Here's a fun contradiction: we live on a planet with a giant nuclear reactor in the sky that our skin can use to manufacture a critical vitamin, and somehow roughly 42% of American adults are deficient in it. That's according to data from the National Health and Nutrition Examination Survey (NHANES), and the number climbs even higher among people with darker skin tones, older adults, and anyone living above the 37th parallel (hi, everyone north of Richmond, Virginia).

Vitamin D isn't really a vitamin at all. It's a prohormone — a precursor your body converts into a hormone that influences everything from bone metabolism to immune function to mood regulation. And the gap between what most people have and what they probably need is wider than the wellness industry's supplement aisle.

What Vitamin D Actually Does in Your Body

Forget the vague "it's good for you" hand-waving. Vitamin D has specific, well-documented roles:

Calcium absorption and bone health. Without adequate vitamin D, your intestines absorb only 10-15% of dietary calcium. With sufficient levels, that jumps to 30-40%. This is why vitamin D deficiency leads to rickets in children and osteomalacia (soft bones) in adults — your body literally can't use the calcium you're eating.

Immune modulation. Vitamin D receptors exist on virtually every immune cell in your body. A meta-analysis published in the BMJ (2017) analyzing 25 randomized controlled trials with over 11,000 participants found that vitamin D supplementation reduced the risk of acute respiratory infections by 12% overall — and by 70% in those with the lowest baseline levels (below 25 nmol/L). That's not a typo.

Mood and mental health. A systematic review and meta-analysis in the Journal of Affective Disorders (2020) found a statistically significant association between low vitamin D levels and depression. The relationship is clearest in people who are genuinely deficient rather than those with already-adequate levels.

Muscle function. Vitamin D plays a role in muscle protein synthesis. Deficiency is associated with proximal muscle weakness — the kind that makes getting out of a chair feel harder than it should.

The D2 vs. D3 Debate, Settled

Vitamin D comes in two supplemental forms:

  • Vitamin D2 (ergocalciferol): Derived from fungi and plants. This is what most prescriptions use.
  • Vitamin D3 (cholecalciferol): Derived from animal sources (lanolin, fish liver oil) or lichen for vegan options. This is what your skin produces from UVB radiation.

The research favors D3. A meta-analysis in the American Journal of Clinical Nutrition (2012) concluded that D3 is approximately 87% more potent at raising and maintaining serum 25(OH)D concentrations compared to D2. D3 also has a longer shelf life and more stable blood levels.

Unless you have a specific reason to choose D2 (vegan preference without lichen-derived D3 available), D3 is the better pick.

How Much Do You Actually Need?

This is where things get contentious. The RDA set by the Institute of Medicine is:

  • 600 IU/day for adults 19-70
  • 800 IU/day for adults over 70

But many researchers and clinicians argue these numbers are too conservative. The Endocrine Society's Clinical Practice Guideline (2011) suggests that adults may need 1,500-2,000 IU/day to consistently maintain serum 25(OH)D above 30 ng/mL — the threshold most experts consider sufficient.

The Tolerable Upper Intake Level is 4,000 IU/day, though some studies have used doses up to 10,000 IU/day without toxicity. That said, more is not automatically better. Vitamin D is fat-soluble, which means it accumulates in your body. Toxicity — while rare — causes hypercalcemia (elevated blood calcium), which can damage the kidneys, heart, and blood vessels.

The smartest approach: get your 25(OH)D level tested, then dose accordingly.

Testing: The Number You Want to Know

The blood test you need is serum 25-hydroxyvitamin D [25(OH)D]. Not 1,25-dihydroxyvitamin D — that's the active hormone and doesn't reflect your stores.

Here's how to read your results:

25(OH)D Level Status
Below 12 ng/mL Deficient
12-20 ng/mL Insufficient
20-50 ng/mL Sufficient (IOM)
30-50 ng/mL Sufficient (Endocrine Society)
Above 50 ng/mL Potentially excessive
Above 150 ng/mL Toxic

Most functional and integrative practitioners aim for 40-60 ng/mL. Whether you target the IOM or Endocrine Society range depends on your individual health picture, but getting above 30 ng/mL is a reasonable minimum for most adults.

