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Simon Parrott – Vitacuity Ltd
Vitamin D deficiency is extremely common in the UK and may be a hidden cause of persistent tiredness. Your body uses vitamin D to power the energy-producing structures inside muscle cells, and when levels drop, these energy factories work less efficiently. Vitamin D also helps control your immune system, and deficiency can leave it in a constant state of low-grade inflammation, draining your energy further.
Research shows promising results: over 84% of otherwise healthy women with fatigue had low vitamin D, and restoring their levels improved their tiredness. Studies measuring energy production directly in muscle tissue found it improved when vitamin D was restored. People with long COVID or chronic fatigue syndrome showed particularly striking improvements, with 82% no longer meeting diagnostic criteria after vitamin D treatment. However, some well-designed studies found no benefit, and the effect depends on actually being deficient to start with. If your vitamin D is already adequate, more supplementation won’t necessarily help.
Daily doses of 2,000 to 4,000 IU of vitamin D3 are both safe and effective for winter supplementation in the UK, and far cheaper and safer than staying deficient. Iron and other nutrients also matter for energy levels, so if you’re seriously tired, it’s worth checking multiple factors with your doctor. But if you’re persistently exhausted, vitamin D supplementation is one of the most practical, low-cost first steps to try.
Verdict: Restoring vitamin D to normal levels if you’re deficient may meaningfully improve fatigue, though results depend on your starting point and individual factors like iron status.
You probably blame your tiredness on a bad night’s sleep, too much screen time, or just the relentless pace of modern life. And maybe that’s partly right. But what if there’s a quieter, more biological reason you’re dragging yourself through the afternoons, one that a simple supplement could help address? What if the exhaustion you’ve been normalising is, at least in part, your cells running out of something they fundamentally need to produce energy?
Vitamin D deficiency is one of the most common nutritional problems in the UK, yet it remains one of the most overlooked explanations for persistent fatigue. Vitacuity reviewed over 1.77 million research papers and identified the most relevant studies on this connection. The picture that emerges is genuinely interesting, not without nuance, but compelling enough that anyone feeling chronically tired should take it seriously.
Vitamin D isn’t really a vitamin. It’s a hormone, one your body makes when sunlight hits your skin, and which every cell in your body has receptors for. That’s a clue to how fundamental it is.
When vitamin D levels drop, one of the things that suffers is your mitochondria, the tiny structures inside your cells that convert food into usable energy (ATP). Think of mitochondria as your body’s power stations. Research published in *Clinical Endocrinology* found that vitamin D-deficient adults showed measurably impaired mitochondrial oxidative function in skeletal muscle, and that correcting their vitamin D levels improved it [13]. In plain English: when you’re low in vitamin D, your muscles’ ability to generate energy is compromised. That’s not a metaphor for feeling a bit flat. It’s a measurable biological dysfunction.
There’s an immune angle too. A detailed 2014 review explained that vitamin D deficiency disrupts the production of cathelicidin, an antimicrobial peptide involved in how your immune system clears out infections and cellular debris. Without it, your immune system runs hot and inefficiently, leading to chronic low-grade inflammation. The result? Persistent fatigue, as your body diverts energy into a perpetually activated immune response [6].
So when researchers talk about vitamin D and fatigue, they’re not talking about a vague correlation. There are credible, plausible biological mechanisms at work.
A 2020 prospective study of 1,200 otherwise healthy Indian women presenting to primary care with fatigue found that 84.8% had low vitamin D levels [12]. These weren’t women with chronic disease, autoimmune conditions, or obvious explanations for their exhaustion, they were otherwise healthy people whose main complaint was feeling tired. After receiving cholecalciferol therapy (60,000 IU weekly for five weeks), their fatigue assessment questionnaire scores improved significantly [12].
Evidence grade: Promising, This was a large observational study without a placebo control group, so we can’t rule out the role of expectation or other lifestyle changes. But the scale of the finding (over a thousand participants, 84.8% with low vitamin D) is striking.
One of the most mechanistically interesting studies came from a 2013 longitudinal investigation using magnetic resonance spectroscopy, a technique that allows researchers to watch mitochondria working in real time, inside living muscle tissue [13]. Vitamin D-deficient adults who were treated with cholecalciferol showed improved mitochondrial oxidative phosphorylation after treatment. The study also reported reductions in muscle cramps and fatigue symptoms alongside the biochemical improvements.
