Vitamin D and Fatigue: Is There a Connection?

You feel exhausted. You are sleeping enough, eating reasonably well, and your blood test came back “normal.” Yet the tiredness does not lift.

If your doctor did not specifically test your vitamin D level, this is worth asking about. Vitamin D deficiency is one of the most common and most consistently overlooked causes of persistent fatigue – and it is significantly underdiagnosed because it is simply not included in most standard blood panels by default.

An estimated 1 billion people worldwide have insufficient vitamin D levels. In northern latitudes, figures are even higher – with some national surveys showing deficiency in 40-60% of adults during winter months. Yet many of these individuals present to their doctors with fatigue, muscle weakness, low mood, and brain fog without ever receiving a vitamin D test.

The connection between vitamin D and energy is real, multimechanistic, and increasingly well-supported by clinical research. This article explains exactly how the relationship works – and what you can do about it.

What Is Vitamin D – and Why Is It Different From Other Vitamins?

Vitamin D is not a vitamin in the conventional sense. It is a fat-soluble steroid prohormone – a compound that the body converts into an active hormone (calcitriol, or 1,25-dihydroxyvitamin D) that regulates the expression of hundreds of genes across virtually every tissue in the body.

Vitamin D receptors (VDRs) are found in nearly every cell type – including muscle fibers, neurons, immune cells, bone cells, pancreatic beta cells, and the mitochondria of skeletal muscle. This extraordinary tissue distribution explains why vitamin D deficiency produces such a wide and seemingly unrelated constellation of symptoms – including fatigue, muscle weakness, bone pain, depressed mood, impaired immunity, and poor concentration.

The body produces vitamin D primarily through UVB radiation from sunlight converting 7-dehydrocholesterol in the skin into previtamin D3. This is then hydroxylated first in the liver and then in the kidneys to produce the active form. The entire activation chain can be disrupted at multiple points – by insufficient sun exposure, liver disease, kidney disease, obesity (which sequesters vitamin D in fat tissue), or genetic variants in vitamin D metabolism enzymes.

Very few foods naturally contain meaningful amounts of vitamin D. Good dietary sources include fatty fish (salmon, mackerel, sardines), egg yolks, beef liver, and fortified dairy and plant milks – but dietary intake alone rarely meets adult requirements without supplementation or adequate sun exposure.

How Vitamin D Deficiency Causes Fatigue: The Mechanisms

The relationship between vitamin D and energy is not a single pathway. It operates through at least five distinct biological mechanisms – each of which, independently, can produce significant fatigue.

1. Mitochondrial Dysfunction in Skeletal Muscle

This is the most direct mechanism. Vitamin D receptors are expressed in skeletal muscle mitochondria – the organelles responsible for producing ATP, the molecule that powers every cellular function. When vitamin D is deficient, mitochondrial respiratory capacity in muscle cells is measurably reduced.

A landmark study found that vitamin D deficiency was directly associated with reduced mitochondrial oxidative phosphorylation in human skeletal muscle – alongside elevated oxidative stress markers and impaired antioxidant defense. This translates directly into the physical fatigue and muscle weakness characteristic of vitamin D deficiency: the muscles simply cannot produce enough ATP to sustain normal activity. Find out more about how oxidative stress impairs cellular energy production in this article on SVKHerbal.

2. Disrupted Serotonin and Dopamine Synthesis

Vitamin D regulates the expression of tryptophan hydroxylase 2 (TPH2) – the rate-limiting enzyme in serotonin synthesis in the brain. When vitamin D is low, TPH2 expression falls, serotonin production decreases, and the result is a neurochemical environment characterized by low mood, reduced motivation, and mental fatigue.

Vitamin D also influences dopaminergic neuron development and function by regulating tyrosine hydroxylase – the rate-limiting enzyme in dopamine synthesis. Low dopamine is associated with apathy, reduced reward motivation, poor concentration, and the particular flat, flat tiredness that distinguishes mood-related fatigue from simple sleep deprivation.

