Vitamin D is often called the “sunshine vitamin” — but it’s actually a steroid hormone that influences almost every cell in the body. And it’s one that the majority of people in northern Europe are significantly deficient in, especially during winter.

Why Is Vitamin D Unique?

Unlike most vitamins, the body synthesises vitamin D itself — via UVB radiation hitting the skin. The problem in Scandinavia is that between October and April, the sun’s angle is too low for UVB to reach the earth’s surface. The result: up to 80% of northern Europeans have insufficient vitamin D levels during winter.

Vitamin D and Training

  • 💪 Muscle function — VDR receptors in skeletal muscle regulate calcium influx during contraction. Deficiency causes muscle weakness and increased injury risk
  • 🧪 Testosterone — An RCT (Pilz et al., 2011) found that 3332 IU D3 daily increased testosterone by 25% in deficient men
  • 🛡️ Immune function — Intense training temporarily suppresses immunity. Vitamin D activates macrophages and T-cells to counteract exercise-induced immunosuppression
  • 🦴 Bone health — Required for calcium absorption. Deficiency raises stress fracture risk in runners and high-impact athletes
  • 💤 Sleep and mood — Low vitamin D is linked to higher depression risk and poorer sleep quality — both directly impacting recovery

Optimal Dosing

  • ☀️ Maintenance (May–September): 1,000–2,000 IU/day
  • ❄️ Winter (October–April): 2,000–4,000 IU/day
  • 📉 Confirmed deficiency (<50 nmol/L): 4,000–5,000 IU/day for 8–12 weeks, then reduce

Always choose vitamin D3 (cholecalciferol), not D2. Take it with a fat-containing meal for maximum absorption. Many combine it with vitamin K2 (MK-7) to direct calcium to the right tissues.

Summary

Vitamin D is more hormone than vitamin, and deficiency is the norm for people in northern climates. For active individuals the consequences are real: reduced muscle strength, lower testosterone, impaired immunity and higher injury risk. 2,000–4,000 IU of D3 daily through winter is one of the simplest, most cost-effective optimisations you can make for health and performance.