Vitamin D in Winter: Why D3 and K2 Matter as the Days Get Shorter

As the clocks go back, a closer look at the sunshine vitamin and the partner nutrient that helps put calcium to good use.

On 25 October, the clocks go back by an hour. Mornings become lighter a little earlier, evenings draw in sooner, and that familiar autumn feeling takes hold. The afternoon walk gets shorter, the sofa becomes more inviting, and suddenly it is dark before dinner.

The clock change itself does not alter the total amount of daylight you receive. However, it marks a point in the year when sunlight becomes less useful for making vitamin D in the UK. By late October, the sun is too low in the sky for your skin to produce meaningful amounts of vitamin D from UVB radiation. This continues through winter and into early spring.¹

As the days shorten, it is worth giving some thought to a nutrient that often gets overlooked until spring returns.

Why vitamin D deserves attention as the days shorten

Vitamin D is sometimes associated with sunshine and summer holidays, but its role in the body goes well beyond enjoying a sunny afternoon.

It contributes to the normal absorption and utilisation of calcium and phosphorus, helping to maintain normal bones and teeth. It also contributes to normal muscle function and normal immune system function.² These are everyday processes that matter throughout the year, even when opportunities to make vitamin D from sunlight become limited.

The difficulty is that relatively few foods naturally contain substantial amounts of vitamin D. Oily fish, egg yolks and some fortified foods contribute to intake, while certain mushrooms, particularly wild varieties or those exposed to ultraviolet light, can provide vitamin D2. Even so, getting enough from diet alone can be challenging.

This is why UK public health advice recommends that adults and children over four consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D during autumn and winter. People who have limited sun exposure or other risk factors may need to consider supplementation throughout the year.¹

For anyone who spends much of the working week indoors, lives further north, or tends to cover most of their skin outdoors, planning ahead becomes particularly relevant.

Recommended intake at a glance

Autumn and winter: UK advice is for adults and children over four to consider 10 µg (400 IU) of vitamin D a day.¹
All year round: may be relevant for people who spend little time outdoors, cover most of their skin when outside, or have darker skin.¹
Upper level: the adult upper level for routine daily vitamin D intake is 100 µg (4,000 IU) from all sources combined.⁴
Vitamin K: no upper level has been set, as its potential for harm is low. Anyone taking warfarin or a similar blood-thinning medicine should speak to their GP before taking vitamin K.³

Vitamin D3 and K2: how do they work together?

Vitamin D3 and vitamin K2 are often discussed together, and there is a good reason for that. Both are involved in processes that help maintain healthy bones, although they have different roles.

Vitamin D helps the body absorb calcium from food. Without sufficient vitamin D, calcium absorption becomes less efficient, making it harder to maintain normal calcium levels and support bone health.

Vitamin K2, meanwhile, helps activate proteins involved in the body’s handling of calcium. One of these is osteocalcin, a protein involved in bone mineralisation.³ Another is matrix Gla protein (MGP), which helps regulate calcium deposits and may help prevent calcium from building up in soft tissues and blood vessels.^3,5

Think of them as two parts of a larger system. Vitamin D helps make dietary calcium available to the body, while vitamin K-dependent proteins have roles in how calcium is handled in different tissues. This is one reason the combination attracts interest in nutritional research.

In a small clinical trial, people with type 2 diabetes took vitamin D3, vitamin K2, or both together every day for three months. The group taking both nutrients had a positive change in a blood marker linked to how vitamin K helps activate osteocalcin, a protein involved in bone health.⁶

These findings add to growing interest in how vitamins D3 and K2 may work together to support the body’s normal use of calcium. Alongside a varied diet, regular physical activity and other healthy habits, the pair may offer useful nutritional support for normal calcium metabolism and the maintenance of normal bones.

Where to find vitamin D3 and K2 in food

Food is a useful starting point, although the table below shows why diet alone can struggle to cover vitamin D in winter. Oily fish leads the way for vitamin D, while natto is in a league of its own for K2.

Food	Portion	Vitamin D (µg)	Vitamin K2 (µg)
Natto (fermented soya beans)	85 g	0	850 (MK-7)
Rainbow trout, farmed, cooked	85 g	16.2	not listed
Salmon (sockeye), cooked	85 g	14.2	0.3
Sardines, canned in oil	2 sardines	1.2	not listed
Egg (vitamin D is in the yolk)	1 large	1.1	4
Tuna, light, canned in water	85 g	1.0	not listed
Chicken breast, roasted	85 g	0.1	13
Beef mince, cooked	85 g	trace	6
Cheddar cheese	40 g	0.4	4
Fortified foods (some cereals, spreads, plant milks)	1 serving	varies: check the label	rarely added

Approximate values from US food composition data.^2,3 Vitamin K2 values for meat, eggs and dairy are mainly the MK-4 form; hard cheeses also contain longer-chain menaquinones that are not captured here, and amounts vary with animal feed and fermentation. For comparison, UK advice for vitamin D in autumn and winter is 10 µg a day.