Evidence grade: Promising, Small sample, but this study used objective measurement of cellular energy production rather than self-reported symptoms. That makes the findings particularly meaningful, even with the sample size caveat.
A double-blind, randomised placebo-controlled trial, the gold standard of clinical research, examined the effect of vitamin D3 supplementation on self-perceived fatigue in vitamin D-deficient adults [8]. Participants receiving vitamin D3 reported significant improvements in fatigue compared to placebo.
Evidence grade: Promising, This was a controlled trial specifically targeting fatigue as the primary outcome. However, the full detail of sample size and dose is not available from the abstract alone, which limits how strongly we can interpret the findings.
Two recent studies, including a 2025 retrospective study and a 2026 RCT, looked specifically at people who developed myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) following COVID-19 infection or vaccination [1][2].
The 2025 retrospective study is particularly striking. Of 28 patients who developed ME/CFS after COVID vaccination, 27 had insufficient or deficient vitamin D at their first visit (mean level: 16 ± 4 ng/mL, well below the recommended threshold) [2]. After vitamin D replacement therapy combining dietary guidance, sun exposure recommendations, and oral supplementation, mean vitamin D levels rose to 28 ± 5 ng/mL. ME/CFS diagnostic symptoms dropped dramatically, from an average of 10.3 ± 2.1 symptoms to just 3.3 ± 2.0. Crucially, 23 of 28 patients (82%) no longer met ME/CFS diagnostic criteria after treatment. Sleep problems showed the greatest improvement (71%), followed by autonomic symptoms (68%) [2].
The 2026 RCT [1] explored a similar population, people with ME/CFS following COVID-19 or vaccination, in a properly randomised design.
Evidence grade: Promising, moving toward strong for this specific population. The retrospective study [2] has no control group, which means we can’t rule out natural recovery. But the 82% response rate, combined with the incoming RCT data [1], makes this one of the more compelling areas of active vitamin D research. The researchers themselves note they are preparing further controlled trials.
A 2025 pilot study directly compared supplementation doses of 1,000, 2,000, 4,000 and 8,000 IU in healthy young adults over two winter seasons [3]. Key findings:
– 1,000 IU/day raised vitamin D levels to the recommended range (above 75 nmol/L), but levels dropped back below this after a 30-day break. – 2,000 IU/day maintained levels within the recommended range even after a break in supplementation. – 4,000 IU/day produced a more rapid rise and maintained levels above 100 nmol/L, a threshold the researchers associated with broader health benefits. – 8,000 IU/day showed a more pronounced rise and fall, suggesting diminishing returns at very high doses rather than sustained benefit.
The study concluded that 2,000 IU is a solid daily maintenance dose for winter, while 4,000 IU may offer additional benefit for those seeking optimal (not just adequate) levels [3].
Evidence grade: Promising, Pilot study with 35 participants. But the dose-response data aligns with established pharmacokinetics and provides genuinely useful practical guidance.
Honesty matters here. A 2026 cross-sectional study of 228 adults found no significant correlation between serum vitamin D levels and fatigue severity scores [4]. In the same study, the strongest predictors of fatigue were haemoglobin and ferritin levels, suggesting iron status may be an equally or more important driver of fatigue in some populations.
Evidence grade: Conflicted. This study excluded people with chronic disease, medication use, and prior supplementation, meaning it was looking at a relatively healthy, non-deficient population. The other studies reviewed here tended to enrol people with established vitamin D deficiency or clinical fatigue conditions. This distinction matters enormously. If your vitamin D is already adequate, topping it up further may not reduce fatigue. If you’re genuinely deficient, as the majority of the UK population is during winter months, restoring levels to normal may make a meaningful difference. These findings don’t contradict each other; they’re asking slightly different questions.
A parallel-group, double-blind, randomised placebo-controlled trial published in 2015 gave chronic fatigue syndrome patients either 100,000 IU of oral vitamin D3 or a matching placebo every two months for six months [7]. The result? No significant improvement in the Piper Fatigue Scale scores (adjusted treatment effect 0.2 points; 95% CI -0.8 to 1.2; p = 0.73). No improvement in vascular outcomes either.
