Between October and March, the UK sun is too weak for most people to make enough vitamin D through skin alone. That matters for athletes: vitamin D supports bone health, immune function, and muscle strength — yet deficiency is common in indoor trainers who work office jobs.
What the NHS recommends
The NHS advises 10 µg (400 IU) of vitamin D daily for everyone over 4 during autumn and winter. Some groups need it year-round — including people who cover skin, have darker skin tones, or spend little time outdoors.
Higher-strength sports products (1,000–4,000 IU) are widely sold; if you use them, avoid doubling up with multivitamins without checking total intake.
D3 vs D2 — and why K2 appears on labels
- Vitamin D3 (cholecalciferol) — preferred form; better at raising blood levels
- Vitamin D2 (ergocalciferol) — plant-derived; less potent per unit
- Vitamin K2 — often paired with D3; supports calcium routing to bone rather than soft tissue
Signs you might be low (but test if unsure)
- Persistent fatigue unrelated to sleep debt
- Frequent illness during heavy training blocks
- Bone or muscle aches without clear injury
A blood test (25-OH vitamin D) is the only reliable check — ask your GP if symptomatic.
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Building a starter stack? Read Your First Supplement Stack. Shop vitamin D and vitamins & minerals.
Do not exceed 100 µg (4,000 IU) daily unless under medical supervision. Vitamin D is fat-soluble — more is not always better.