Osavi Vitamin D3 + K2

Vitamin D3· evidence: Strong

Vitamin D3 (cholecalciferol) is the form the skin makes from sunlight. In the UK there is not enough strong sunlight from about October to March, so many people have low levels over winter. Good-quality trials show supplements reliably raise blood levels and correct deficiency, and that matters for bone health in people who are low.

The NHS advises everyone to consider 10 µg (400 IU) a day in autumn and winter, and all year round for people who get little sun, cover their skin, have darker skin or live in care homes. For people who already have adequate levels, large trials have not shown that extra vitamin D prevents falls, fractures or other illness.

In short: a modest daily dose is sensible and cheap insurance for most UK adults in winter. Very high doses are not better and should only be taken on medical advice.

Typical dose: 10 µg (400 IU) a day is the NHS advice for adults and children over 1. Doses of 25 µg (1,000 IU) to 50 µg (2,000 IU) are common and safe for adults. Babies under 1 need 8.5 to 10 µg a day unless they have more than 500 ml of formula.

Safety: Do not take more than 100 µg (4,000 IU) a day without medical advice (FSA/NHS); children aged 1 to 10 should not have more than 50 µg, and babies under 1 no more than 25 µg. Long-term very high intake can raise blood calcium, which can damage the kidneys and heart. People with kidney disease, sarcoidosis or high calcium levels should speak to their GP first.

Research we looked at:
NHS. Vitamin D. nhs.uk/conditions/vitamins-and-minerals/vitamin-d
Scientific Advisory Committee on Nutrition (SACN). Vitamin D and Health. 2016.
EFSA NDA Panel. Scientific opinion on the tolerable upper intake level for vitamin D. EFSA Journal 2023;21(8):8145.
Bolland MJ, Grey A, Avenell A. Effects of vitamin D supplementation on musculoskeletal health: a systematic review, meta-analysis, and trial sequential analysis. Lancet Diabetes Endocrinol 2018;6(11):847-858.

Vitamin K2 (MK-7)· evidence: Limited

Vitamin K2 (menaquinone, usually as MK-7 in supplements) is one of the forms of vitamin K. The body needs vitamin K to make proteins involved in blood clotting and bone. Most adults get enough vitamin K from green vegetables, vegetable oils and some fermented foods, and true deficiency is rare in healthy adults.

Vitamin K2 is often sold alongside vitamin D3 on the idea that it helps direct calcium into bone. Trials of K2 on bone density and fractures are small and mixed, and the effect in well-nourished people is uncertain. Claims about arteries or heart health are not established.

Adding K2 to a vitamin D supplement is reasonable but not necessary for most people. It mainly adds cost.

Typical dose: Adults need about 1 µg per kg of body weight a day (around 70 µg for a 70 kg adult), mostly met by diet. Supplements usually give 45 to 200 µg of MK-7.

Safety: Vitamin K can stop warfarin and similar anticoagulants (vitamin K antagonists) working properly. Anyone on warfarin must not take vitamin K supplements without talking to their GP or anticoagulant clinic. The FSA notes that 1 mg or less a day from supplements is unlikely to cause harm in other people.

Research we looked at:
NHS. Vitamin K. nhs.uk/conditions/vitamins-and-minerals/vitamin-k
EFSA NDA Panel. Dietary reference values for vitamin K. EFSA Journal 2017;15(5):4780.
Mott A, Bradley T, Wright K, et al. Effect of vitamin K on bone mineral density and fractures in adults: an updated systematic review and meta-analysis of randomised controlled trials. Osteoporos Int 2019;30(8):1543-1559.