Vitamin D3 + K2
Essential for testosterone synthesis: most UK men are deficient, especially in winter.
What is Vitamin D3 + K2?
Vitamin D3 (cholecalciferol) is technically a hormone precursor rather than a vitamin. It is produced in the skin when exposed to UVB radiation, which is effectively absent in the UK from October to March. Vitamin K2 (MK-7 form) works synergistically, directing calcium to bones rather than arteries: important when supplementing D3 at meaningful doses.
What does the evidence say?
A 2011 randomised controlled trial by Pilz et al. in Hormone and Metabolic Research found men supplementing 3,332 IU of vitamin D3 daily for 12 months had 25.2% higher testosterone than the placebo group. A 2010 population study of 2,299 men (Wehr et al.) found a significant positive correlation between vitamin D status and testosterone levels. The NHS estimates that around 1 in 5 people in the UK have low vitamin D levels, rising to over 40% in darker-skinned individuals.
Multiple high-quality randomised controlled trials with consistent results and independent replication.
Dosage guide
| Effective dose | 2,000–4,000 IU of D3 per day for most UK adults in winter |
| Maximum dose | 10,000 IU/day (NHS upper safe limit); 4,000 IU is a reasonable cap without testing |
| Timing | With a fat-containing meal. D3 is fat-soluble. Morning or midday is preferable to avoid potential sleep disruption. |
Best form to buy
D3 (cholecalciferol) not D2 (ergocalciferol). D3 is significantly more effective at raising serum 25(OH)D levels. Pair with K2 as MK-7 (not MK-4) for cardiovascular and bone benefits. Oil-based softgels absorb better than dry tablets.
Who benefits most
Essentially every UK man, particularly from October to April. Men with darker skin tones, office workers, and those over 50 are at higher risk of deficiency and benefit most.
Side effects and safety
Safe at recommended doses. Toxicity (hypercalcaemia) is possible at very high doses over extended periods but rare below 10,000 IU/day. If supplementing long-term at higher doses, get a blood test (25-OH vitamin D).
“Non-negotiable for UK men in winter. This isn't a performance supplement, it's a basic health requirement. Get tested, a private vitamin D test costs about £25. If you're below 75 nmol/L, fix it.”