Vitamin D deficiency is one of the most common nutritional shortfalls in the UK — yet it’s also one of the easiest to address once identified. Around one in five UK adults has low vitamin D levels, rising to one in three during winter months. Many don’t know it.
The symptoms are often subtle, overlap with other conditions, and are easy to dismiss. But the NHS recognises vitamin D deficiency as a significant public health concern, particularly for people who spend little time outdoors, have darker skin, or are over 65.
Here’s what to look for, why it happens, and what to do if you think you might be affected.
How common is vitamin D deficiency in the UK?
The Scientific Advisory Committee on Nutrition (SACN) reported in 2016 that approximately 20% of UK adults have serum vitamin D levels below 25 nmol/L (the threshold considered deficient) at some point during the year, with prevalence highest in late winter and spring.
The UK’s latitude means that between October and March, sunlight doesn’t carry enough UVB energy to trigger vitamin D synthesis in the skin — for more detail, see our article on vitamin D in winter. This makes dietary and supplemental sources the only viable options during these months for most of the population.
Groups at highest risk include people with darker skin (who need more sun exposure to produce the same amount of D3), people who cover their skin for cultural or religious reasons, older adults (who synthesise D3 less efficiently), people who are housebound, and those with gut conditions affecting fat absorption (since vitamin D is fat-soluble).
Vitamin D deficiency symptoms: what the NHS recognises
Vitamin D deficiency can be asymptomatic, particularly in its early stages. When symptoms do appear, they tend to be non-specific and easily attributed to other causes. The most commonly reported include:
- Fatigue and low energy. One of the most frequently reported symptoms, though also one of the hardest to attribute specifically to vitamin D without a blood test.
- Bone pain and muscle aches. Vitamin D is essential for calcium absorption and bone mineralisation. Deficiency causes the bone condition osteomalacia in adults, presenting as diffuse bone pain, particularly in the lower back, hips, and legs.
- Muscle weakness. Vitamin D receptors are present in muscle tissue. Deficiency impairs muscle function and is associated with increased fall risk in older adults.
- Low mood. Vitamin D receptors are present in mood-regulating areas of the brain. Low vitamin D is associated with higher rates of depression, particularly in winter.
- Frequent illness. Vitamin D plays a key role in immune regulation. Deficiency is associated with increased susceptibility to respiratory infections.
- Impaired wound healing. Vitamin D influences inflammatory response and skin repair.
Severe, long-standing deficiency can cause rickets in children and osteomalacia in adults, conditions involving softening of the bones. These are less common in the UK now but still occur, particularly in children from South Asian backgrounds and in older adults.
Vitamin D deficiency and fatigue
Fatigue is one of the most frequently cited symptoms of vitamin D deficiency, and there is reasonable mechanistic support for the link. Vitamin D influences mitochondrial function (the cellular machinery responsible for energy production) and deficiency has been associated with reduced aerobic capacity in some studies.
A review published in Nutrients (2020) found that vitamin D deficiency was significantly associated with fatigue and that correction of deficiency improved fatigue scores in several clinical studies, particularly in people whose baseline levels were below 50 nmol/L.
That said, fatigue has many causes. A blood test is the only reliable way to know whether low vitamin D is contributing to yours.
Vitamin D deficiency and muscle pain
Muscle pain is one of the most common reasons people eventually get their vitamin D tested — and for good reason. Vitamin D receptors (VDRs) are present throughout skeletal muscle tissue, and the vitamin plays a direct role in muscle cell function, protein synthesis, and calcium handling within muscle fibres.
When vitamin D levels drop below 25–30 nmol/L, the effects on muscle can be significant. A systematic review published in Pain and Therapy (2022) found a consistent association between vitamin D deficiency and chronic musculoskeletal pain, including diffuse muscle aches, proximal muscle weakness (difficulty climbing stairs or rising from a chair), and a heightened sensitivity to pressure pain.
The pain pattern is typically widespread rather than localised. It often affects the thighs, upper arms, and lower back — areas with high concentrations of type II (fast-twitch) muscle fibres, which appear to be particularly sensitive to vitamin D status. This can mimic conditions like fibromyalgia, and misdiagnosis is not uncommon. A 2014 study in BMC Musculoskeletal Disorders found that a significant proportion of patients initially diagnosed with non-specific musculoskeletal pain had undiagnosed vitamin D deficiency.
Joint pain can also feature. While vitamin D deficiency doesn’t cause joint inflammation directly, it impairs the body’s ability to regulate inflammatory pathways. People with both low vitamin D and existing joint conditions (such as osteoarthritis) tend to report more severe symptoms.
The practical question is whether correcting the deficiency resolves the pain. The evidence is encouraging but not uniform. A meta-analysis in Nutrients (2021) found that vitamin D supplementation significantly reduced pain scores in people who were deficient at baseline, with the strongest improvements seen in those with the lowest starting levels. People whose levels were already adequate saw little benefit, which reinforces that this is a deficiency-driven problem, not a case for megadosing.
If you’re experiencing unexplained muscle aches, weakness, or fatigue — particularly during or after winter — a 25(OH)D blood test is a reasonable first step. Addressing confirmed deficiency with a well-absorbed vitamin D3 supplement alongside adequate magnesium (which is itself involved in muscle relaxation) covers both of the most common nutritional contributors to muscle pain.
