your lowest point (sun exposure has been minimal for months). Test summer/early fall for your highest. Interpretation should consider seasonal variability.

Retest 8-12 weeks after starting supplementation to verify you've reached target. Steady-state vitamin D takes about 8 weeks to establish at a given intake.

Evidence-Based Benefits

Bone health. Well-established. Adequate vitamin D is essential for calcium absorption and bone mineralization. Deficiency causes osteomalacia (adult) or rickets (children). Correction prevents these.

Falls in the elderly. Vitamin D supplementation in deficient elderly reduces fall rate, likely via muscle function effects.

Immune function. Reasonable evidence for reduced respiratory infection frequency in deficient individuals. Less dramatic effect in sufficient individuals.

Mood. Some evidence for seasonal affective disorder and general depression, particularly in deficient individuals. Correction often produces improvement.

Musculoskeletal symptoms. Deficient individuals often report generalized aches, muscle weakness, and fatigue that resolve with correction.

Less Clearly Supported

Cardiovascular events. Observational associations strong; randomized trial evidence more equivocal. The VITAL trial showed no major cardiovascular benefit in already-sufficient adults supplemented with 2000 IU daily.

Cancer prevention. Observational associations with lower cancer rates, but randomized trials have been mostly negative for preventing cancer incidence, though some signals for reducing cancer mortality.

Autoimmune disease prevention. VITAL trial showed 22% reduction in autoimmune disease incidence over 5 years with 2000 IU daily. Promising.

Cognitive decline. Observational associations present; intervention data mixed.

Fertility, metabolic disease, etc. Active research areas, evidence still developing.

Who's at Risk of Deficiency

• Northern latitudes (above ~40° — Boston, New York, Seattle)
• Indoor workers or limited sun exposure
• Darker skin (more melanin filters UVB)
• Age over 50 (reduced skin synthesis efficiency)
• Obesity (vitamin D sequestered in adipose tissue)
• Malabsorption conditions (celiac, IBD, gastric bypass)
• Kidney disease (impaired conversion to active form)
• Liver disease (impaired first hydroxylation)
• Certain medications (anticonvulsants, corticosteroids)
• Exclusive breastfeeding (for infants)

For middle-aged American men, assume you're possibly deficient until proven otherwise. Testing is cheap; correction is easy.

Dosing

Supplementation usually uses cholecalciferol (D3), more effective at raising blood levels than ergocalciferol (D2).

General maintenance (sufficient individuals): 1000-2000 IU daily

Correcting insufficiency (blood level 20-30 ng/mL): 2000-4000 IU daily for 8-12 weeks, then retest

Correcting deficiency (blood level <20 ng/mL): 5000-10000 IU daily for 8-12 weeks OR high-dose loading (50,000 IU weekly for 8 weeks), then maintenance dose

Upper tolerable intake limit (Institute of Medicine): 4000 IU daily in adults, though evidence suggests higher doses are safe

Toxicity threshold: Typically requires sustained intake above 10,000-50,000 IU daily for months, producing blood levels above 150 ng/mL with hypercalcemia. Rare.

The K2 Question

Some popular protocols recommend vitamin K2 alongside D3, arguing that D3 alone can mobilize calcium without directing it to bone, potentially contributing to vascular calcification.

The evidence for routine K2 co-supplementation is weaker than supplement marketing suggests. Most benefit is seen in K-deficient individuals or those on certain medications (warfarin — in which case K supplementation is contraindicated). For most supplementing D3, K2 is probably not necessary but also not harmful.

If you want to add K2: 100-200 mcg MK-7 form daily is typical. Cost is minor; upside is uncertain; downside is minor.

Magnesium and Vitamin D

Magnesium is required for vitamin D activation. Severely magnesium-deficient individuals may not respond to vitamin D supplementation as expected. Since magnesium deficiency is also common, addressing both together makes sense in many cases. We'll cover magnesium in the next article.

Sunlight Alternative

You can raise vitamin D from sun exposure. Rough rule: 10-20 minutes of midday sun exposure on face, arms, and legs (unclothed portions) in summer at moderate latitudes produces substantial vitamin D. Duration needs to increase at higher latitudes and with darker skin.

Practical issues:

• Sun exposure has skin cancer risk trade-offs
• UVB needed for D synthesis is only significant when the sun is high (within a few hours of solar noon)
• Winter at higher latitudes (above ~35°) has essentially no vitamin D-producing UVB
• Sunscreen (correctly applied SPF 30+) blocks most D synthesis
• Glass blocks UVB — time indoors or in cars doesn't contribute

For most men, some sun exposure contributes to vitamin D status, but dependable year-round sufficiency usually requires supplementation, dietary intake, or both.

Food Sources

Limited in typical American diets:

• Wild salmon: 400-600 IU per 3 oz serving
• Farmed salmon: 100-200 IU per 3 oz (lower)
• Sardines: 100-200 IU per 3 oz
• Cod liver oil: 400-1000 IU per teaspoon (potent source)
• Egg yolks: 40 IU per yolk
• Fortified milk: 100-130 IU per cup
• Fortified cereals: 40-100 IU per serving

Eating salmon twice a week and drinking fortified milk daily might provide ~200-500 IU/day — sufficient for some, inadequate for others. Supplementation closes the gap.

Practical Protocol

Test once to know where you stand. Based on result:

1. Under 30 ng/mL: Supplement 2000-5000 IU D3 daily with food (D3 is fat-soluble). Retest in 8-12 weeks.
2. 30-50 ng/mL: Supplement 1000-2000 IU D3 daily for maintenance.
3. Over 50 ng/mL: If regularly getting sun or eating fatty fish, minimal supplementation needed. Retest in 6-12 months.

Cost: $10 for a 6-month supply of quality D3. Possibly the cheapest high-impact supplement available.

Who Shouldn't Supplement Without Testing

Most healthy adults can supplement modest doses (1000-2000 IU/day) without testing, as toxicity risk is low at these doses. However, specific situations where testing first makes sense:

• Kidney stone history (hypercalcemia risk)
• Hyperparathyroidism or calcium metabolism disorders
• Sarcoidosis or granulomatous diseases (altered D metabolism)
• Already supplementing with unclear dose
• Very high clinical suspicion of deficiency (plan for aggressive correction)

The Summary

Vitamin D deficiency is common and cheap to correct. Test once. If low, supplement to reach 30-50 ng/mL. Maintain with 1000-2000 IU daily afterward. The intervention is a handful of dollars a month and addresses a widespread deficiency with real health consequences.

This is the supplement that's worth taking even for men skeptical of supplementation generally, because deficiency is a specific problem with known consequences, and correction is straightforward.