Evidence grade: Conflicted. Here’s what’s important to understand. The supplementation protocol was intermittent megadosing (100,000 IU every two months), not daily supplementation, a very different physiological pattern than consistent daily dosing. The mean baseline vitamin D in participants was 46 nmol/L, which, while below optimal, is not the severe deficiency (often below 25 nmol/L) seen in studies that did show improvement. With 50 participants, the study was also underpowered to detect modest effects. Taken together, this study tells us that intermittent high-dose vitamin D in moderately deficient CFS patients doesn’t dramatically move the needle, it doesn’t tell us that daily supplementation in genuinely deficient individuals won’t help.
The research on vitamin D and fatigue is genuinely interesting, but it’s not a closed case. Here’s where the gaps are:
Most studies are small. Even the positive trials tend to involve dozens, not hundreds or thousands, of participants. The dose-response pilot study had just 35 volunteers [3]. The ME/CFS retrospective study had 28 patients [2]. We need larger, longer, better-powered trials to be confident about effect sizes.
Confounding is a real issue. People who are vitamin D deficient are often also sedentary, spend less time outdoors, have poorer diets, and may be dealing with other deficiencies simultaneously, iron being a notable example [4]. It’s genuinely difficult to isolate vitamin D’s role from this cluster of factors.
Dose and delivery protocol remain unresolved. The 2015 RCT [7] used intermittent megadosing and found nothing. The 2025 dose-response study [3] found that 2,000–4,000 IU daily works better than erratic high doses. But we don’t yet have large RCTs in fatigued, deficient populations using optimal daily dosing protocols.
The CFS and long COVID findings are early. The 82% response rate in the 2025 retrospective study [2] is attention-grabbing, but without a control group, natural recovery can’t be excluded. The 2026 RCT [1] should provide much clearer answers once its full data is published.
We don’t fully understand the causal direction. Does low vitamin D cause fatigue? Or do fatigued, unwell people go outside less and therefore make less vitamin D? The biological mechanisms from the mitochondrial research [13] and immune pathway review [6] suggest genuine causality, but observational data alone can’t prove it.
Vitamin D is not the only factor. The 2026 cross-sectional study [4] is a useful reminder that fatigue is multi-factorial. Haemoglobin and ferritin were stronger predictors of fatigue severity than vitamin D in that healthy population. A broader nutritional view, reviewed in a 2023 nutrient therapy analysis that identified 50 studies with beneficial findings [9], suggests that fatigue often reflects multiple deficiencies, not just one.
So what would a sensible, well-read friend actually tell you to do with all of this?
First, accept the UK reality. In the UK, meaningful sun exposure for vitamin D synthesis is essentially impossible between October and April, and often unreliable even in summer. Deficiency is the norm, not the exception. The NHS recommends everyone consider supplementing in winter. The research here suggests that’s the bare minimum, not the ceiling.
Second, supplement confidently. Vitamin D is fat-soluble, but toxicity at normal supplementation doses is genuinely rare. The dose-response research [3] shows that 2,000 IU/day is sufficient to maintain levels in the recommended range through winter, while 4,000 IU/day achieves higher optimal levels. These are well within established safe ranges. You’re not going to tip into toxicity at 2,000–4,000 IU/day, the risk of staying deficient is far greater than the risk of supplementing at these doses.
Third, if you’re persistently fatigued, don’t assume one cause. The 2026 cross-sectional study [4] was a useful reminder: if you’re seriously tired, low ferritin and haemoglobin may be an equally important, or even more important, piece of the puzzle. These require a blood test to assess properly. But vitamin D supplementation is cheap, safe, and practically guaranteed to help if you’re deficient, which, statistically, you probably are.
Fourth, don’t expect miracles if you’re not deficient. The evidence consistently points to improvement when genuinely low levels are restored to normal. If your vitamin D is already adequate, adding more is unlikely to transform your energy levels. The benefit is in correcting deficiency, not megadosing beyond it.
Practically speaking: Take 2,000–4,000 IU of vitamin D3 daily, particularly from October through April. Take it with your fattiest meal of the day, it’s fat-soluble and absorbs far better this way. If you’ve been feeling persistently exhausted, it’s also worth asking your GP to check ferritin, haemoglobin, and vitamin D, not because you need a test to justify supplementing vitamin D (you don’t), but because it will help you understand the fuller picture of what’s driving your fatigue.
