Brittica guides

Vitamin D3 and K2: benefits, UK doses and how to take them together

Last updated: 4 November 2026

It is November. The clocks have gone back, you leave for work in the dark and come home in the dark, and your skin has more or less stopped making vitamin D until spring. That is not a scare story. It is simple physics: from October to March, UK sunlight is too weak for your skin to make the vitamin D you need. Government figures show that 23% of adults aged 19 to 64 already have low blood levels.⁹ And the average Briton gets only about 3 micrograms a day from food, against the 10 micrograms that experts recommend.¹⁰

So vitamin D is worth thinking about. But walk into any pharmacy or scroll any supplement site and you meet a second name stuck to it: K2. Vitamin D3 and K2 now outsell plain vitamin D in many shops. Do you need both? How much is safe? And what do those huge “500%” figures on labels mean?

This guide answers all of it in plain English, with the UK rules, the real numbers and none of the hype. We use reputable search engines and analytics software to see what people in the UK ask about vitamin D3 and K2, and we built each section around those questions.

What does vitamin D3 do?

Vitamin D helps your body absorb and use calcium, which is why it matters so much for bones, teeth and muscles.* It also contributes to the normal function of your immune system.*

D3 (cholecalciferol) is the same form your skin makes in sunlight. D2 (ergocalciferol) comes from plants and fungi. Both count, and labels should tell you which one you are getting.

Signs you may be low in vitamin D

Low vitamin D creeps up quietly. A severe lack causes rickets in children and osteomalacia in adults, a softening of the bones that brings bone pain and muscle weakness.¹ Milder shortfalls usually have no clear symptoms at all, which is exactly why the NHS gives everyone the same autumn and winter advice rather than waiting for people to feel unwell.

How much vitamin D should I take in the UK?

The UK’s Scientific Advisory Committee on Nutrition (SACN) set a reference intake of 10 micrograms a day, all year round, for everyone aged 4 and over.³ The NHS turns that into simple advice: between October and March, everyone, including pregnant and breastfeeding women, should consider a daily supplement containing 10 micrograms.¹

Some people are advised to take 10 micrograms every day of the year: people who are rarely outdoors, people in care homes, people who usually cover most of their skin outside, and people with dark skin, such as those of African, African-Caribbean or South Asian background.¹

How much is too much vitamin D?

The NHS is clear: do not take more than 100 micrograms (4,000 IU) a day as it could be harmful. That applies to adults, including older people and pregnant women, and to 11 to 17 year olds. For children aged 1 to 10 the limit is 50 micrograms.¹ Taking too much over a long time can raise calcium in the blood (hypercalcaemia), which can weaken bones and damage the kidneys and heart.¹

The European Food Safety Authority reviewed this limit in 2023 and kept 100 micrograms as the safe upper level for adults.⁴

Vitamin D 1,000 IU, 25 micrograms, 4,000 IU: what do they mean?

Here is the conversion that clears up most of the confusion: 1 microgram = 40 IU. So 10 micrograms is 400 IU, 25 micrograms is 1,000 IU and 100 micrograms is 4,000 IU.¹

Why do labels say “500%” when the NHS says 10 micrograms?

Labels in Great Britain show a %NRV (nutrient reference value). For vitamin D, the labelling NRV is only 5 micrograms, half the SACN figure. So a 10 microgram product shows 200%, and a 25 microgram product shows 500%.

A big percentage does not mean you are over a safe limit. The number to compare against is the 100 microgram maximum.^1,4 For vitamin K, the NRV is 75 micrograms, so 100 micrograms shows as 133%.

When is the best time to take vitamin D?

Time of day matters less than food. Vitamin D is fat-soluble, and in one small clinical study, people who switched to taking it with their largest meal raised their blood levels by about half.¹¹ Breakfast, lunch or dinner all work, as long as there is some fat on the plate.

What does vitamin K2 do?

Vitamin K contributes to normal blood clotting and to the maintenance of normal bones.* Your body uses it to activate proteins that need vitamin K to work, including osteocalcin in bone.⁷

“Vitamin K” is really a family. K1 (phylloquinone) is the main form in green leafy vegetables. K2 is a group called menaquinones, named MK-4 to MK-13 by the length of their side chain. MK-4 and MK-7 are the two you usually see on labels. MK-7 is found in natto, a Japanese fermented soya food, and in smaller amounts in some cheeses.⁷

How much vitamin K do I need?

The NHS says adults need about 1 microgram a day for each kilogram of body weight, so a 65 kg adult needs about 65 micrograms. Most people get that from broccoli, spinach and other leafy greens, vegetable oils and cereal grains.² For supplements, the Department of Health and Social Care advises that 1 mg (1,000 micrograms) or less a day is unlikely to cause any harm.²

MK-4 vs MK-7: which form of K2 is better?

The forms behave differently in your blood. In a 2007 study published in Blood, K1 and MK-7 were both absorbed well, but MK-7 stayed in circulation far longer. That gave steadier blood levels and a 7 to 8-fold build-up with daily intake.⁵ MK-4 clears from the blood within hours, which is why MK-4 products tend to use much bigger doses spread through the day.

That is why most modern D3 and K2 products, including ours, use MK-7: one small daily dose keeps levels steady.

Vitamin K2 benefits: what the research shows

The best-known trial gave 180 micrograms of MK-7 a day for three years to 244 healthy postmenopausal women. Compared with placebo, the MK-7 group lost less bone mineral density at the spine and hip with age.¹² That is one well-run study, in one group of people, so treat it as encouraging rather than proof that K2 does the same for everyone.

Should you take vitamin D3 and K2 together?