A 2025 systematic review confirmed that vitamin D deficiency is significantly associated with depression and depressive fatigue in adults, with supplementation producing measurable improvements in mood and energy in deficient individuals.

3. Chronic Low-Grade Inflammation

Vitamin D is a potent immunomodulator. Its active form, calcitriol, suppresses pro-inflammatory cytokine production – particularly TNF-alpha, IL-6, and IL-1 beta – by inhibiting the NF-kB signaling pathway. When vitamin D is insufficient, this anti-inflammatory brake is removed, allowing chronic low-grade systemic inflammation to persist unchecked.

Chronic inflammation drives fatigue through multiple pathways: elevated cytokines cross the blood-brain barrier, activating the brain’s immune cells (microglia) and producing “sickness behavior” – the deep tiredness, social withdrawal, reduced motivation, and cognitive slowing experienced during illness. In the context of vitamin D deficiency, this inflammatory state can persist at a subclinical level for years, producing chronic fatigue without obvious signs of illness.

4. Impaired Calcium Signaling and Neuromuscular Function

Vitamin D is essential for calcium homeostasis – it promotes calcium absorption in the gut and regulates its distribution across tissues. Calcium is not only a structural component of bone; it is the primary intracellular signaling ion for muscle contraction, neurotransmitter release, and nerve conduction.

When vitamin D is low, neuromuscular calcium signaling is impaired. Muscle fibers contract less efficiently. The nerve-muscle junction functions suboptimally. The result is the characteristic proximal muscle weakness – difficulty climbing stairs, rising from a chair, lifting the arms – that clinicians recognize as a hallmark of severe vitamin D deficiency, but which begins in milder form at deficiency levels well above the clinical threshold.

5. Disrupted Sleep Quality and Circadian Rhythm

Vitamin D receptors are expressed in the suprachiasmatic nucleus (SCN) – the brain’s master circadian clock. Vitamin D influences the expression of key clock genes including CLOCK and BMAL1, and modulates melatonin production pathways. When vitamin D is low, sleep architecture is disrupted – particularly deep NREM sleep and REM sleep – and the circadian regulation of cortisol, growth hormone, and inflammatory cytokines becomes erratic.

Studies consistently show that vitamin D deficient individuals report poorer sleep quality, shorter sleep duration, and higher rates of daytime sleepiness – independent of other factors. This creates a compounding fatigue loop: low vitamin D impairs sleep, impaired sleep worsens fatigue, and the reduced physical activity that results from fatigue further limits outdoor sunlight exposure and vitamin D synthesis.

Who Is Most at Risk of Vitamin D Deficiency and Fatigue?

Certain populations face significantly elevated risk of both vitamin D deficiency and its energy consequences:

• People with limited sun exposure – office workers, night shift workers, those living in northern latitudes, and anyone who spends most of the day indoors. UVB rays that synthesize vitamin D do not penetrate glass and are largely absent at latitudes above 35 degrees in winter months
• Older adults – skin efficiency at synthesizing vitamin D from sunlight declines by approximately 75% between ages 20 and 70. Kidney activation efficiency also falls with age
• People with darker skin tones – melanin reduces UVB absorption, requiring significantly longer sun exposure to produce equivalent vitamin D
• People with obesity – vitamin D is fat-soluble and becomes sequestered in adipose tissue, dramatically reducing its bioavailability in the circulation
• People with gastrointestinal disorders – Crohn’s disease, celiac disease, and gastric bypass all impair fat-soluble vitamin absorption, including vitamin D
• Pregnant and breastfeeding women – fetal demand and restricted sunlight during pregnancy compound deficiency risk
• Vegetarians and vegans – the richest dietary sources of vitamin D are animal products. Plant-based eaters relying solely on diet are at high risk
• People on certain medications – anticonvulsants, glucocorticoids, and some antifungals accelerate vitamin D catabolism

This population overlap matters. A 50-year-old woman working indoors in a northern city, following a mostly plant-based diet, who has experienced weight gain in recent years, may be experiencing four simultaneous risk factors for vitamin D depletion.

