What Vitamin D Should I Take: Form, Dose by Age

Vitamin D3 (cholecalciferol) is the preferred supplemental form for most people, and the dose that makes sense depends heavily on your age, body weight, and baseline blood level. For healthy adults, most guidelines cluster around 600 to 2,000 IU per day, while infants need 400 IU and older adults often benefit from 700 to 1,000 IU or more. But the picture gets more complicated once you factor in the different forms available, how well your body absorbs them, what medications you take, and whether genetic variation might make you a poor responder to standard doses.

Vitamin D3 Versus D2

The two main supplemental forms are vitamin D3 (cholecalciferol, derived from animal sources or lichen) and vitamin D2 (ergocalciferol, derived from fungi). Both raise blood levels, but D3 does so more effectively. A meta-analysis of 20 head-to-head studies found that D3 raised total blood levels of 25(OH)D roughly 40% more than D2 when given in equivalent daily doses.¹ A clinical comparison in a primary care setting confirmed the pattern: a D3 injection raised serum concentrations about twice as much as an equivalent D2 injection.²

D2 is not useless. It still raises its own specific metabolite in the blood, and for people who need a plant-based or vegan option, it works. But if you have no dietary restriction that rules D3 out, it is the stronger choice. Most over-the-counter supplements sold today are D3, and that is a reasonable default.

Calcifediol for Faster Results or Absorption Problems

There is a third option that fewer people know about: calcifediol, also called 25-hydroxyvitamin D or calcidiol. This is the form your liver normally converts vitamin D into before it circulates in the blood. Taking calcifediol essentially skips that conversion step. Its intestinal absorption is close to 100%, it enters the bloodstream directly through the portal vein rather than traveling through the slower lymphatic system, and on a weight-for-weight basis it is roughly three times more potent than regular vitamin D in mildly deficient people.³

A randomized trial in postmenopausal women showed the practical difference clearly. After one month, women taking calcifediol had nearly double the increase in blood vitamin D compared with those taking standard cholecalciferol. By four months, about 35% of the calcifediol group had reached levels above 30 ng/mL, compared with just 8% in the cholecalciferol group.⁴ For people with fat malabsorption, liver disease, or a history of bariatric surgery, calcifediol can be a better strategy than taking higher and higher doses of regular D3.³ It is available by prescription in some countries and over the counter in others.

Does It Matter What You Take It With?

Vitamin D is fat-soluble, and eating some fat alongside your supplement improves absorption. One study found that taking vitamin D with a fat-containing meal increased the peak plasma level by about 32% compared with taking it on an empty stomach.⁵ The type of fat did not seem to matter much; monounsaturated and polyunsaturated fats performed similarly. Interestingly, though, a separate study looking at a large single dose (50,000 IU) found that while a low-fat meal actually produced a slightly higher peak absorption than a high-fat meal, the long-term blood level response at 30 and 90 days did not differ between the groups.⁶

The practical takeaway: taking your vitamin D with a meal that contains some fat is a good habit, but do not stress about exactly how much fat. Even a modest amount helps. If you forget and take it on an empty stomach, you are still absorbing most of it over the long run.

Beyond the classic soft-gel capsule, newer delivery systems have emerged, including sublingual sprays, orodispersible tablets, and specialized lipid-matrix formulations. A trial comparing a phospholipid-based orodispersible vitamin D3 preparation against standard chewable tablets and soft-gel capsules found that the novel formulation achieved higher and more sustained blood levels.⁷ These specialized formats may matter most for people with gastrointestinal conditions that impair normal absorption; for most healthy adults, a standard capsule or tablet taken with food works fine.

Dose by Age and Life Stage

There is no single dose that fits everyone, and official recommendations vary somewhat between countries and expert bodies. Here is a practical breakdown of what the evidence supports:

Infants (0–12 months): 400 IU per day of vitamin D3 is the widely agreed-upon dose for preventing deficiency. Studies confirm this is sufficient to keep blood levels in a healthy range.⁸ This applies to both breastfed and formula-fed babies, though formula-fed infants who drink enough fortified formula may already be covered.
Children and adolescents (1–18 years): Guidelines generally recommend 400 to 600 IU per day, ideally met through a combination of diet, sunlight, and supplementation when needed.⁹
Adults (19–64 years): Most guidelines suggest 600 to 1,000 IU daily for maintenance. Some researchers argue that reaching optimal blood levels (above 30 ng/mL) may require 2,000 IU or more per day in people who start out deficient or get very little sun exposure.¹⁰
Older adults (65+): Evidence supports at least 700 to 1,000 IU per day for fall and fracture prevention, with good daily adherence.¹¹ A meta-analysis of randomized trials found that doses in the 700 to 2,000 IU range were associated with lower fall risk in older adults.¹²
Breastfeeding mothers: An alternative to supplementing your infant directly is for the mother to take 6,400 IU of D3 per day, which raises breast milk vitamin D content enough to meet the baby’s needs. A randomized trial showed this approach was as effective as giving the infant 400 IU directly, and it was safe for the mother.¹³