No UK body says you must. NHS and SACN advice covers vitamin D on its own, and the NHS does not recommend a vitamin K supplement for most adults.^1,2,3

So why do so many people take them as a pair? Because they work on the same team. Vitamin D helps you absorb calcium, and vitamin K helps activate the proteins your bones use. Both contribute to the maintenance of normal bones.* Taking them in one product means one capsule instead of two, and it is easy to keep up through a long British winter.

Think of it as a two-step job. Vitamin D3 gets the calcium in. Vitamin K2 activates osteocalcin, the protein that binds calcium straight into your bones and nowhere else, so it steers calcium into the bones where you want it.⁷ One capsule covers both steps.

How to choose the best vitamin D3 and K2 in the UK

Check	What to look for	Why it matters
Vitamin D dose	Micrograms per daily serving, not per capsule	10 µg is the NHS advice; stay under 100 µg in total1
Vitamin D form	D3 (cholecalciferol)	The same form your skin makes in sunlight
Everything you take	Add up multivitamins, drops and combined products	The 100 µg limit is for your total intake1
K2 form	MK-7 named on the label	Steadier blood levels from a small daily dose5
K2 dose	Micrograms per daily serving	1,000 µg or less a day is unlikely to cause harm2
Claims	Bones, immune system, muscle function, calcium absorption, blood clotting	Anything beyond this list is not authorised in GB6

Where Ca-AKG+ fits

If you want D3 and K2 but would rather your daily capsules did more, Ca-AKG+ was built for you. Two capsules give you:

• Vitamin D3 25 micrograms (1,000 IU, 500% NRV). Above the NHS’s 10 microgram advice, well below the 100 microgram maximum.
• Vitamin K2 as MK-7 100 micrograms (133% NRV). The long-lasting form.
• Magnesium glycinate 125 mg, which contributes to the reduction of tiredness and fatigue.*
• 1,000 mg calcium alpha-ketoglutarate, plus ubiquinol CoQ10, PQQ, spermidine, pine bark and olive leaf, chosen for the processes behind healthy ageing. Read the 14 ageing switches to see why.

Already taking a vitamin D supplement? Count the 25 micrograms in Ca-AKG+ towards your total. Ca-AKG+ contains wheat and is not for use in pregnancy, while breastfeeding or under 18. It contains vitamin K, so it is not the right choice if you take warfarin.

Frequently asked questions

Can I take vitamin D3 and K2 together?

Yes. They are often combined in one product. UK guidance does not require it, but many people choose the pair because both contribute to normal bones.*

How much vitamin D should I take a day?

The NHS says 10 micrograms (400 IU) a day from October to March, and all year for some groups. Adults should not take more than 100 micrograms (4,000 IU) a day.¹

Is 1,000 IU (25 micrograms) of vitamin D too much?

No. It is more than the 10 microgram daily advice but well within the 100 microgram adult maximum set by the NHS and EFSA.^1,4 Just add up every product you take.

What is the difference between MK-4 and MK-7?

Both are forms of vitamin K2. MK-7 stays in the blood much longer, so a small daily dose gives steadier levels.⁵

What are the side effects of vitamin D3 and K2?

At normal doses, most people notice nothing. The main risk with vitamin D is taking too much for too long, which raises blood calcium.¹ The main issue with vitamin K is its effect on warfarin.⁸

Can I take vitamin K2 if I’m on warfarin?

The NHS advises not to start vitamin K supplements while taking warfarin without checking with your doctor first.⁸

What foods contain vitamin K?

K1 is in green leafy vegetables such as broccoli and spinach, vegetable oils and cereal grains. K2 is in natto and, in smaller amounts, some cheeses, meat and dairy.^2,7

*Vitamin D contributes to the maintenance of normal bones and teeth, normal muscle function, normal absorption and utilisation of calcium, and the normal function of the immune system. Vitamin K contributes to the maintenance of normal bones and to normal blood clotting. Magnesium contributes to the reduction of tiredness and fatigue. Food supplements should not replace a varied, balanced diet and a healthy lifestyle.

References

1. NHS. Vitamin D. Page last reviewed 3 August 2020 (accessed 4 November 2026).
2. NHS. Vitamin K. Page last reviewed 3 August 2020 (accessed 4 November 2026).
3. Scientific Advisory Committee on Nutrition. SACN vitamin D and health report. GOV.UK, 21 July 2016.
4. EFSA Panel on Nutrition, Novel Foods and Food Allergens. Scientific opinion on the tolerable upper intake level for vitamin D. EFSA Journal 2023;21(8):8145.
5. Schurgers LJ, Teunissen KJF, Hamulyák K, Knapen MHJ, Vik H, Vermeer C. Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Blood 2007;109(8):3279-83.
6. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. GOV.UK.
7. NIH Office of Dietary Supplements. Vitamin K: fact sheet for health professionals.
8. NHS. Taking warfarin with other medicines and herbal supplements.
9. Public Health England. PHE publishes new advice on vitamin D. GOV.UK, 21 July 2016 (National Diet and Nutrition Survey data).
10. University of Sheffield. British scientists recommend we need to triple our daily vitamin D intake. 21 July 2016.
11. Mulligan GB, Licata A. Taking vitamin D with the largest meal improves absorption and results in higher serum levels of 25-hydroxyvitamin D. J Bone Miner Res. 2010;25(4):928-930.
12. Knapen MHJ, Drummen NE, Smit E, Vermeer C, Theuwissen E. Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporos Int. 2013;24(9):2499-2507.
13. Keyword research: reputable search engine and analytics data (Google Search UK; DataForSEO; OpenSEO), November 2026.

For more detailed information, contact info@brittica.co.uk.