Recognizing Vitamin D Deficiency: The Full Symptom Picture

Vitamin D deficiency is frequently missed because its symptoms are non-specific and overlap with many other conditions. The classic presentation includes:

• Persistent fatigue – unresponsive to adequate sleep. Described as a heavy, pervasive tiredness that is present on waking and does not resolve through the day
• Muscle weakness – particularly proximal muscle weakness: difficulty rising from chairs, climbing stairs, lifting above the head. Caused by impaired mitochondrial ATP production and calcium signaling in muscle fibers
• Bone pain and joint aches – dull, diffuse aching in the lower back, hips, pelvis, and legs. In severe cases, this progresses to osteomalacia – actual softening of the bone matrix
• Depressed mood, anxiety, and emotional flatness – driven by impaired serotonin and dopamine synthesis and chronic neuroinflammation
• Brain fog and poor concentration – reduced cerebral oxygenation from low vitamin D-driven neurovascular changes, combined with neuroinflammatory effects
• Frequent infections – reduced immune cell regulation increases susceptibility to respiratory infections, particularly in winter when both vitamin D levels and infection rates peak simultaneously
• Hair loss – vitamin D receptors in hair follicle keratinocytes regulate the hair growth cycle. Deficiency disrupts this, producing diffuse telogen effluvium
• Slow wound healing – vitamin D is required for the production of antimicrobial peptides at wound sites and for inflammatory resolution

The critical point is that many of these symptoms – fatigue, low mood, muscle weakness, brain fog – can appear and persist for months or years before vitamin D deficiency is identified. They are often attributed to stress, poor sleep, aging, or depression – and treated accordingly, without resolution.

Find out more about the wider consequences of vitamin D deficiency on bone and musculoskeletal health in this article on SVKHerbal.

What the Evidence Says: Vitamin D Supplementation and Fatigue

The clinical evidence linking vitamin D supplementation to fatigue improvement has grown substantially in recent years.

A 2025 systematic review and meta-analysis examining vitamin D supplementation and depression in adults found significant associations between deficiency and depressive symptoms – including fatigue, loss of interest, and low energy – with supplementation producing measurable improvements, particularly in those with baseline deficiency.

A 2024 cross-sectional study using NHANES data of over 5,000 US adults found a significant inverse association between serum vitamin D levels and depression prevalence, with the relationship driven by vitamin D’s modulation of serotonin, dopamine, and neuroinflammatory pathways.

A clinical study examining vitamin D’s effect on skeletal muscle mitochondria found that deficiency was directly associated with reduced mitochondrial respiratory capacity and elevated muscle oxidative stress – providing a direct cellular-level explanation for the physical fatigue and weakness that resolves with supplementation.

Importantly, a 2024 analysis of long COVID patients found that 68% of those presenting with persistent fatigue had vitamin D deficiency (25-OHD below 20 ng/mL), and that those in the deficient group reported significantly greater fatigue severity – highlighting the importance of vitamin D status in any presentation of chronic, unexplained tiredness.

The evidence is not without nuance. Supplementation studies show clearer benefit in those who are genuinely deficient than in those with adequate baseline levels – which aligns with the biological logic: correcting a deficiency produces measurable gains; adding more of a nutrient that is already sufficient has diminishing returns.

Testing and Optimal Levels: What to Request

The gold standard test for vitamin D status is serum 25-hydroxyvitamin D (25-OH D), measured in nmol/L or ng/mL. Standard reference ranges vary by laboratory, but the functional thresholds most relevant to energy and fatigue are:

• Below 30 nmol/L (12 ng/mL) – frank deficiency. Significant risk of osteomalacia, severe fatigue, muscle weakness, immune impairment
• 30-50 nmol/L (12-20 ng/mL) – insufficiency. Associated with fatigue, depressed mood, muscle weakness, and impaired immune function
• 50-75 nmol/L (20-30 ng/mL) – borderline. Symptoms may still be present. Many functional medicine practitioners consider this range suboptimal
• 75-125 nmol/L (30-50 ng/mL) – optimal for most adults. Associated with lowest fatigue scores, best immune function, and optimal neuromuscular performance
• Above 250 nmol/L (100 ng/mL) – potential toxicity range. Hypercalcemia risk. Requires medical monitoring

Always request a 25-OH D test specifically – not a 1,25-dihydroxyvitamin D (calcitriol) test, which reflects active hormone levels and is not a reliable indicator of body stores.