People who are severely deficient, obese, or taking certain medications may need substantially higher doses under medical supervision, sometimes including loading protocols of 50,000 IU once or more per week.¹⁴

Daily, Weekly, or Monthly Dosing

If swallowing a pill every day is not your thing, you can take a weekly or monthly dose instead. One randomized trial compared daily, weekly, and monthly regimens that all added up to the same total amount (equivalent to 1,000 IU per day) and found statistically equivalent increases in blood vitamin D across all three schedules.¹⁵

That said, another study in nursing home residents found daily dosing produced a somewhat larger increase in blood levels than weekly, and weekly beat monthly.¹⁶ The differences were modest, and the takeaway is practical: the best schedule is the one you will actually stick to. If you remember a daily pill, do that. If a weekly dose fits your routine better, that works too. Very large monthly or quarterly bolus doses (like 100,000 IU or more at once) are less ideal for reasons beyond absorption, as research on respiratory infections suggests that infrequent large boluses may not deliver the same benefits as regular smaller doses.

What Blood Level to Aim For

The standard blood test measures 25-hydroxyvitamin D, or 25(OH)D. Most experts agree that below 20 ng/mL counts as deficient, 21 to 29 ng/mL is insufficient, and levels at or above 30 ng/mL are generally considered adequate.¹⁷ Some researchers argue for a higher target of 40 to 60 ng/mL, particularly for benefits beyond bone health, but the evidence for pushing levels that high is less settled.

Routine screening is not recommended for the general population in most guidelines. Testing makes more sense if you have risk factors for deficiency: dark skin, limited sun exposure, obesity, malabsorption conditions, kidney disease, or you take medications that interfere with vitamin D metabolism. If your doctor orders the test and your level comes back low, a retest after two to three months of supplementation helps confirm that your chosen dose is working.

Cofactors That Affect How Vitamin D Works

Vitamin D does not operate in isolation. Every enzyme involved in converting vitamin D to its active form requires magnesium as a cofactor. If you are low on magnesium, your body may struggle to activate the vitamin D you are taking.¹⁸ Magnesium deficiency is common, particularly in older adults and people who eat highly processed diets, so it is worth considering whether your magnesium intake is adequate before assuming your vitamin D supplement is not working.

Vitamin K is the other nutrient that comes up frequently alongside vitamin D. Both play roles in calcium metabolism: vitamin D helps you absorb calcium, while vitamin K2 helps direct that calcium into bones rather than soft tissue. Evidence from both animal and human studies suggests that having adequate levels of both vitamins is more beneficial for bone and cardiovascular health than having just one.¹⁹ A recent randomized trial in patients with severe coronary artery calcification found that combined vitamin K2 and D3 supplementation slowed the progression of calcification compared with placebo.²⁰ You do not necessarily need a combo supplement; eating leafy greens (for K1) and fermented foods or taking K2 separately alongside your D3 achieves the same thing.

When the Standard Dose Is Not Enough

Several conditions make it harder for your body to use vitamin D normally, and people in these groups often need higher doses or different forms.

Obesity is one of the biggest factors. Vitamin D is fat-soluble and gets sequestered in body fat, meaning less circulates in the blood. People with a higher body mass index typically need more supplementation to reach the same blood level as a leaner person.¹⁴ Bariatric surgery compounds the problem by creating malabsorption; post-surgical patients may need extremely high doses, sometimes 50,000 IU one to three times per week, with monitoring.²¹

People on certain medications also face challenges. Corticosteroids can impair vitamin D metabolism by increasing the activity of enzymes that break it down. Antiepileptic drugs like phenytoin, phenobarbital, and carbamazepine are known to accelerate the breakdown of vitamin D into its inactive form. Orlistat, a weight-loss drug that blocks fat absorption, can also reduce uptake of fat-soluble vitamins including D.²² Additionally, thiazide diuretics combined with calcium and vitamin D can sometimes push calcium levels dangerously high in elderly patients or those with kidney issues.²³ If you take any of these medications, your doctor should be involved in deciding your vitamin D dose.

