Vitamin D supplements: who needs them and how much

By the DieticiansWorldwide.com editorial team · 17 October 2026 · Supplements & industry

Why vitamin D is different from other vitamins

Most vitamins come almost entirely from food. Vitamin D is the exception. Your skin makes it when ultraviolet B light from the sun hits it, and very few foods contain much of it naturally. The US National Institutes of Health Office of Dietary Supplements (ODS) lists fatty fish, fish liver oils, beef liver, egg yolks and cheese as the main natural sources, and notes that fortified foods such as milk and breakfast cereals supply most of the vitamin D in American diets (NIH ODS, Vitamin D fact sheet, updated June 2025).

Vitamin D's main established job is helping the body absorb calcium and keep calcium and phosphate at the right levels for healthy bones and muscles. Without enough of it, bones can become thin, brittle or misshapen.

That combination, a nutrient made mainly from sunlight but needed for bone health, is why vitamin D is one of the few supplements that public-health bodies actively recommend for large groups of healthy people. It is also why so many people take far more than they need. This guide sets out who benefits, how much is enough, and where the line sits between helpful and harmful.

For a wider look at when supplements make sense and how the industry works, see our supplements hub.

Who needs a vitamin D supplement

The answer depends on where you live, the season, your skin, your age and how much time you spend outdoors.

UK guidance

The NHS says most people can make enough vitamin D from sunlight from about late March or early April to the end of September. Outside that window, everyone should consider a daily supplement of 10 micrograms (mcg) (NHS, Vitamin D).

The NHS advises taking 10 mcg every day of the year if you:

• are rarely outdoors, for example because you are frail or housebound;
• live in a care home or other institution;
• usually wear clothes that cover most of your skin when outdoors;
• have dark skin, for example if you have an African, African-Caribbean or South Asian background.

It also recommends a supplement all year for all children aged 1 to 4, and 8.5 to 10 mcg a day for babies up to 1 year, unless they are having more than 500 ml of infant formula a day, since formula is already fortified.

US guidance

ODS sets out the US Recommended Dietary Allowances, which assume minimal sun exposure. They are 15 mcg (600 IU) a day from age 1 to 70, including during pregnancy and breastfeeding, and 20 mcg (800 IU) from age 71. Infants have an Adequate Intake of 10 mcg (400 IU).

ODS lists the groups most likely to have inadequate vitamin D status as breastfed infants, older adults, people with limited sun exposure, people with dark skin, people with conditions that limit fat absorption, and people with obesity or who have had gastric bypass surgery. If you are in the US, our United States country guide explains how to find a registered dietitian there.

The Endocrine Society's 2024 guideline

In 2024 the Endocrine Society published a clinical practice guideline on using vitamin D to prevent disease in people without a specific medical reason for treatment. It suggested routine ("empiric") supplementation for four groups: children and adolescents aged 1 to 18, adults aged 75 and over, people who are pregnant, and people with high-risk prediabetes. It suggested against taking vitamin D above the standard recommended intakes to lower disease risk in healthy adults under 75, and it suggested against routine blood testing for vitamin D in all the populations it considered, including people with obesity or darker skin (Demay et al., Journal of Clinical Endocrinology and Metabolism, 2024).

The panel also said that the optimal doses for these groups remain unclear, because the trial doses varied so much.

How much to take: a comparison by country and age

Sources: NHS and NIH ODS, both linked above. One microgram equals 40 IU, so 10 mcg is 400 IU and 25 mcg is 1,000 IU.

The NHS's maximum safe amounts are slightly simpler than the US upper limits: no more than 100 mcg (4,000 IU) a day for adults, including during pregnancy and breastfeeding and in older age, and for children aged 11 to 17; no more than 50 mcg (2,000 IU) a day for children aged 1 to 10; and no more than 25 mcg (1,000 IU) a day for babies under 12 months.

The UK and US figures differ because they were set for different purposes. The UK figure is a minimum for the population when sun exposure is low; the US RDA assumes almost no sun at all and is set to cover nearly everyone. Both are well below the upper limits.

Is 1,000 IU a day okay?

For most healthy adults, yes. 1,000 IU is 25 mcg, which is above the UK's 10 mcg advice and the US RDA, but only a quarter of the adult upper limit of 100 mcg (4,000 IU) set in both countries. Many single-ingredient supplements are sold at this strength.