The immune system, muscles and the winter months

Bones are only part of the picture.

Vitamin D contributes to normal immune system function, which remains active whether you are enjoying a summer barbecue or waiting for the kettle to boil on a dark November morning. Maintaining adequate vitamin D status is therefore relevant throughout the year, particularly when sufficient sunlight is no longer available to contribute to vitamin D production.

Muscle function is also dependent on vitamin D, which matters for everything from climbing stairs to getting out of a low chair or carrying the shopping home.

And what about mood? Shorter days can affect how you feel. Less daylight can also influence your body clock and sleep patterns, which may help explain why researchers have taken an interest in vitamin D and mood during the darker months. While more research is needed to understand the connection fully, maintaining adequate vitamin D levels is one part of supporting your overall health through winter.

It is worth keeping the bigger picture in mind. Getting outdoors during daylight, staying physically active and maintaining a regular sleep routine remain useful habits as the seasons change. A vitamin D supplement can form part of a sensible winter routine, without the expectation of covering every job.

Neutrient D3+K2: one simple addition to your daily routine

This is where Neutrient D3+K2 comes in.

Vitamin D3 is the same form of vitamin D that the body makes in the skin following UVB exposure, while MK-7 is a form of vitamin K2 that has attracted research interest because it remains in the circulation for relatively longer.

Neutrient D3+K2 doesn’t need to add another step to your day. Simply take your daily serving with a spoonful of yoghurt or stir it into overnight oats, then enjoy it straight away. Pairing it with a breakfast that contains a little fat may help your body absorb these fat-soluble vitamins, and tying it to your usual morning routine makes it easier to remember.

Neutrient D3+K2 at a glance

Vitamin D3: 50 µg (2,000 IU) per daily serving
Vitamin K2 as MK-7: 50 µg per daily serving
Carrier: MCT oil with vitamin E, suited to these fat-soluble nutrients
Free from: sweeteners, flavourings, alcohol and glycerin; suitable for vegans

Supporting your vitamin D intake through winter

The clocks going back can feel like an announcement that summer is officially over. There may be fewer opportunities for long evenings outdoors, but there are still plenty of ways to look after yourself as the seasons change.

Keep moving, get outside in daylight when you can, eat a varied diet and pay attention to nutrients that become harder to obtain from sunlight alone.

For vitamin D, waiting until the depths of winter to think about intake may mean overlooking an easy opportunity to plan ahead. Neutrient D3+K2 offers a straightforward way to include both nutrients in a daily routine, supporting the body's normal vitamin D and vitamin K functions as the days grow shorter.

The clocks will go forward again in spring. Until then, a little preparation now can help keep your nutritional routine on track.

Frequently Asked Questions

Which foods contain vitamin K2?

Vitamin K2 is found in fermented foods, such as natto, a traditional Japanese food made from fermented soybeans, as well as in some cheeses and animal-derived foods. The amounts vary considerably depending on the food and how it is produced.

Can you take vitamin D3 all year round?

Yes, vitamin D supplements can be taken throughout the year when needed. In the UK, year-round supplementation may be particularly relevant for people who spend little time outdoors, cover most of their skin when outside, or have darker skin. The appropriate dose depends on individual circumstances and total vitamin D intake.

When should you take a vitamin D3 and K2 supplement?

Choose a time that fits comfortably into your daily routine and follow the product directions. Consistency is generally more useful than choosing a particular time of day.

Written by: Jacqueline Newson BSc (Hons) Nutritional Therapy

References

NHS. Vitamin D. Advice on vitamin D supplementation during autumn and winter. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
NIH Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
NIH Office of Dietary Supplements. Vitamin K: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/VitaminK-HealthProfessional/
EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific opinion on the tolerable upper intake level of vitamin D. EFSA Journal. 2012;10(7):2813.
Theuwissen E, Smit E, Vermeer C. The role of vitamin K in soft-tissue calcification. Advances in Nutrition. 2012;3(2):166-173.
Aguayo-Ruiz JI, et al. Effect of supplementation with vitamins D3 and K2 on undercarboxylated osteocalcin and insulin serum levels in patients with type 2 diabetes mellitus: a randomized, double-blind, clinical trial. Diabetology & Metabolic Syndrome. 2020.