Vitamin D for Adults: How Much You Need and How to Get It

Why Vitamin D Matters for Adult Health

Vitamin D is a fat-soluble vitamin that helps your body absorb calcium and phosphorus—two minerals that are critical for building and maintaining healthy bones. Without enough vitamin D, your body cannot efficiently use the calcium you consume, which over time can weaken bones and increase the risk of fractures, especially in older adults.

Beyond bone health, researchers are studying vitamin D for its potential influence on immune function, muscle strength, and other systems, but the evidence for benefits beyond the skeleton is not yet conclusive. Large clinical trials have not consistently shown that vitamin D supplementation improves outcomes such as falls or fractures in healthy adults who already have sufficient levels.

• Supports calcium and phosphorus absorption.
• Helps maintain healthy bones and teeth.
• The VITAL trial, a large study of over 25,000 adults, found no reduction in fractures or falls with daily vitamin D supplementation in healthy adults with adequate levels at baseline.

Sources: Harvard T.H. Chan School of Public Health, Office of Dietary Supplements, National Institutes of Health (NIH), Stanford Lifestyle Medicine

Recommended Intake and Optimal Blood Levels

The National Academy of Medicine has set the Recommended Dietary Allowance (RDA) for vitamin D at 600 IU (15 mcg) per day for adults ages 19–70, and 800 IU (20 mcg) per day for adults over 70. These intakes are designed to maintain blood levels of 25-hydroxyvitamin D (25(OH)D) at or above 50 nmol/L (20 ng/mL), a level that is sufficient for bone health in most people.

However, there is ongoing debate about what constitutes an “optimal” level. While the National Academies considers 50 nmol/L sufficient, some experts recommend higher levels for certain health outcomes. The 2024 Endocrine Society guidelines do not recommend routine testing in healthy adults, but they do suggest that adults with prediabetes may benefit from supplementation. Rather than aiming for a specific number, it is wise to discuss your individual risk factors and health goals with a healthcare provider.

• Adults 19–70 years: 600 IU/day
• Adults >70 years: 800 IU/day
• Serum 25(OH)D ≥50 nmol/L (20 ng/mL) is considered sufficient for bone health.
• Levels below 30 nmol/L (12 ng/mL) are considered deficient; 30–50 nmol/L (12–20 ng/mL) is inadequate.

Sources: Harvard T.H. Chan School of Public Health, Office of Dietary Supplements, National Institutes of Health (NIH), Harvard Health Publishing

Sources: Sunlight, Food, and Supplements

Your skin can produce vitamin D when exposed to ultraviolet B (UVB) rays from the sun. This is a natural and primary source, but many people do not achieve adequate levels from sun exposure alone, especially during winter months, when living at higher latitudes, or if you use sunscreen regularly. The amount you synthesize also depends on your skin tone, age, and time of day.

Food sources of vitamin D are limited. Fatty fish like salmon, trout, tuna, and mackerel, as well as fish liver oils and egg yolks, contain natural vitamin D. In the United States, many foods—including cow’s milk, breakfast cereals, and some plant-based milks—are fortified with vitamin D, which helps many adults meet their needs. Still, it can be hard to get the full recommended intake from diet alone, so supplements are often the most reliable way to reach your target.

• Sunlight (UVB) is a natural source, but your location and lifestyle affect how much you produce.
• Natural food sources: fatty fish, fish liver oils, egg yolks.
• Fortified foods: milk, cereals, some plant-based milks.
• Supplements help fill the gap when sun and food are insufficient.

Sources: Office of Dietary Supplements, National Institutes of Health (NIH), Harvard T.H. Chan School of Public Health, Stanford Lifestyle Medicine

Vitamin D2 vs. D3: What's the Difference?

Vitamin D comes in two forms: vitamin D2 (ergocalciferol), which is derived from yeast and some plants, and vitamin D3 (cholecalciferol), which is produced in human skin and found in animal-based foods. Both can raise blood levels of 25(OH)D, but most research suggests that vitamin D3 is more effective at raising and maintaining those levels over time.