It is not okay for every age. For babies under 12 months, 1,000 IU (25 mcg) is the NHS maximum, and a child's dose should follow age-specific advice. And if you also take a multivitamin, fortified drinks and a calcium-plus-D tablet, the amounts add up. Count every source before you assume you are well under the limit.

The Endocrine Society point is worth repeating: for healthy adults under 75, there is no trial evidence that going above standard intakes prevents disease. A modest daily dose is a reasonable insurance policy in winter; a high dose is not a health upgrade.

Food first, or supplement first?

Dietitians usually start with food. That is the position ODS takes too: nutritional needs should be met mainly through food, with fortified foods and supplements used where food alone cannot do it.

With vitamin D, food alone often cannot. The US food table from ODS shows how few foods contribute meaningful amounts:

Source: NIH ODS (Table 3), from USDA FoodData Central. Fortification levels vary by country, so check labels where you live.

Oily fish twice a week, eggs and fortified foods help, but outside the summer months most people in northern countries will struggle to reach 10 mcg a day from food. That is why the supplement advice exists.

To see whether the rest of your diet covers the basics (oily fish, dairy or fortified alternatives, fruit and vegetables), try our healthy eating check.

D2 or D3: which form is best?

Supplements contain one of two forms. Vitamin D2 (ergocalciferol) is made by UV-irradiating ergosterol from yeast. Vitamin D3 (cholecalciferol) is usually made from lanolin in sheep's wool; vegan D3 from lichen is also available.

ODS says both forms raise blood levels of 25-hydroxyvitamin D, the main marker of vitamin D status, and seem to be equally able to cure rickets. However, research shows D3 raises blood levels more and keeps them higher for longer. For that reason, D3 is the more common choice. D2 suits people who want a plant-based product and cannot find lichen-derived D3.

Both forms are well absorbed. ODS notes that taking vitamin D with a meal containing some fat improves absorption, but some is absorbed even without fat.

Vitamin D3 with K2: is the combination worth it?

D3-plus-K2 products are heavily marketed with the idea that K2 "directs" calcium into bones rather than arteries. The biology behind the claim is real: vitamin K is needed to activate proteins such as osteocalcin in bone and matrix Gla-protein in blood vessels, and ODS notes that matrix Gla-protein is being researched because it might help reduce abnormal calcification (NIH ODS, Vitamin K fact sheet).

The leap from biology to buying advice is the problem. None of the national guidance reviewed for this article (NHS, NIH ODS or the 2024 Endocrine Society guideline) recommends adding K2 to a vitamin D supplement. At the doses usually recommended for healthy people, there is no established reason to need it.

There is also a safety issue. ODS warns that vitamin K can have a serious and potentially dangerous interaction with warfarin and similar anticoagulants, which work by blocking vitamin K. People taking these medicines need to keep their vitamin K intake consistent, because sudden changes can raise or lower the drug's effect. If you take warfarin, do not start a K2 product without talking to your anticoagulation clinic.

Signs of very low vitamin D

Mild insufficiency usually causes no obvious symptoms. Severe, long-term deficiency affects bones. ODS describes:

• In children: rickets, where bone fails to mineralise properly, causing soft bones and skeletal deformities. Severe rickets can also cause bone pain, failure to thrive, developmental delay, seizures from low calcium, muscle spasms and dental problems. Prolonged exclusive breastfeeding without a vitamin D supplement can cause rickets in infants.
• In adolescents and adults: osteomalacia, where existing bone is poorly mineralised, leading to weak bones. Signs are similar to rickets and include bone pain and deformities, muscle spasms and, in severe cases, seizures.

Tiredness, low mood and frequent infections are often blamed on low vitamin D online. Low levels are associated with some of these, but ODS notes that clinical trials have not shown that vitamin D supplements prevent or treat depression. Persistent tiredness has many causes, including iron or B12 problems; our guide to vitamin B12 supplements covers one of them. See a doctor rather than self-diagnosing.

Testing

According to ODS, the US Food and Nutrition Board considers a blood level of 50 nmol/L (20 ng/mL) or more sufficient for most people, and says deficiency risk rises below 30 nmol/L (12 ng/mL). The Endocrine Society suggests against routine testing in people without a specific medical reason, so a test is usually something your doctor orders if you have symptoms or a condition affecting absorption, not a routine check.