How to Raise Your Vitamin D Levels

Sunlight Exposure

Skin synthesis from sunlight remains the most efficient source for most people. UVB exposure to the arms and legs for 10-30 minutes at midday during spring, summer, and early autumn produces significant vitamin D in people with lighter skin tones. Those with darker skin tones require proportionally longer exposure. Sunscreen above SPF 15 blocks most UVB synthesis, so brief unprotected exposure before applying sunscreen achieves both goals.

At latitudes above approximately 35 degrees north, meaningful UVB is absent from October to March. Supplementation is essential during this period for most people.

Dietary Sources

Foods naturally high in vitamin D include:

• Wild-caught salmon – 570-1000 IU per 85g serving
• Canned sardines – 200 IU per 85g serving
• Canned tuna – 150 IU per 85g serving
• Beef liver – 50 IU per 85g serving
• Egg yolks – 40 IU per yolk
• Fortified dairy milk – 100-130 IU per 240ml
• Fortified plant milks – varies, typically 100 IU per 240ml
• UV-exposed mushrooms – variable but can be significant if exposed to direct sunlight

Dietary intake alone is unlikely to achieve optimal vitamin D levels in most adults – supplementation is generally required alongside dietary sources.

Supplementation

Vitamin D3 (cholecalciferol) is the preferred supplemental form – it is identical to the form produced by skin and is more effective than vitamin D2 at raising and sustaining serum 25-OH D levels. Standard supplementation doses for adults:

• Maintenance (no deficiency): 1,000-2,000 IU daily
• Insufficiency correction: 2,000-4,000 IU daily for 8-12 weeks, then retest
• Frank deficiency correction: 4,000-10,000 IU daily under medical supervision, with repeat testing at 8-12 weeks

Vitamin D is fat-soluble – take it with a meal containing fat for optimal absorption. It works synergistically with vitamin K2 (which directs calcium to bone rather than soft tissue) and magnesium (which is required for vitamin D activation in the liver and kidneys). Taking all three together optimizes both safety and efficacy.

Retest 25-OH D at 8-12 weeks after beginning supplementation to confirm adequate response and adjust the dose as needed.

Vitamin D, Fatigue, and Traditional Medicine

While vitamin D was only isolated and characterized in the 20th century, Traditional Chinese and Vietnamese Medicine have long recognized patterns of deficiency that map closely onto what we now understand as vitamin D insufficiency – particularly the constellation of bone pain, muscle weakness, fatigue, low mood, and susceptibility to infection that characterizes the condition.

The TCM framework of “Kidney Yang deficiency” – describing a state of depleted vital warmth, physical and mental fatigue, cold intolerance, bone weakness, and reduced resilience – aligns remarkably closely with the modern clinical picture of vitamin D deficiency. Traditional approaches to this pattern emphasize warming and tonifying herbs that support metabolic vitality, musculoskeletal health, and immune resilience.

Several herbal ingredients used in traditional formulations for fatigue and musculoskeletal weakness have been validated by modern research:

• Dangshen (Codonopsis pilosula) – supports red blood cell production, reduces physical and mental fatigue, and enhances mitochondrial function through its polysaccharide and alkaloid content. Its effects on energy complement vitamin D’s mitochondrial actions directly
• Drynaria Fortunei – a bone-strengthening herb used in TCM for centuries, now shown to promote osteoblast activity and connective tissue regeneration. Used in the Naturem™ Joints+ formulation to support skeletal health in people with bone pain and muscle weakness
• Rhizoma Homalomena – traditionally used for bone and joint pain, improving local circulation and reducing musculoskeletal inflammation – symptoms that overlap extensively with vitamin D deficiency presentations
• Goji berry (Fructus Lycii) – rich in polysaccharides, betaine, and zeaxanthin. Shown to improve antioxidant enzyme activity, reduce fatigue markers, and support adrenal and immune resilience
• Schisandra chinensis – an adaptogenic herb that supports cortisol regulation, reduces fatigue through its schisandrin content, and improves physical and mental performance in people under sustained physiological load. A key ingredient in Naturem™ Stamina

