For most people, vitamin D3 at 600 to 800 IU daily is the right choice. D3 (cholecalciferol) is the form your skin naturally produces in sunlight, and it raises your blood levels about 40% more effectively than D2 (ergocalciferol), the plant-derived alternative. Beyond picking the right form, the dose you need depends on your age, body size, and how much sun you actually get.
D3 vs. D2: Which Form Works Better
Vitamin D supplements come in two forms: D2 and D3. Both are available over the counter, but they’re not equally effective. A meta-analysis of 20 comparative studies found that D2 raised total blood levels of vitamin D about 40% less than the same dose of D3. That’s a meaningful gap, especially if you’re trying to correct a deficiency.
D2 is derived from fungi and was historically the default in prescriptions. D3 comes from animal sources, typically lanolin (a waxy substance in sheep’s wool). If you’re vegan, you’re not stuck with D2. Lichen-derived D3 is chemically identical to lanolin-based D3, and your body processes both the same way. Look for “vitamin D3 from lichen” on the label.
The bottom line: choose D3 unless you have a specific reason not to. It’s more effective per unit, widely available, and inexpensive.
How Much You Need by Age
The recommended daily amounts, set by the National Academies of Sciences, Engineering, and Medicine, assume you’re getting minimal sun exposure:
- Infants (0 to 12 months): 400 IU (10 mcg)
- Children and adults (1 to 70 years): 600 IU (15 mcg)
- Adults over 70: 800 IU (20 mcg)
- Pregnant or breastfeeding: 600 IU (15 mcg)
These are baseline recommendations designed to maintain bone health in the general population. Many doctors recommend somewhat higher doses, particularly 1,000 to 2,000 IU daily, for people who are already deficient or have limited sun exposure. The tolerable upper limit for adults is 4,000 IU per day, meaning you can safely take up to that amount without medical supervision. Going above 4,000 IU daily on your own is not recommended, as excess vitamin D can cause calcium to build up in your blood, leading to nausea, kidney problems, and other issues over time.
Factors That Change Your Needs
The standard recommendations are just a starting point. Several factors can push your personal needs higher.
Body weight. Vitamin D is fat-soluble, meaning it gets stored in body fat rather than circulating freely. People with higher body fat percentages tend to have lower blood levels of vitamin D on the same dose. If you carry extra weight, you may need a higher daily amount to reach the same blood concentration as someone who is leaner.
Skin tone. Melanin, the pigment that makes skin darker, does reduce your skin’s ability to produce vitamin D from sunlight. Research comparing the lightest and darkest skin types found an inhibition factor of about 1.3 to 1.4, meaning darker skin produces roughly 25 to 30% less vitamin D from the same sun exposure. That’s a modest but real difference that adds up over months, particularly in northern climates.
Where you live. If you’re above about 37 degrees latitude (roughly a line from San Francisco to Richmond, Virginia, in the U.S., or anywhere in the UK and northern Europe), your skin produces little to no vitamin D from sunlight during the winter months. The sun sits too low in the sky for its rays to trigger vitamin D production. Supplementing year-round makes sense if you live in these areas.
Age. Older adults produce vitamin D less efficiently in the skin and are more likely to spend time indoors. That’s why the recommendation jumps to 800 IU after age 70.
What a Blood Test Tells You
The most useful way to dial in your dose is a simple blood test measuring 25-hydroxyvitamin D, often written as 25(OH)D. Your doctor can order this, and some labs offer direct-to-consumer options. Results are reported in nanomoles per liter (nmol/L) or nanograms per milliliter (ng/mL), depending on the lab.
Most experts consider a level below 20 ng/mL (50 nmol/L) deficient and a level of 20 to 29 ng/mL (50 to 72 nmol/L) insufficient. A level of 30 ng/mL or above is generally considered adequate for bone and overall health. There’s no strong evidence that pushing beyond 50 ng/mL provides additional benefits, and levels consistently above 150 ng/mL can become toxic.
If your test reveals a severe deficiency, your doctor may prescribe a short-term loading dose, typically 50,000 IU once a week for 6 weeks, to bring your levels up quickly. After that, you’d switch to a standard daily maintenance dose.
How to Take It for Best Absorption
Vitamin D is fat-soluble, so your body absorbs it best when you take it with a meal that contains some fat. This doesn’t require anything elaborate. A few eggs at breakfast, avocado on toast, or a handful of nuts alongside your supplement is enough. Taking it on a completely empty stomach means less of the vitamin makes it into your bloodstream.
Timing of day doesn’t matter for absorption. Some people prefer taking it in the morning with breakfast simply for consistency. The most important thing is picking a time you’ll remember daily, because vitamin D works best when levels are kept steady rather than taken sporadically.
Vitamin K2 and Other Cofactors
You’ll notice many vitamin D supplements are sold in combination with vitamin K2. There’s a logical reason for this pairing. Vitamin D helps your body absorb calcium from food, while vitamin K2 activates a protein called osteocalcin that directs that calcium into your bones rather than letting it accumulate in your arteries and soft tissues. In theory, taking both together supports better calcium management.
The clinical evidence for this combination is mixed. Some studies show benefits for bone density, others don’t, and the results depend heavily on whether participants already had osteoporosis and what other treatments they were on. A combined D3/K2 supplement is a reasonable choice and unlikely to cause harm, but it’s not strictly necessary for everyone. If you eat leafy greens, fermented foods, or cheese regularly, you’re likely getting enough K2 from your diet.
Magnesium also plays a supporting role in vitamin D metabolism. Your body uses magnesium to convert vitamin D into its active form. If you’re low in magnesium (common in adults eating a typical Western diet), your vitamin D supplement may not work as efficiently. Magnesium-rich foods include nuts, seeds, dark chocolate, and leafy greens.
What Vitamin D Actually Does (and Doesn’t Do)
Vitamin D’s clearest, most established role is in bone health. It helps your intestines absorb calcium and phosphorus, the minerals your skeleton needs to stay strong. Without enough vitamin D, children can develop rickets (soft, weak bones) and adults can develop osteomalacia, a painful softening of the bones.
Beyond bones, there’s reasonable evidence that adequate vitamin D levels support immune function. Maintaining sufficient levels appears to lower the risk of respiratory infections. This is one area where the science is fairly consistent.
For other conditions, the picture is less clear. The VITAL trial, one of the largest randomized controlled studies ever conducted on vitamin D, followed nearly 26,000 healthy adults taking 2,000 IU of D3 daily for over five years. It found no significant benefit for cancer, cardiovascular disease, bone density, body composition, or fracture risk in generally healthy people who weren’t deficient. This doesn’t mean vitamin D is useless. It means that for people who already have adequate levels, taking more doesn’t appear to add protection against these conditions. The benefit of supplementation is clearest when you’re actually deficient.