What vitamin D does and does not do

Vitamin D is promoted for almost everything. The trial evidence is narrower. ODS summarises it as follows:

• Bones: with calcium, it may increase bone density in some postmenopausal women and older men, but it is not clear whether it reduces falls and fractures. The large VITAL trial gave 50 mcg (2,000 IU) of D3 a day to 25,871 generally healthy men aged 50 and over and women aged 55 and over for a median of 5.3 years and found no reduction in total, hip or non-vertebral fractures compared with placebo. Most participants already had adequate levels.
• Heart disease, type 2 diabetes, depression, multiple sclerosis: observational studies link low levels with higher risk, but trials have generally not shown that supplements prevent these conditions, especially in people who are not deficient.
• Cancer: trials have generally not shown a reduction in cancer incidence.
• Weight loss: the research does not support vitamin D supplements for weight loss.

The practical takeaway: vitamin D prevents and treats deficiency, which protects bones. It is not a general-purpose disease shield.

The risks of too much

Vitamin D is fat-soluble, so excess is stored rather than flushed out. The NHS warns that taking too much over a long period can cause hypercalcaemia (too much calcium in the blood), which can weaken bones and damage the kidneys and heart. You cannot overdose from sunlight.

ODS adds that toxicity is almost always caused by supplements, and in extreme cases can lead to kidney failure, calcification of soft tissues, abnormal heart rhythms and death. The US Food and Nutrition Board said toxicity symptoms are unlikely below 250 mcg (10,000 IU) a day, but noted that even intakes below the upper limit might have adverse effects over time, and recommended avoiding blood levels above about 125 to 150 nmol/L (50 to 60 ng/mL).

Medicine interactions

ODS lists several interactions to discuss with your pharmacist or doctor:

• Orlistat (a weight-loss drug) can reduce absorption of vitamin D from food and supplements.
• Statins may reduce the body's own vitamin D production, and high vitamin D intakes might reduce the potency of atorvastatin, lovastatin and simvastatin.
• Corticosteroids such as prednisone can reduce calcium absorption and impair vitamin D metabolism.
• Thiazide diuretics, combined with vitamin D supplements, might lead to high blood calcium, especially in older adults and people with kidney problems.

The NHS also notes that some people with medical conditions may need lower amounts, so if you have kidney disease or a condition that affects calcium levels, take vitamin D only on medical advice. Our guide to magnesium glycinate covers another mineral supplement often taken alongside it.

Choosing a product: what labels do and do not guarantee

The US Food and Drug Administration says it is not authorised to approve dietary supplements for safety and effectiveness before they are marketed, and that firms can often sell them without notifying the FDA (FDA). Rules are stricter in some countries, but in many markets the label is the manufacturer's own claim.

Independent testing programmes fill part of the gap. The US Pharmacopeia, for example, says its verification programme audits, reviews and tests products so that "what's on the label is what's in the bottle" (USP). A seal like that tells you about quality and contents. It does not tell you whether you need the product or whether the dose is right for you. Our guide to third-party tested supplements explains how the main schemes compare.

A sensible checklist:

1. Check the dose in mcg and IU. For most adults, 10–25 mcg (400–1,000 IU) a day is plenty.
2. Prefer D3 unless you need a plant-based D2 or lichen-derived D3.
3. Avoid stacking. Add up multivitamins, fortified foods and combination products.
4. Look for independent testing if quality assurance matters to you.
5. Be wary of extras. K2, "immune" blends and high-dose "mega" products add cost and, in some cases, interaction risks.

The same scepticism applies to other popular products; our review of berberine as "nature's Ozempic" is a good example of marketing running ahead of evidence.

When to get personal advice

A vitamin D supplement at standard doses is one of the simplest and safest steps in nutrition. Personal advice is worth getting if you are pregnant, have a baby or young child, are over 75, take any of the medicines above, have kidney, liver or gut conditions, have had weight-loss surgery, or have been told your level is low and need a treatment plan. A registered dietitian can look at your diet, sun exposure, medicines and test results together and tell you what dose, if any, makes sense. If you are working on wider diet changes at the same time, such as eating more fibre, our high-fibre foods list is a good companion read.

Book a session with a dietitian