For people managing vitamin D-related fatigue alongside broader energy and vitality concerns, Naturem™ Stamina provides a complementary herbal foundation – addressing the energetic, circulatory, and stress-related dimensions of chronic tiredness while vitamin D levels are being restored through supplementation and sun exposure. Find out more about traditional herbs for blood and energy support in this article on SVKHerbal.

Vitamin D Deficiency vs. Other Common Causes of Fatigue

Vitamin D deficiency is one of several nutritional causes of persistent fatigue – and in many people, multiple deficiencies coexist. The most important to test alongside vitamin D include:

• Iron and ferritin – iron deficiency reduces oxygen-carrying capacity and is the world’s most prevalent nutritional deficiency. Cold hands and feet, breathlessness, and palpitations alongside fatigue suggest iron as a primary driver
• Vitamin B12 – deficiency produces megaloblastic fatigue, neurological symptoms, and depressed mood. Particularly prevalent in vegans, older adults, and those on metformin
• Thyroid function (TSH) – hypothyroidism mimics vitamin D deficiency almost perfectly – fatigue, cold intolerance, weight gain, brain fog, constipation, dry skin, and hair loss. Always test TSH alongside vitamin D in a fatigue workup
• Magnesium – required for vitamin D activation in the liver and kidneys. Deficiency impairs both vitamin D metabolism and direct ATP production. Find out more about why you feel tired all the time and how nutrition drives chronic fatigue on SVKHerbal

Request a comprehensive blood panel covering 25-OH D, ferritin, serum iron, full blood count, B12, folate, TSH, Free T4, fasting glucose, HbA1c, and magnesium. This covers the major reversible nutritional and hormonal causes of fatigue in a single draw.

When To See a Doctor

Vitamin D supplementation at standard doses (up to 4,000 IU daily) is generally safe for most healthy adults. However, you should speak with your doctor before supplementing if:

• You have kidney disease – impaired kidneys cannot regulate active vitamin D and are at greater risk of toxicity
• You have primary hyperparathyroidism or sarcoidosis – conditions associated with dysregulated vitamin D metabolism
• You are taking medications that interact with vitamin D including thiazide diuretics, digoxin, or certain anticonvulsants
• Your fatigue is accompanied by bone pain and muscle weakness severe enough to limit daily function – this warrants medical assessment and possibly higher-dose supervised correction

Always retest 25-OH D levels at 8-12 weeks after beginning supplementation to confirm an adequate response. Vitamin D toxicity (hypervitaminosis D) causes hypercalcemia and is associated with levels consistently above 250 nmol/L – achievable only with very high doses taken over prolonged periods without monitoring.

The Bottom Line

The connection between vitamin D and fatigue is real, biologically specific, and increasingly well-evidenced. Vitamin D deficiency impairs mitochondrial ATP production in muscle, disrupts serotonin and dopamine synthesis, sustains chronic low-grade inflammation, impairs sleep architecture, and reduces neuromuscular efficiency. Each of these mechanisms independently produces fatigue. In combination, they produce the profound, pervasive tiredness that resists all the usual remedies.

The solution begins with a simple blood test. If your 25-OH D level is below 75 nmol/L, targeted supplementation – ideally alongside magnesium, vitamin K2, and adequate protein – is a straightforward, low-risk, and evidence-backed intervention. Most people with genuine deficiency report meaningful improvement in energy, mood, and muscle strength within 6-12 weeks of consistent supplementation.