Genetic Variation in Vitamin D Response

Not everyone responds to the same dose of vitamin D equally, and genetics is part of the reason. Variants in the GC gene, which encodes the protein that carries vitamin D through the blood, have a measurable effect. People with certain common variants at the GC locus had nearly three times the odds of remaining below 50 nmol/L (about 20 ng/mL) after supplementation, compared with people carrying the more favorable version of the gene.²⁴ Variants in CYP2R1, the gene for one of the liver enzymes that activates vitamin D, also influence baseline blood levels.²⁵

What this means practically: if you supplement consistently and your blood levels still do not budge, genetics may be involved. The solution is usually to increase the dose or switch forms (say, to calcifediol) rather than to give up. Genetic testing for vitamin D response is available through some direct-to-consumer panels, but it is not standard clinical practice yet. A simpler approach is to just test your blood level after a few months of supplementation and adjust accordingly.

Safety and the Risk of Taking Too Much

Vitamin D toxicity is real but uncommon at normal supplemental doses. It happens when excessive intake causes a buildup of 25(OH)D that starts activating calcium absorption beyond what the body can handle, leading to dangerously high blood calcium levels.²⁶ The tolerable upper intake level set by most authorities is 4,000 IU per day for adults, though many researchers consider short-term use of higher doses safe under medical supervision. Cases of toxicity in the literature almost always involve doses far above this, often through dosing errors or the use of ultra-concentrated formulations.

You cannot get vitamin D toxicity from sunlight, because the skin self-regulates and stops producing excess. Nor can you realistically reach toxic levels from food alone. The risk is exclusively from supplements, and it is easily avoided by not taking mega-doses without medical monitoring.

Vitamin D and Respiratory Infections

One of the most debated health claims for vitamin D is that it protects against respiratory infections. An influential 2017 meta-analysis of individual participant data found an overall protective effect, with daily or weekly supplementation reducing the odds of acute respiratory infection by about 19%. The benefit was strongest among people who were deeply deficient at baseline, where the risk dropped by roughly 70%.²⁷ Bolus dosing, such as large monthly or quarterly doses, showed no benefit.

However, an updated meta-analysis including more recent trials found that the overall confidence interval for the protective effect now includes the possibility of no benefit at all, meaning the evidence has weakened as more data has accumulated.²⁸ A large trial during the COVID-19 pandemic tested cod liver oil (a modest source of vitamin D) against placebo in over 34,000 adults and found no difference in rates of SARS-CoV-2 infection, serious COVID-19, or other respiratory infections.²⁹

The honest read of the data is that correcting a genuine deficiency probably helps your immune defenses, but taking vitamin D when you are already at adequate levels is unlikely to act as a shield against colds or flu. The respiratory-infection angle should not be the primary reason you supplement.

Sunlight, Skin Color, and When Supplements Are Non-Negotiable

Your skin makes vitamin D when exposed to UVB radiation from sunlight, and for much of human history this was the primary source. The amount you produce depends on latitude, season, time of day, cloud cover, clothing, and skin pigmentation. Research at UK latitudes found that people with very dark skin (Fitzpatrick type V) needed about 25 minutes of midday sun exposure from March through September, with forearms and lower legs uncovered, to meet vitamin D requirements. This is two to three times the exposure needed by people with very fair skin to achieve the same increase in blood levels.³⁰

A controlled study directly comparing vitamin D production across skin types (Fitzpatrick II through VI) found that melanin’s inhibitory effect was smaller than often assumed, with an inhibition factor of about 1.3 to 1.4 between the lightest and darkest skin types under the same UVB dose.³¹ The catch is that in real life, people with darker skin can tolerate more sun before burning and often need longer exposure to get the same UVB dose. At higher latitudes, especially during winter months when UVB is weak or absent, supplementation becomes necessary regardless of skin color.

The evolutionary backstory here is worth a mention. As early humans migrated out of equatorial Africa into regions with less intense UV radiation, lighter skin pigmentation evolved to allow sufficient vitamin D synthesis in lower-UV environments. Darker skin near the equator protected against folate degradation from excessive UV. This vitamin D-folate tradeoff is one of the best-supported models for the global distribution of skin color in indigenous populations.³² In modern life, where many people spend most of their time indoors regardless of latitude, the evolutionary calibration no longer matches reality, which is part of why vitamin D deficiency is so widespread.

Medications That Interfere With Vitamin D

Beyond the medications already mentioned that deplete vitamin D, it is worth knowing about the reverse interaction: vitamin D supplementation can affect how certain drugs work. Concurrent vitamin D use appeared to lower blood concentrations of atorvastatin (a common statin), though the clinical significance of this is unclear.²³ Several drug classes can activate a liver receptor called the pregnane X receptor, which speeds up the breakdown of vitamin D.³³ If you take multiple medications, particularly anticonvulsants, corticosteroids, or HIV antiretroviral therapy, periodic blood-level checks are a sensible precaution to make sure your supplementation is keeping pace with what your medications may be undoing.

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