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If you live in the UK and you're not supplementing vitamin D, the odds are you're deficient for roughly six months of the year. The NHS defines deficiency as below 25 nmol/L. Functional medicine practitioners and most sports scientists put the optimal range at 100-150 nmol/L.
The gap between "not deficient" and "optimal" is enormous, and most men are sitting somewhere in the bottom half of it wondering why their energy is poor, their mood is flat, and their strength gains have stalled.
Vitamin D3 is part of the fix. But taking D3 without K2 is a half-measure that creates its own problems, specifically around where calcium ends up in your body. Most supplement guides skip this part. This one won't.
My first vitamin D test came back at 42 nmol/L, technically "sufficient" by NHS standards, nowhere near optimal. I started supplementing 4,000 IU daily with K2. Three months later I was at 118 nmol/L. The subjective change was real: better sleep, steadier mood, noticeably more energy in the mornings. Could have been placebo. Could have been the dozen other things I was doing. But the data was clear enough that I haven't stopped since.
The UK Vitamin D Problem
The UK sits between approximately 50 and 60 degrees north latitude. For around 6-7 months of the year (typically October to April), the sun angle is too low for UVB rays to penetrate the atmosphere effectively enough to trigger significant vitamin D synthesis in skin.
Even in summer, most people with desk jobs aren't getting meaningful sun exposure during peak hours. Add in sunscreen use, darker skin tone (which requires longer sun exposure for the same synthesis), and the picture gets bleaker.
The NHS recommends 400 IU daily for the general population. This dose is sufficient to prevent severe deficiency but will not get most people to optimal blood levels, particularly if starting from a low baseline.
Why Vitamin D Matters for Men Specifically
Vitamin D functions more like a hormone than a vitamin. It interacts with receptors in virtually every tissue in the body, including Leydig cells in the testes, the cells responsible for testosterone production.
Beyond testosterone, adequate vitamin D is associated with improved muscle function, immune regulation, mood stability, and reduced all-cause mortality. In men over 40, where multiple systems are simultaneously under pressure, getting vitamin D right removes a significant source of drag on everything else.
Why K2 Matters: The Calcium Traffic Problem
Vitamin D dramatically increases calcium absorption from the gut. This is mostly a good thing, it's why vitamin D is essential for bone health. But there's a problem: absorbed calcium needs to go somewhere specific (bones, teeth) and avoid somewhere specific (arteries, soft tissue).
Vitamin K2 (specifically the MK-7 form) activates two key proteins:
Osteocalcin: binds calcium into bone matrix. Without sufficient K2, osteocalcin is inactive and calcium doesn't get incorporated into bone effectively.
Matrix Gla Protein (MGP): the most potent known inhibitor of arterial calcification. Without sufficient K2, MGP is inactive and calcium can deposit in arterial walls.
Taking high-dose vitamin D without K2 increases calcium absorption but doesn't ensure it goes where it's supposed to. Over time, this can contribute to arterial stiffness.
Vitamin D gets calcium into your body. Vitamin K2 directs it to your bones and away from your arteries. They work as a system. Supplementing one without the other is a half-measure at best.
D3 vs D2: Which Form to Take
Always D3 (cholecalciferol). It's the form your skin synthesises from sunlight and is significantly more effective at raising blood levels than D2 (ergocalciferol).
K2 Forms: MK-4 vs MK-7
K2 comes in several forms. MK-7 (menaquinone-7) is the one to look for:
MK-7 has a long half-life in the blood (days), meaning a single daily dose maintains stable levels. It's derived from fermented foods (traditionally natto). Better evidence for cardiovascular benefit.
MK-4 has a very short half-life (hours), requiring multiple daily doses to maintain levels. Some research suggests advantages for bone health specifically.
For a single daily supplement, MK-7 is the practical choice.
Optimal Doses
Vitamin D3: For maintenance in someone already at good levels, 2,000 IU daily is a reasonable starting point. For men building from a deficient baseline, 4,000 IU daily for 3 months (followed by retesting) is more appropriate. Doses above 10,000 IU daily long-term require medical supervision.
Vitamin K2 (MK-7): 100-200 mcg daily alongside D3 supplementation. This is higher than older recommendations, newer evidence supports the higher end.
Fat intake: both D3 and K2 are fat-soluble. Take them with the meal that contains the most fat for optimal absorption.
The thing about vitamin D testing that frustrates me: GPs rarely test for it unless you ask specifically. When I ask, I often have to argue for it. It costs pennies as part of a private panel. If you haven't tested in the past 12 months, add it to your next bloodwork order. You cannot meaningfully optimise the dose without knowing your baseline.
What to Buy in the UK
For most men, a combination D3+K2 supplement in the 2,000-4,000 IU D3 / 100-200 mcg K2 range covers the bases. The Bulk option gives a higher K2 dose at good value. MyProtein's combination sits at a reasonable middle ground.
How to Know if It's Working
Test. That's the only way to know.
Start supplementing, then retest after 3 months. If you started at 42 nmol/L and you're now at 110 nmol/L, the dose is working. If you're at 55 nmol/L, increase the dose or check you're taking it with fat.
Target range for optimal function: 100-150 nmol/L. Above 250 nmol/L is the toxicity threshold, you'd need sustained very high doses to get there.
You can check vitamin D as part of a broader male health panel through services like Medichecks, who include it in their standard male health screens. My full guide to bloodwork for men over 40 covers exactly what to test and what to do with the results.
Where This Fits in the Stack
Vitamin D3+K2 sits alongside magnesium as one of the highest-priority supplementation decisions for men over 40. Both are cheap, widely deficient in the UK population, and have outsized effects on testosterone, sleep, and metabolic health when corrected.
Zinc rounds out the trifecta of micronutrients most commonly deficient in men with low-normal testosterone. If your diet isn't heavy in oysters, red meat, and pumpkin seeds, it's worth testing.
Get your levels. Supplement what's low. Retest. That's the whole protocol.
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