Vitamin D technically isn't even a vitamin but a prohormone – a precursor to a hormone. And that's exactly why it has such far-reaching effects on your body. For athletes, it's especially relevant: it directly influences muscle strength, immune function, and recovery.
The Problem: Chronic Undersupply
In Central and Northern Europe, your body can only synthesize Vitamin D through the skin from approximately May to September – and only between about 10:00 AM and 3:00 PM under direct UVB radiation. During the remaining months, sun intensity is too low. Through diet, you can only cover about 20% of your needs (fatty fish, egg yolk, mushrooms).
This means: the majority of the population in our latitudes is undersupplied in winter. For athletes who train mostly indoors, the risk is even higher.
Symptoms of Vitamin D Deficiency
A deficiency often develops gradually and non-specifically – which is why it's frequently overlooked:
• Frequent infections – Vitamin D is central to immune defense
• Fatigue and exhaustion – chronic, even with sufficient sleep
• Muscle pain and weakness – especially relevant for athletes
• Bone and joint pain – back and hips commonly affected
• Mood swings/depressive mood – especially in winter (winter blues)
• Slower wound healing
• Hair loss – can be a sign, but many other causes possible
Why Athletes Are Particularly Affected
1. Increased demand – Intense training stresses the immune system and increases Vitamin D needs
2. Indoor training – Those who train in the gym rather than outdoors get less sun
3. Muscle strength – Vitamin D directly influences muscle contraction. A deficiency can measurably reduce your strength numbers
4. Injury prevention – Sufficient Vitamin D supports bone health and can reduce stress fracture risk
Reference Values: What's Normal?
• Severe deficiency: < 20 ng/ml (50 nmol/l) – medical treatment recommended
• Insufficiency: 20–29 ng/ml (50–72 nmol/l) – supplementation advisable
• Normal: 30–50 ng/ml (75–125 nmol/l) – optimal range
• Elevated: > 50 ng/ml (125 nmol/l) – not necessary, in exceptions unproblematic
• Toxic: > 150 ng/ml (375 nmol/l) – dangerous
Important: A blood test (25-hydroxyvitamin D) is the only reliable method to determine your status. Symptoms alone aren't sufficient for diagnosis.
Dosage: How to Supplement Correctly?
Examine.com recommends 2,000 IU (50 μg) per day as a starting value for confirmed deficiency – offering a good balance of efficacy and safety. A follow-up blood test should be done after 2–3 months.
### Practical Tips
• Vitamin D3 (cholecalciferol) is superior to D2 – it raises blood levels more effectively
• Take with fat – Vitamin D is fat-soluble, so best taken with a meal containing fat
• Combine with K2 – Vitamin K2 (MK-7, 100–200 μg) ensures calcium is built into bones, not deposited in blood vessels. More here: supplement combinations
• Maximum safe dose: 4,000 IU/day as the upper limit for most people. Higher doses only under medical supervision
The Best Vitamin D Sources
Natural sources (difficult to meet needs):
• Fatty fish (salmon, mackerel, herring) – the best food source
• Cod liver oil – extremely rich, but not for everyone
• Egg yolk – contains Vitamin D, but in small amounts
• Mushrooms (grown under UV light) – the only relevant plant source
Supplementation (recommended October–March):
• Vitamin D3 drops – easy to dose, good absorption through oil base
• Vitamin D3 capsules – practical, often combined with K2
• Vitamin D3 tablets – cheapest option
Conclusion
Vitamin D is not a "lifestyle supplement" but, in our latitudes, a necessary addition – especially for athletes from October to March. 2,000 IU Vitamin D3 daily, ideally with K2 and a fatty meal. Have your blood levels checked at the doctor once a year. The investment of a few cents per day can measurably improve your muscle strength, immune function, and mood.