The Ultimate Vitamin D Guide: Dosage, Deficiency, FAQs & Choosing the Right Supplement

Vitamin D is essential for bone health, immune function, and overall wellbeing. However, because sunlight exposure in the UK is limited for large parts of the year, millions of people experience low levels.

Below are the most frequently asked questions regarding vitamin D dosage, deficiency risks, and how to choose the right supplement.

Vitamin D in Winter: How Much Do You Actually Need?

UK Government advice is that everyone should consider taking a daily vitamin D supplement during the autumn and winter (1).

How much vitamin D should I take in autumn and winter?

During autumn and winter, when sunlight is insufficient for vitamin D production, all adults and children over four years old (including pregnant and breastfeeding women) should take a daily 10 microgram (400IU) vitamin D supplement. This recommendation addresses the difficulty of obtaining adequate vitamin D solely from diet during these months (1).

From late March/early April to the end of September, most individuals can achieve sufficient vitamin D levels through sun exposure and a balanced diet.

How many IU of vitamin D should I take daily?

The correct dosage depends on your lifestyle, health status, and time of year. Nature's Best provides options tailored to different requirements:

Dosage (IU & µg)	Best Suited For	Expert / Official Context
400IU (10µg)	General baseline support	Recommended by the UK Department of Health for autumn and winter.
1000IU (25µg)	Year-round maintenance (our most popular option)	Ideal if you spend limited time in the sun during spring and summer.
2000IU (50µg)	Higher risk of deficiency, higher BMI, or extra bone support	The maximum daily dose recommended by Nature's Best for routine supplementation.

Note: The NHS advises adults not to exceed 100µg (4000IU) of vitamin D per day from supplements (2).

What is the difference between Vitamin D2 and Vitamin D3?

Vitamin D exists in two primary dietary forms:

• Vitamin D2 (Ergocalciferol): Found primarily in mushrooms and some fortified foods. It is generally not as bioavailable as D3.
• Vitamin D3 (Cholecalciferol): Found in certain fish and dairy products and naturally synthesised by the skin upon exposure to sunlight.

Because Vitamin D3 is widely agreed to be the superior, more bioavailable form, all Nature's Best vitamin D supplements exclusively use Vitamin D3.

How do you convert International Units (IU) to micrograms (µg)?

Vitamin D measurements can sometimes cause confusion because labels use different metrics.

• The Conversion Rule: Each microgram (µg / mcg) is equivalent to 40IU.
• Quick Reference Guide:
  • 10µg = 400IU
  • 25µg = 1000IU
  • 50µg = 2000IU

Tip: Always check whether a label says micrograms (µg or mcg) versus milligrams (mg). Vitamin D is only required in minute quantities and should never be supplied in milligrams.

Why are most vitamin D supplements dissolved in oil?

Vitamin D is a fat-soluble vitamin, meaning it requires fat to be properly absorbed by the body.

• Most liquid-gel or capsule formats from Nature's Best (such as the 1000IU, Vegan 1000IU, and 2000IU variants) are dissolved in sunflower oil to maximise bioavailability.
• If you are taking a tablet format (such as the 400IU tablet), it is recommended to take it with a meal containing fat or alongside an omega-3 supplement.

Who is at risk of having low vitamin D levels?

UK Government Research indicates that roughly 1 in 6 adults in the UK suffer from low blood levels of vitamin D (3). While anyone can become deficient, high-risk groups include:

• Individuals with limited outdoor exposure or those who habitually cover their skin.
• People with darker skin tones (who require more sunlight synthesis to produce equivalent vitamin D levels).
• Older adults, whose skin is less efficient at synthesising vitamin D.

How long does it take for supplements to change your vitamin D levels?

Consistency is key when taking nutritional supplements. Scientific research demonstrates that:

• The most significant increase in serum vitamin D levels typically occurs between 3 to 9 months of consistent daily supplementation.
• Scientific data estimates that for every daily 100IU (2.5mcg) supplement taken, blood levels [measured as serum 25(OH)D] are expected to rise by roughly 1 ng/mL.

Why take a Vitamin K2 and Vitamin D3 supplement?

Vitamin K2 and D3 are commonly combined in a single supplement because they perform complementary roles in calcium metabolism. Vitamin D3 increases calcium absorption in the gut and raises blood calcium levels. Vitamin K2 activates proteins that direct that calcium into bones and teeth and away from arteries and soft tissue.

Main benefits of a K2 + D3 supplement:

• Bone density support: D3 enables calcium absorption; K2 helps bind that calcium into bone matrix.
• Cardiovascular support: K2 may reduce arterial calcification by preventing calcium buildup in blood vessel walls.
• Immune function: D3 plays a role in regulating immune response.
• Fills common nutrient gaps: D3 deficiency is widespread due to limited sun exposure; K2 is scarce in most Western diets.

Who should be cautious: People taking blood thinners should consult a doctor before supplementing, since K2 can interact with anticoagulant medications.

If you have underlying health conditions or are tracking a diagnosed deficiency, always consult a qualified healthcare practitioner or nutritionist to tailor your timeline and dosage.

References

1. NHS (2020). Vitamin D – Vitamins and Minerals. [online] NHS. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/ [Accessed 14 Sept. 2026].
2. Department of Health and Social Care (2022). New review launched into vitamin D intake to help tackle health disparities. [online] GOV.UK. Available at: https://www.gov.uk/government/news/new-review-launched-into-vitamin-d-intake-to-help-tackle-health-disparities [Accessed 14 Sept. 2026].
3. NHS (2020). Vitamin D – Vitamins and Minerals. [online] NHS. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/ [Accessed 14 Sept. 2026].