Vitamin D3 and K2: Why You Probably Need Both

July 06 2026, by SANUSq Research team
8 min reading time

Vitamin D3 and K2 for stronger bones and protected arteries

It's July. You've been outdoors. You feel like you're covered on vitamin D — and you probably aren't. Worse, even if you're supplementing, taking vitamin D3 on its own can quietly send calcium to the wrong places.

Written by SANUSq Research Team | Medically reviewed by Dr Ajay Kumar, MD
Published: 6 July 2026 | Updated: 11 July 2026

Vitamin D deficiency is consistently identified as one of the most prevalent nutritional shortfalls across Europe — and it doesn't vanish when the weather improves (Cashman et al., 2016). In summer it simply becomes easier to ignore. The reasons it persists are less obvious than most people expect, and they matter for far more than bone health alone.

Why summer sun doesn't guarantee enough vitamin D

Your skin only manufactures vitamin D when UVB radiation reaches it at a sufficient angle and intensity. Several ordinary, everyday factors quietly block or blunt that process — even at the height of summer.

Sunscreen. Sunscreen filters out UVB — the same wavelength the skin needs to synthesise vitamin D. Under experimental conditions, correctly applied sunscreen almost entirely blocks the skin's vitamin D production (Matsuoka et al., 1987). In everyday life the effect is smaller, because most people apply far less than the test amount — but diligent daily sun protection and reliable vitamin D status don't always go together.

Time of day. UVB only reaches the ground at a useful angle between roughly 10am and 3pm. The early-morning and late-afternoon sun most people actually catch on a workday produces almost none. A simple rule: if your shadow is longer than you are tall, the sun is too low to help.

Latitude. For anyone living above about 35°N — which covers most of Europe — UVB is too weak for meaningful vitamin D synthesis for several months of the year (Webb, Kline & Holick, 1988). Summer widens the window, but it stays narrow for anyone working indoors.

How much skin is exposed. The forearms and lower legs that summer clothing typically leaves bare produce far less vitamin D than the torso or back. Surface area matters considerably.

Skin tone. Melanin acts as a natural sunscreen. Darker skin tones need substantially longer UV exposure to produce the same amount of vitamin D (Clemens et al., 1982).

Stacked together, these factors mean a meaningful share of adults — especially those who work indoors, wear SPF, or live at higher latitudes — simply aren't producing enough vitamin D even in July. The symptoms of a shortfall are diffuse enough (fatigue, low mood, recurrent infections, muscle weakness) that they're routinely blamed on something else.

The only reliable indicator of vitamin D status is a serum 25(OH)D blood test. Sunshine in the forecast and colour on your shoulders are not substitutes.

The K2 problem that most vitamin D supplements ignore

Here's the part even people who supplement tend to miss: taking vitamin D3 on its own can solve one problem and quietly create another.

Vitamin D3 stimulates calcium absorption from the gut — that's the mechanism behind its bone benefits. But absorbed calcium then has to be directed: into bones and teeth, where it belongs, and away from arterial walls, kidneys and soft tissue, where it does harm. Think of D3 as opening the door for calcium, and K2 as the traffic controller deciding where it goes.

Vitamin K2 does that directing by activating two proteins: osteocalcin, which binds calcium into the bone matrix, and matrix Gla protein (MGP), the body's primary defence against arterial calcification (Schurgers et al., 2008). Without enough K2, extra calcium mobilised by D3 is absorbed but not properly routed. The long-running Rotterdam Study found that people with the highest dietary intake of menaquinone (K2) had a 57% lower risk of dying from coronary heart disease than those with the lowest intake (Geleijnse et al., 2004).

This is exactly why the SANUSq formula pairs the two. The MK-7 form of K2 we use has a much longer half-life than the shorter MK-4 form — MK-7 stays active in the bloodstream for around three days, versus only a few hours for MK-4, giving far more stable levels from a single daily dose (Schurgers et al., 2007).

How liposomal vitamin D3 and K2 work together — D3 for bones, muscles and immunity; K2 for bone density and healthy arteries

D3 without K2 increases calcium absorption but not calcium direction. K2 is the co-pilot, not an optional extra.

A note for statin users. Statins work by inhibiting the mevalonate pathway — the same metabolic route the body uses to make CoQ10, and one that also supplies a building block needed to convert vitamin K1 into K2. Statin-related CoQ10 depletion is well documented, and there is a plausible, actively researched mechanism by which statins may also lower vitamin K2. It's worth being aware of, and worth a conversation with your doctor. If you missed our CoQ10 article, the same pathway is why.

The product: why liposomal delivery matters here

Liposomal Vitamin D3+K2 uses the same phospholipid encapsulation as the rest of the SANUSq range — and it matters more than usual for these two nutrients. D3 and K2 are both fat-soluble, and fat-soluble vitamins are notoriously erratic in conventional tablets and capsules: how much you absorb depends heavily on the fat content of the meal you take them with, and varies widely as a result. Liposomal delivery sidesteps that by wrapping the vitamins inside phospholipid vesicles that the gut lining absorbs directly, for more consistent uptake.

Liposomal Vitamin D3+K2 — a powerful combination for bone and cardiovascular health

Liposomal Vitamin D3+K2 | D3+K2 3-pack bundle
See current promotions and bundle discount offers.

Also relevant: the Immune System bundle

Vitamin D is one of the central pillars of immune function — it helps regulate both innate and adaptive immunity and drives production of the body's own antimicrobial peptides, such as cathelicidin (Hewison, 2011). If immune resilience is your main priority alongside bone and cardiovascular health, the SANUSq Immune System bundle pairs Liposomal D3+K2 with complementary immune-support products at a bundle price.

Immune System bundle

Frequently asked questions

Can't I just get enough vitamin D from the sun in summer?

For many people, no. Sunscreen, time of day, latitude, how much skin is exposed and your skin tone all limit how much your body actually makes — and their effects stack. Plenty of people who spend time outdoors in July still test as deficient.

How do I know if I'm deficient?

The only reliable way is a serum 25(OH)D blood test. Symptoms of a shortfall — fatigue, low mood, frequent infections, muscle weakness — are too vague to diagnose on their own and are easily attributed to other causes.

Why do I need to take K2 with D3?

D3 increases how much calcium you absorb; K2 directs that calcium into bone and away from arteries. Taking D3 without adequate K2 raises calcium absorption without ensuring it ends up where it should. The two work as a pair.

What's the difference between MK-7 and MK-4?

Both are forms of vitamin K2. MK-7 has a far longer half-life — it stays active in the body long enough for once-daily dosing to give steady coverage, whereas MK-4 clears within hours. SANUSq uses MK-7.

I'm on statins — does this apply to me?

Possibly. Statins act on the mevalonate pathway, which the body also uses to make CoQ10 and to help produce K2, so statin users may be lower in these nutrients. It's worth discussing with your doctor.

Is it safe to take D3 and K2 together every day?

For most healthy adults, yes — they're designed to be taken together, and K2 helps ensure the calcium mobilised by D3 is handled correctly. If you take anticoagulant medication such as warfarin, speak to your doctor first, as vitamin K interacts with these drugs.