Roughly one billion people worldwide have insufficient vitamin D levels, and most of them don't know it. The symptoms are vague enough to be blamed on stress, age, or "just winter." Here is what's worth taking seriously, what to test, and how to fix it.
The most common symptoms
1. Persistent fatigue
Not "I had a bad night" tired — tired despite sleep, tired in the morning, tired by 3 p.m. Vitamin D is involved in mitochondrial function; chronic deficiency reduces cellular ATP production.
2. Bone and lower-back pain
Aching long bones, dull lumbar pain unrelated to a strain, sensitivity when pressing on the sternum or shins. Vitamin D regulates calcium absorption; without it, bone mineralisation suffers (osteomalacia in adults, rickets in children).
3. Muscle weakness and cramps
Difficulty climbing stairs, slow rise from a chair, frequent calf cramps. Severe deficiency causes proximal muscle weakness — the kind doctors test by asking you to stand from squatting without using your hands.
4. Low mood, seasonal depression
Vitamin D receptors are present in brain regions involved in mood regulation. Multiple meta-analyses link low 25-OH-D to depressive symptoms, particularly in winter.
5. Frequent infections
More colds than peers, slow recovery, chronic sinusitis. Vitamin D modulates innate immunity and the antimicrobial peptide cathelicidin.
6. Slow wound healing & hair thinning
Less specific, but reported in clinical deficiency. Hair regrowth after correction is well-documented in alopecia areata patients with low D.
The only test that matters
25-hydroxyvitamin D, also written 25-OH-D or calcidiol. Blood draw, no fasting required, ~€20–€40 in most labs. Do not request "1,25-dihydroxyvitamin D" — that's the active form, but the body keeps it in normal range until deficiency is severe, masking the problem.
Reading your result
| 25-OH-D level | Status | Action |
|---|---|---|
| < 12 ng/mL (30 nmol/L) | Severe deficiency | Medical correction (loading dose) |
| 12–20 ng/mL | Deficiency | Supplementation, retest in 3 months |
| 20–30 ng/mL | Insufficiency | Increase sun + diet, possibly supplement |
| 30–60 ng/mL | Optimal | Maintain |
| > 100 ng/mL | Risk of toxicity | Stop supplementing, retest |
How to correct it
- Sun first. 10–30 minutes of midday sun on bare arms and legs, 2–3 times a week, restores most people in summer (skin type and latitude dependent — Sun Day calculates this for you).
- Diet, secondary. Fatty fish (salmon, mackerel, sardines), egg yolks, fortified milk. Realistically you cannot eat your way to optimal levels.
- Supplementation, when needed. Cholecalciferol (D3) is preferred over D2. Typical maintenance dose: 800–2000 IU/day. Severe deficiency: medical loading regimen, not self-prescribed.
- Take with fat. Vitamin D is fat-soluble; absorption doubles when taken with a meal containing fat.
How Sun Day fits in
Sun Day estimates your daily synthesised vitamin D from real UV-B reaching your skin (skin type, exposed area, time of day, altitude, cloud cover). It tells you when you've had enough sun to cover the day's need — and when you haven't, so supplementation can fill the gap.
Track what your skin actually makes
Stop guessing. Get a daily vitamin D estimate based on real UV data.