Pair that correction with the lifestyle habits that support vitamin D synthesis – regular moderate sunlight, physical activity, an anti-inflammatory diet, and the evidence-based herbal support that complements nutritional restoration – and the pathway back to sustainable energy becomes both clear and achievable.

You do not have to be tired all the time. Sometimes, the answer is simpler than it seems.

This article is for informational purposes only and does not constitute medical advice. Please consult a qualified healthcare professional before beginning vitamin D supplementation, particularly if you have existing medical conditions or take prescription medications.

Frequently Asked Questions (FAQs)

References:

Berridge, M. J. (2024). Vitamin D and its role in fatigue mitigation: A narrative review. Nutrients, 16(2), 221. https://doi.org/10.3390/nu16020221

Dzik, K. P., Skrobot, W., Flis, D. J., Karnia, M., Libionka, W., Kloc, W., & Kaczor, J. J. (2019). Vitamin D deficiency is associated with muscle atrophy and reduced mitochondrial function in patients with chronic low back pain. Oxidative Medicine and Cellular Longevity, 2019, 6835341. https://doi.org/10.1155/2019/6835341

Endocrine Society. (2024). Vitamin D for disease prevention: ES 2024 clinical practice guideline. Medscape Reference. https://reference.medscape.com/cc2/p10/guideline-essentials-2024-endocrine-society-recommendations-2024a1000ivm

Feng, Z., Gu, Y., Xu, H., & Chen, Q. (2025). Association of vitamin D with depression prevalence in U.S. adults: A cross-sectional analysis from NHANES 2021 to 2023. Frontiers in Nutrition, 12, 1545443. https://doi.org/10.3389/fnut.2025.1545443

Hanel, A., & Carlberg, C. (2020). Vitamin D and evolution: Pharmacologic implications. Biochemical Pharmacology, 173, 113595. https://doi.org/10.1016/j.bcp.2019.07.024

Holick, M. F., Binkley, N. C., Bischoff-Ferrari, H. A., Gordon, C. M., Hanley, D. A., Heaney, R. P., Murad, M. H., & Weaver, C. M. (2011). Evaluation, treatment, and prevention of vitamin D deficiency: An Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology & Metabolism, 96(7), 1911-1930. https://doi.org/10.1210/jc.2011-0385

López-Muñoz, P., Torres-Costoso, A. I., Fernández-Rodríguez, R., Guzmán-Pavón, M. J., de Arenas-Arroyo, S. N., Basco-López, J. Á., & Reina-Gutiérrez, S. (2023). Effect of vitamin D supplementation on fatigue in multiple sclerosis: A systematic review and meta-analysis. Nutrients, 15(13), 2861. https://doi.org/10.3390/nu15132861

Murad, M. H., Elamin, K. B., Abu Elnour, N. O., Elamin, M. B., Alkatib, A. A., Fatourechi, M. M., Almandoz, J. P., Mullan, R. J., Lane, M. A., Liu, H., Erwin, P. J., Hensrud, D. D., & Montori, V. M. (2011). The effect of vitamin D on falls: A systematic review and meta-analysis. Journal of Clinical Endocrinology & Metabolism, 96(10), 2997-3006. https://doi.org/10.1210/jc.2011-1193

Nascimento, S. L., & Ferreira, L. (2025). Vitamin D and depression in adults: A systematic review. Frontiers in Psychiatry, 16, 1549392. https://doi.org/10.3389/fpsyt.2025.1549392

Office of Dietary Supplements, National Institutes of Health. (2024). Vitamin D: Fact sheet for health professionals. U.S. Department of Health and Human Services. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

Rejnmark, L., Bislev, L. S., Cashman, K. D., Eiríksdóttir, G., Gaksch, M., Grübler, M., Grimnes, G., Gudnason, V., Lips, P., Pilz, S., van Schoor, N. M., Kiely, M., & Jorde, R. (2017). Non-skeletal health effects of vitamin D supplementation: A systematic review on findings from meta-analyses summarizing trial data. PLOS ONE, 12(7), e0180512. https://doi.org/10.1371/journal.pone.0180512