Sunlight: The Original Source (With Caveats)

Your skin produces vitamin D3 when UVB rays hit a cholesterol compound (7-dehydrocholesterol) in the epidermis. In theory, 10-30 minutes of midday sun on large skin areas (arms, legs, back) can produce 10,000-25,000 IU.

In practice, it's complicated:

  • Latitude matters. Above 37 degrees N (roughly the line from San Francisco to Richmond, VA), UVB intensity is too low for meaningful vitamin D synthesis from November through February.
  • Skin pigmentation matters. Melanin acts as a natural sunscreen. People with darker skin need 3-5 times more sun exposure to produce equivalent vitamin D.
  • Sunscreen matters. SPF 30 reduces vitamin D synthesis by about 95%. This doesn't mean you should skip sunscreen — skin cancer is real — but it's part of why supplementation is often necessary.
  • Age matters. A 70-year-old produces about 75% less vitamin D from the same sun exposure as a 20-year-old.

Food Sources: Necessary but Usually Not Sufficient

Very few foods naturally contain significant vitamin D:

  • Wild-caught salmon: 600-1,000 IU per 3.5 oz serving
  • Sardines: 300 IU per 3.5 oz
  • Egg yolks: 40 IU per yolk (from conventionally raised hens; pastured hens may have 3-4x more)
  • UV-exposed mushrooms: Variable, up to 400 IU per serving
  • Fortified milk: 100 IU per cup
  • Cod liver oil: 1,360 IU per tablespoon

Unless you're eating wild salmon and sardines daily, food alone probably won't get you to best levels. Fortification helps but doesn't close the gap for most people.

The Vitamin D + K2 Connection

Vitamin D increases calcium absorption. Vitamin K2 directs that calcium into your bones and teeth — and away from your arteries and soft tissues. Taking high-dose vitamin D without adequate K2 may theoretically increase the risk of arterial calcification, though definitive human data is still accumulating.

A reasonable insurance policy: pair your D3 supplement with 100-200 mcg of vitamin K2 (MK-7 form), especially if you're taking more than 2,000 IU/day. More on this in our vitamin K2 deep-dive.

Who's at Highest Risk for Deficiency?

  • People with limited sun exposure (office workers, those who cover most skin)
  • Individuals with darker skin pigmentation
  • Older adults (reduced synthesis and absorption)
  • People with obesity (vitamin D gets sequestered in fat tissue)
  • Those with malabsorption conditions (celiac, Crohn's, gastric bypass)
  • Breastfed infants (breast milk is low in vitamin D)
  • People on certain medications (anticonvulsants, glucocorticoids, antifungals)

When to Talk to a Pro

Consult a healthcare provider if:

  • Your serum 25(OH)D is below 20 ng/mL — you may need a loading dose
  • You have kidney or liver disease (these organs convert vitamin D to its active form)
  • You take medications that affect vitamin D metabolism
  • You have a history of kidney stones (vitamin D increases calcium absorption, which can be a factor)
  • You're considering doses above 4,000 IU/day
  • You're pregnant or planning pregnancy

FAQ

Can you get enough vitamin D from food alone? For most people, no. The richest dietary sources (fatty fish, cod liver oil) provide meaningful amounts, but hitting 2,000+ IU daily from food alone requires deliberate effort. Supplementation is usually the most reliable strategy, especially in winter months.

Is it possible to get too much vitamin D from the sun? No. Your body self-regulates vitamin D production from UVB exposure. Once you've made enough, the precursor compounds break down into inactive metabolites. Toxicity only occurs from supplementation, not sun exposure.

How long does it take to correct a deficiency? With adequate supplementation (typically 2,000-5,000 IU/day for moderate deficiency), most people see significant improvement in serum levels within 8-12 weeks. Severe deficiency may require a prescription loading protocol of 50,000 IU weekly for 6-8 weeks.

Should I take vitamin D with fat? Yes. Vitamin D is fat-soluble, meaning it absorbs significantly better when taken with a meal containing dietary fat. A study in the Journal of the Academy of Nutrition and Dietetics (2015) found that taking vitamin D with the largest meal of the day increased serum levels by about 50% compared to taking it on an empty stomach.

A note from Living & Health: We're a lifestyle and wellness magazine, not a doctor's office. The information here is for general education and entertainment — not medical advice. Always talk to a qualified healthcare professional before making changes to your health routine, especially if you have existing conditions or take medications.