Vitamin D is involved in immune function, bone health, mood regulation, muscle strength, and possibly even cancer prevention. It’s also one of the most common nutrient deficiencies worldwide — an estimated 1 billion people have insufficient levels, including roughly 42% of American adults.
The tricky part: figuring out how much you actually need. Government recommendations, medical organizations, and supplement companies all give different numbers. Here’s what the research actually says.
What Vitamin D Does#
Vitamin D functions more like a hormone than a typical vitamin. When your skin is exposed to UVB radiation from sunlight, it synthesizes vitamin D3 (cholecalciferol), which then travels to the liver and kidneys for conversion into its active form, calcitriol.
Calcitriol’s most established role is regulating calcium and phosphorus absorption, which is critical for bone health. But vitamin D receptors exist in virtually every cell in your body, which is why research has linked it to:
- Immune function — Vitamin D activates T cells and modulates both innate and adaptive immunity. Deficiency is associated with increased susceptibility to infections.
- Bone health — Without adequate D, you absorb only 10-15% of dietary calcium. With adequate D, absorption increases to 30-40%.
- Mood and mental health — Observational studies consistently link low vitamin D to higher rates of depression, though causation is still debated.
- Muscle function — Deficiency is associated with muscle weakness and increased fall risk, especially in older adults.
- Cardiovascular health — Some evidence links low vitamin D to increased cardiovascular risk, though supplementation trials have shown mixed results.
How Much Do You Need?#
This is where it gets contentious. Different organizations recommend different amounts:
| Organization | Daily Recommendation |
|---|---|
| IOM (Institute of Medicine) | 600 IU (ages 1-70), 800 IU (71+) |
| Endocrine Society | 1,500-2,000 IU (adults) |
| Vitamin D Council | 5,000 IU (adults) |
| Many functional medicine practitioners | 2,000-5,000 IU |
The IOM’s recommendation (600 IU) is designed to maintain a blood level of 20 ng/mL, which they consider sufficient for bone health. The Endocrine Society targets 30 ng/mL, which requires more. Many researchers and clinicians now aim for 40-60 ng/mL for optimal health, which typically requires 2,000-5,000 IU daily.
Our Take#
For most adults with suboptimal levels, 2,000-5,000 IU daily is a reasonable, evidence-supported range. This dose is well below the tolerable upper intake level of 4,000 IU (IOM) or 10,000 IU (Endocrine Society) and has a strong safety profile in clinical trials.
That said, the best approach is to test your blood levels and dose accordingly. A one-size-fits-all recommendation ignores the massive individual variation in vitamin D metabolism.
Who’s at Risk for Deficiency?#
Several factors dramatically affect your vitamin D status:
Skin Color#
Melanin acts as a natural sunscreen. People with darker skin need 3-5 times more sun exposure to produce the same amount of vitamin D as lighter-skinned individuals. Studies show deficiency rates of 82% in Black Americans and 69% in Hispanic Americans, compared to about 40% in white Americans.
Geographic Location#
If you live above the 37th parallel (roughly north of Atlanta, Georgia; Albuquerque, New Mexico; or San Francisco, California), the sun angle is too low to produce adequate UVB radiation from approximately November through March. During these months, supplementation is essentially necessary.
Age#
The skin’s ability to synthesize vitamin D declines with age. A 70-year-old produces about 75% less vitamin D from the same sun exposure as a 20-year-old.
Body Weight#
Vitamin D is fat-soluble, meaning it gets sequestered in fat tissue. People with higher body fat levels often need significantly more vitamin D to maintain adequate blood levels. Some researchers recommend obese individuals take 2-3 times the standard dose.
Lifestyle#
If you work indoors, wear sunscreen consistently, or cover most of your skin for cultural or religious reasons, you’re producing minimal vitamin D from sunlight. Window glass blocks UVB rays, so sitting by a sunny window doesn’t count.
