How Much Vitamin D Do You Actually Need? (It's Complicated)

Suppi Research Team · February 28, 2026 · 10 min read

The short answer is: probably more than the official recommendation, but less than the mega-doses some corners of the internet suggest.

The RDA for vitamin D is 600 IU per day for adults up to age 70 and 800 IU for adults over 70. These numbers were set by the Institute of Medicine (now the National Academies) in 2011. The Endocrine Society, on the other hand, has suggested that many adults need 1,500-2,000 IU daily to maintain blood levels above 30 ng/mL. Many physicians routinely recommend 1,000-4,000 IU.

Why the disagreement? It comes down to what you think vitamin D should do and what blood level you're targeting.

What Vitamin D Levels Actually Mean

When your doctor tests vitamin D, they're measuring 25-hydroxyvitamin D, written as 25(OH)D. This is the storage form. Here's how the numbers break down:

The IOM set "sufficient" at 20 ng/mL, primarily for bone health. The Endocrine Society argues that 30 ng/mL is a better target for broader health outcomes. This single threshold disagreement is why you'll see different dose recommendations depending on the source.

An estimated 42% of US adults have 25(OH)D levels below 20 ng/mL. That number jumps to 82% in Black Americans and 69% in Hispanic Americans, largely due to differences in melanin (which reduces vitamin D synthesis from sunlight).

D3 vs D2: This Actually Matters

Vitamin D comes in two supplemental forms:

• D3 (cholecalciferol) — The form your skin makes from sunlight. Found in animal sources (fatty fish, egg yolks, lanolin from sheep's wool).
• D2 (ergocalciferol) — Derived from fungi/yeast. The form used in many prescription vitamin D supplements and fortified foods.

They're not interchangeable. A 2012 meta-analysis in the American Journal of Clinical Nutrition found that D3 is significantly more effective at raising and sustaining 25(OH)D levels than D2. At equivalent doses, D3 was roughly 87% more potent in raising serum levels. D3 also has a longer half-life in the body.

Unless you're vegan (some D3 comes from lichen-based sources now), always choose D3. If your doctor prescribes high-dose D2 (like 50,000 IU weekly), ask about D3 alternatives. Many pharmacies now stock D3 in high-dose capsules.

How Much You Need: The Real-World Answer

Forget the official recommendations for a moment. In practice, here's what it takes to reach a blood level of 30+ ng/mL for most adults:

These aren't pulled from nowhere. The Endocrine Society's 2011 clinical practice guideline recommends 1,500-2,000 IU/day for adults at risk of deficiency, and suggests that obese individuals may need 2-3 times more to achieve the same blood levels. A 2014 meta-regression analysis in Nutrients found that each additional 1,000 IU of D3 raises 25(OH)D by approximately 10 ng/mL in adults, though this varies by baseline levels and body weight.

Risk Factors for Deficiency

Some groups are far more likely to be deficient and benefit from testing:

• Darker skin — Melanin reduces UVB-mediated vitamin D synthesis by up to 90%. A person with very dark skin needs 5-10x more sun exposure to produce the same amount of vitamin D as someone with very light skin.
• Northern latitudes — If you live above the 37th parallel (roughly north of Los Angeles / Atlanta in the US), your skin produces almost no vitamin D from sun exposure during November through February. In Seattle or London, that window stretches even longer.
• Office workers / indoor lifestyles — UVB doesn't penetrate glass. If you're inside from 9-5, your sun exposure may be near zero on workdays.
• Older adults — The skin's ability to produce D3 decreases with age. A 70-year-old's skin produces roughly 25% of the vitamin D a 20-year-old's does.
• Obesity — Vitamin D gets sequestered in fat tissue, reducing its bioavailability. Studies consistently show that obese individuals have lower 25(OH)D levels at the same intake.
• Breastfed infants — Breast milk is low in vitamin D. The AAP recommends 400 IU/day for all breastfed infants.
• GI conditions — Crohn's, celiac, and other conditions affecting fat absorption reduce vitamin D uptake.

If you tick two or more of these boxes, a blood test is worth getting. It's a cheap test—typically $20-50 with insurance—and it removes all guesswork.

Take It With Fat

Vitamin D is fat-soluble. This has a direct practical implication: taking it with a meal that contains fat significantly increases absorption.

A 2015 study in the Journal of the Academy of Nutrition and Dietetics found that taking vitamin D with a fat-containing meal increased absorption by approximately 50% compared to taking it on an empty stomach. The fat doesn't need to be a lot—even 11g (roughly a tablespoon of olive oil or a handful of nuts) made a significant difference.

Take your vitamin D with breakfast or lunch if those meals contain some fat. Taking it with dinner works too, though some people report it interferes with sleep (this is anecdotal, not well-studied).

Vitamin D and K2: Do You Need Both?

You've probably seen "D3 + K2" supplements and wondered if the K2 is necessary.

The theory: vitamin D increases calcium absorption. Vitamin K2 directs that calcium into bones rather than letting it deposit in arteries. Without K2, the extra calcium from D supplementation could theoretically contribute to arterial calcification.

The evidence is mixed. A 2017 review in the International Journal of Endocrinology found that the combination may be more effective for bone health than either alone, but the data on cardiovascular protection is still inconclusive. The Rotterdam Study did find that higher K2 intake was associated with reduced coronary calcification, but this was observational, not a controlled trial of supplements.

Our take: if you're taking moderate doses of D3 (1,000-4,000 IU), K2 supplementation isn't strictly necessary if you eat fermented foods, cheese, or eggs regularly (natural K2 sources). At higher doses or if your diet is low in K2, adding 100-200mcg of MK-7 (the most bioavailable form of K2) is a reasonable precaution. It's very safe and inexpensive.

Toxicity: Where's the Danger Zone?

Vitamin D toxicity (hypervitaminosis D) is real but rare. It almost always involves either accidental mega-dosing or prolonged intake well above 10,000 IU/day.

The mechanism: too much D leads to hypercalcemia (excess calcium in the blood), which can cause nausea, vomiting, muscle weakness, kidney stones, and in severe cases, kidney failure and cardiac arrhythmias.

The IOM set the tolerable upper intake level (UL) at 4,000 IU/day for adults. This is a conservative number with a built-in safety margin. Studies have shown that up to 10,000 IU/day for 5 months doesn't cause toxicity in most healthy adults, but there's no good reason to take that much unless your doctor prescribes it for severe deficiency.

Blood levels associated with toxicity typically start above 100 ng/mL (some cases reported above 150 ng/mL). If you're taking 1,000-4,000 IU daily, you're extremely unlikely to reach these levels.

One notable case series involved patients who took manufacturing-error supplements containing 50,000+ IU per pill instead of the labeled 1,000 IU. Quality control matters. Buy from reputable brands that do third-party testing.

Practical Takeaways

• Get a 25(OH)D blood test if you haven't in the past 1-2 years, especially if you have risk factors.
• For most adults, 1,000-2,000 IU of D3 daily is reasonable for maintenance.
• If deficient, 4,000-5,000 IU daily for 8-12 weeks under medical guidance, then retest.
• Always take with a fat-containing meal.
• Choose D3, not D2.
• Don't exceed 4,000 IU/day without medical supervision.
• Consider K2 (100-200mcg MK-7) if your diet is low in fermented foods and you're taking higher D3 doses.