This is one of the most practical, low-cost, low-risk interventions in the fatigue space. The evidence isn’t flawless, but it’s solid enough, the mechanism is credible, the cost is negligible, and the downside risk of supplementing at normal doses is essentially zero.
Restore your levels. Give it eight to twelve weeks. Then see how you feel.
[1] Vitamin D in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome After COVID-19 or Vaccination: A Randomized Controlled Trial (2026). DOI: 10.3390/nu18030521 | https://pubmed.ncbi.nlm.nih.gov/41683343/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12899809/
[2] Efficacy of vitamin D replacement therapy on 28 cases of myalgic encephalomyelitis/chronic fatigue syndrome after COVID-19 vaccination (2025). DOI: 10.1016/j.nut.2025.112718 | https://pubmed.ncbi.nlm.nih.gov/40090177/
[3] Comparison of Vitamin D3 Supplementation Doses of 1,000, 2,000, 4,000 and 8,000 IU in Young Healthy Individuals (2025). DOI: 10.21873/invivo.13848 | https://pubmed.ncbi.nlm.nih.gov/39740917/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11705152/
[4] Association of serum 25-hydroxyvitamin D levels with fatigue severity in adults: A cross-sectional study (2026). DOI: 10.14744/nci.2026.09623 | https://pubmed.ncbi.nlm.nih.gov/42516797/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC13402490/
[5] Chronic cholecalciferol supplementation and adequate vitamin D status are associated with reduced risk of Long COVID (2026). DOI: 10.1007/s40618-026-02858-8 | https://pubmed.ncbi.nlm.nih.gov/41870866/
[6] Review: Vitamin D3 deficiency results in dysfunctions of immunity with severe fatigue and depression in a variety of diseases (2014). https://pubmed.ncbi.nlm.nih.gov/24425848/
[7] Effect of intermittent vitamin D3 on vascular function and symptoms in chronic fatigue syndrome, a randomised controlled trial (2015). DOI: 10.1016/j.numecd.2014.10.007 | https://pubmed.ncbi.nlm.nih.gov/25455721/
[8] Effect of vitamin D3 on self-perceived fatigue: A double-blind randomized placebo-controlled trial (2016). https://pubmed.ncbi.nlm.nih.gov/28033244/
[9] Nutrient Therapy for the Improvement of Fatigue Symptoms (2023). https://pubmed.ncbi.nlm.nih.gov/37432282/
[10] Effect of vitamin D3 supplementation in winter on physical performance of university students: a one-month randomized controlled trial (2023). DOI: 10.1080/15502783.2023.2258850 | https://pubmed.ncbi.nlm.nih.gov/37735799/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10540648/
[12] Can self-perceived easy fatigability be a predictor of vitamin D deficiency in young Indian women? (2020). DOI: 10.4103/jfmpc.jfmpc_862_19 | https://pubmed.ncbi.nlm.nih.gov/32318457/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7113952/
[13] Improving the vitamin D status of vitamin D deficient adults is associated with improved mitochondrial oxidative function in skeletal muscle (2013). https://pubmed.ncbi.nlm.nih.gov/23393184/
[14] The effects of vitamin D3 supplementation on serum total 25[OH]D concentration and physical performance: a randomised dose-response study (2013). https://pubmed.ncbi.nlm.nih.gov/23410885/
[15] Erratum: Effect of vitamin D3 on self-perceived fatigue: A double-blind randomized placebo-controlled trial: Erratum (2017). DOI: 10.1097/MD.0000000000006038 | https://pubmed.ncbi.nlm.nih.gov/31305701/ | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5279116/
This article is for informational purposes only and does not constitute medical advice. Food supplements should not be used as a substitute for a varied and balanced diet and healthy lifestyle. If you are pregnant, breastfeeding, taking medication or have a medical condition, consult your doctor before taking any supplement. These statements have not been evaluated by the Food and Drug Administration (FDA) or the Medicines and Healthcare products Regulatory Agency (MHRA). This product is not intended to diagnose, treat, cure, or prevent any disease.
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