Food Sources of Vitamin D#
Very few foods naturally contain vitamin D in meaningful amounts:
| Food | Vitamin D per Serving |
|---|---|
| Wild-caught salmon (3 oz) | 570-600 IU |
| Farmed salmon (3 oz) | 100-250 IU |
| Canned sardines (3 oz) | 175 IU |
| Canned tuna (3 oz) | 150 IU |
| Egg yolk (1 large) | 40 IU |
| Fortified milk (1 cup) | 100-120 IU |
| Fortified orange juice (1 cup) | 100 IU |
| UV-exposed mushrooms (3 oz) | 400 IU |
Even with a diet rich in fatty fish and fortified foods, it’s nearly impossible to reach 2,000-5,000 IU from food alone. This is one of the few cases where supplementation is clearly justified for most people.
Testing Your Levels#
A blood test for 25-hydroxyvitamin D (also written as 25(OH)D) is the standard measure. Most labs report results in ng/mL:
- Below 12 ng/mL — Severely deficient. Increased risk of rickets (children), osteomalacia (adults), and significant health effects.
- 12-20 ng/mL — Deficient. Needs correction.
- 20-30 ng/mL — Insufficient. Below optimal for most people.
- 30-50 ng/mL — Sufficient. This is the target range for most.
- 50-80 ng/mL — Upper normal range. Generally considered safe.
- Above 100 ng/mL — Risk of toxicity. Reduce supplementation.
You can get tested through your doctor (often covered by insurance if you have symptoms or risk factors) or through direct-to-consumer lab services.
When to Test#
- Before starting supplementation (to establish a baseline)
- 3 months after starting or changing your dose (to confirm adequacy)
- Annually thereafter (to maintain appropriate levels)
Supplementation Guidelines#
Once you know your levels, here’s a general dosing framework:
- Severely deficient (<20 ng/mL): Consult your doctor. They may prescribe a loading dose of 50,000 IU weekly for 8-12 weeks, followed by a maintenance dose.
- Insufficient (20-30 ng/mL): 2,000-5,000 IU daily for 3 months, then retest.
- Sufficient (30-50 ng/mL): 1,000-2,000 IU daily for maintenance, especially during fall/winter.
- Optimal (>50 ng/mL): You may not need supplementation, but monitor levels.
D3 vs. D2#
Always choose vitamin D3 (cholecalciferol) over D2 (ergocalciferol). D3 is the form your body produces naturally and is significantly more effective at raising and maintaining blood levels. D2 is the plant-derived form — acceptable for strict vegans, but D3 is preferred.
Don’t Forget K2#
If you’re taking more than 2,000 IU of D3 daily, consider pairing it with vitamin K2 (MK-7 form). Vitamin D increases calcium absorption, and K2 helps direct that calcium into bones rather than arteries. The combination is considered optimal by most experts.
For specific product recommendations, see our best vitamin D supplements guide.
Key Takeaways#
- Vitamin D deficiency is extremely common, affecting an estimated 42% of American adults.
- Most people need 2,000-5,000 IU daily, but the best approach is to test your blood levels and dose accordingly.
- You probably can’t get enough from food alone — supplementation makes sense for most people, especially in winter.
- Choose D3 over D2, and consider adding K2 if you take high doses.
- Risk factors for deficiency include darker skin, living above the 37th parallel, older age, higher body fat, and indoor lifestyles.
- Test your 25-hydroxyvitamin D levels at least once, ideally targeting 30-50 ng/mL.
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If you’re looking for specific product recommendations, check out our full best vitamin D supplements roundup, where we rank the top options by form, testing, and value.
Frequently Asked Questions#
How do I know if I’m deficient in vitamin D?
The only reliable way is a blood test measuring 25-hydroxyvitamin D. Common symptoms include fatigue, bone pain, muscle weakness, and frequent illness, but these overlap with many conditions.
Can I get enough from food alone?
It’s very difficult. Even a diet rich in fatty fish and fortified foods rarely reaches 2,000 IU daily. Supplementation is typically necessary.
How long does it take to correct a deficiency?
With 2,000-5,000 IU daily, most people see significant improvement in 2-3 months. Severe deficiency may require higher doses supervised by a doctor.
Is it possible to get too much vitamin D from the sun?
No. Your body self-regulates production from sunlight. Toxicity only occurs from excessive supplementation.
Should I take vitamin D in the summer?
It depends on your sun exposure, skin color, and latitude. Testing your levels is the best way to know whether you need year-round supplementation.