Vitamin D and Fatigue
Fatigue is one of the most common symptoms associated with vitamin D deficiency in observational studies, and one of the outcomes patients most hope supplementation will fix. The reality is nuanced — correction helps some people, particularly the severely deficient, and doesn't help many others.
The ViDiFatiCS trial
Nowak 2016 randomised 120 adults with self-reported fatigue and 25(OH)D < 20 ng/mL to either 100,000 IU cholecalciferol bolus or placebo. At 4 weeks, the vitamin D group had significantly greater fatigue reduction on validated fatigue scales. This trial specifically enrolled the deficient — the population most likely to respond.
Meta-analyses
2020 Cochrane-style pooled analysis of RCTs of vitamin D for fatigue: modest benefit overall, with effect size larger in patients starting below 20 ng/mL and negligible in those starting above 30 ng/mL. Consistent with the general pattern of vitamin D trials.
Chronic fatigue syndrome / ME/CFS
Vitamin D deficiency is common in ME/CFS cohorts, but supplementation trials have not shown reliable disease-modifying benefit. Correcting deficiency is reasonable general care; ME/CFS specific management is a specialist area with distinct interventions (pacing, symptom-directed therapy).
Fatigue causes not fixed by vitamin D
- Poor sleep hygiene or untreated sleep apnoea.
- Iron-deficiency anaemia (especially in menstruating women; see our iron page).
- Untreated hypothyroidism (check TSH).
- Depression or anxiety.
- Overtraining or under-recovery in athletes.
- Occult chronic disease (diabetes, kidney disease, cancer).
- Medications with fatigue as side effect (beta-blockers, antihistamines, statins).
Practical approach
- Persistent fatigue > 1 month: check 25(OH)D, TSH, CBC (for anaemia), ferritin, B12, glucose.
- Vitamin D < 20 ng/mL: correct with 2,000–4,000 IU/day for 8–12 weeks; expect partial improvement in a subset of patients.
- Vitamin D > 30 ng/mL and still fatigued: look elsewhere; more vitamin D won't help.
- Persistent unexplained fatigue: comprehensive medical workup, not another supplement.