Because of this, many clinicians and health organizations recommend vitamin D3 supplements for most people, although you can still meet your needs with D2 if that is what you have available. Check your supplement label to see which form it contains.

• D3 (cholecalciferol) is more effective at raising blood levels.
• D2 (ergocalciferol) is often plant-sourced but less potent.
• Most experts prefer D3 for supplementation.

Sources: Office of Dietary Supplements, National Institutes of Health (NIH), Harvard T.H. Chan School of Public Health, Stanford Lifestyle Medicine, Cleveland Clinic Journal of Medicine

Risk Factors for Deficiency

Vitamin D deficiency is not uncommon. In the United States, about 5.9% of the population is deficient (serum 25(OH)D <30 nmol/L) and 24% are insufficient (<50 nmol/L). Several factors can increase your risk: darker skin pigmentation, older age, limited sun exposure (due to indoor lifestyles, northern latitudes, or always using sunscreen), and obesity. Certain chronic conditions that affect fat absorption can also increase risk.

If you have one or more of these risk factors, it may be worth discussing your vitamin D status with your healthcare provider, even though routine testing is not recommended for everyone.

• Darker skin pigmentation
• Older age
• Limited sun exposure (e.g., indoor work or living far from the equator)
• Obesity
• Chronic conditions that affect fat absorption

Sources: Office of Dietary Supplements, National Institutes of Health (NIH), Harvard T.H. Chan School of Public Health, Frontiers in Nutrition

Testing and Monitoring Your Levels

Testing for vitamin D involves a simple blood test that measures serum 25-hydroxyvitamin D. The National Academy of Medicine defines deficiency as levels below 30 nmol/L (12 ng/mL) and inadequacy as 30–50 nmol/L (12–20 ng/mL). Levels above 50 nmol/L are generally considered sufficient for bone health.

The 2024 Endocrine Society guidelines do not recommend routine testing in healthy adults of any age. They do, however, suggest that adults with prediabetes may benefit from supplementation. If you have concerns about your vitamin D status, ask your healthcare provider whether testing is appropriate for your situation.

• Blood test measures 25(OH)D.
• Deficiency: <30 nmol/L (12 ng/mL).
• Inadequacy: 30–50 nmol/L (12–20 ng/mL).
• Sufficiency for bone health: ≥50 nmol/L (20 ng/mL).
• Routine testing is not recommended for healthy adults.

Sources: Office of Dietary Supplements, National Institutes of Health (NIH), Harvard Health Publishing

Safe Supplementation: Dosage and Upper Limits

When diet and sunlight aren’t enough, supplements can help you reach your target intake. The tolerable upper intake level (UL) for adults is 4,000 IU (100 mcg) per day from all sources—food and supplements combined. Sticking within this range is safe for most people. Taking vitamin D with a meal that contains fat improves absorption.

Vitamin D toxicity is rare and almost always caused by excessive supplement intake, which can lead to hypercalcemia (high calcium levels in the blood). Always speak with a healthcare provider before starting any supplement, particularly if you are taking other medications or have health conditions.

For most healthy adults, a supplement dose of 600–800 IU daily is sufficient, but your doctor may recommend a different amount based on your blood levels and risk factors. Do not exceed 4,000 IU without medical supervision.

• Upper limit: 4,000 IU/day for adults.
• Take with a meal containing fat for better absorption.
• Toxicity is rare and usually from high-dose supplements.
• The VITAL trial used 2,000 IU daily and showed no harm in healthy adults, but also no benefit for fractures or falls.

Sources: Harvard T.H. Chan School of Public Health, Office of Dietary Supplements, National Institutes of Health (NIH), MD Anderson Cancer Center, Cleveland Clinic Journal of Medicine, Stanford Lifestyle Medicine