Vitamin D deficiency and hair loss
Vitamin D receptors are present in hair follicles and play a role in the hair growth cycle, specifically in stimulating follicles to enter the growth phase (anagen). Deficiency has been associated with telogen effluvium, a form of diffuse hair shedding triggered by physiological stress.
Studies including a 2019 paper in the International Journal of Dermatology found lower serum vitamin D levels in people with telogen effluvium and alopecia areata compared to controls. The relationship appears to be correlational rather than straightforwardly causal. Hair loss has many causes, and correcting vitamin D deficiency doesn’t always resolve it.
If you’re experiencing unexplained hair thinning or shedding alongside other symptoms of deficiency, vitamin D is worth investigating. Hair loss from vitamin D deficiency is generally considered reversible once levels are restored, though the timeline varies.
Vitamin D deficiency and nails
Nail changes are a less commonly discussed but genuine sign of nutritional deficiency. Brittle nails, pitting, and slow growth have all been associated with low vitamin D, though the evidence base is thinner than for other symptoms.
The mechanism is indirect: vitamin D supports calcium metabolism and keratin production, both of which affect nail structure. If you notice persistent nail fragility alongside other deficiency symptoms, it’s worth including in the picture when speaking with a GP.
Why magnesium deficiency makes vitamin D deficiency worse
This is an important and often overlooked connection. Vitamin D doesn’t become biologically active on its own. It undergoes two enzymatic conversion steps: first in the liver, then in the kidneys, to become calcitriol, the active form. Both steps require magnesium-dependent enzymes.
A 2018 review in The Journal of the American Osteopathic Association found that magnesium deficiency limits vitamin D activation and that people with low magnesium may appear to have adequate serum vitamin D on a standard blood test while still experiencing functional deficiency, because the body cannot activate what it has.
Surveys consistently find that 50–70% of UK adults consume below the recommended intake of magnesium. This means a significant portion of people taking vitamin D supplements may be getting less from them than they think, purely because their magnesium intake is insufficient. If you’re addressing vitamin D deficiency, a well-absorbed magnesium supplement alongside it is a practical and evidence-informed step. For more on how vitamin D, magnesium, and zinc interact and when to take each, see our article on vitamin D with zinc and magnesium.
Vitamin D and B12 deficiency: why they often occur together
Vitamin B12 deficiency and vitamin D deficiency share several risk groups: older adults, people who eat little animal protein, people with gut absorption issues, and those with limited sun exposure. It’s therefore common for both to be low simultaneously.
Both deficiencies can cause fatigue, weakness, and mood changes, which makes symptom-based diagnosis unreliable. A 2017 study in Nutrients found a significant correlation between vitamin D and B12 levels, particularly in older adults, and suggested that testing for one should prompt testing for the other.
If a GP is investigating fatigue or low mood, asking for both vitamin D and B12 to be checked at the same time is reasonable and cost-effective.
Vitamin D deficiency and weight gain
The relationship between vitamin D and body weight is real but often misunderstood. Vitamin D is fat-soluble, meaning it is stored in body fat. In people with higher body fat, vitamin D gets sequestered in adipose tissue and is less bioavailable in circulation, leading to lower serum levels even at the same dietary intake.
This means deficiency is more common in people with obesity, but it’s largely a consequence of higher body fat rather than a cause of weight gain. The honest position from the research is that correcting vitamin D deficiency is unlikely to directly cause weight loss. However, addressing deficiency may support energy levels, mood, and physical capacity in ways that indirectly support a healthier lifestyle.
Can you get enough vitamin D from sunlight in the UK?
Between April and September, brief sun exposure to uncovered skin can generate useful amounts of vitamin D3. Around 10–20 minutes near midday (between 11am and 3pm) without sunscreen is enough for most lighter-skinned adults. Darker skin requires longer exposure to produce the same amount.
From October through March, UK sunlight doesn’t carry enough UVB energy to trigger vitamin D synthesis regardless of how much time you spend outdoors. Dietary and supplemental sources become the only reliable option for these months.
If you spend time outside regularly in summer but develop symptoms in winter, a blood test around February or March will give you the clearest picture of where your levels sit at their lowest point.
How to test for vitamin D deficiency in the UK
The standard test is a serum 25-hydroxyvitamin D (25(OH)D) blood test. In the UK, this can be requested through your GP, though GPs may apply eligibility criteria before referring on the NHS. Private testing is available from around £30–50 through services including Medichecks and Thriva, and can be done via a finger-prick home kit.
Reference ranges vary slightly between labs, but the following are broadly used:
- Below 25 nmol/L: deficient (SACN definition)
- 25–50 nmol/L: insufficient (at risk of symptoms and long-term bone effects)
- 50–125 nmol/L: adequate for most adults
- Above 125 nmol/L: potentially excessive (risk of toxicity at sustained high levels)
How much vitamin D to take for deficiency
The right dose depends on your starting level and the degree of deficiency. General guidance:
- Prevention / maintenance: 400–2,000 IU daily (NHS recommends 400 IU minimum Oct–Mar for all UK adults)
- Insufficiency (25–50 nmol/L): 1,000–2,000 IU daily is usually sufficient to restore levels over 2–3 months
- Deficiency (below 25 nmol/L): Your GP may prescribe higher loading doses for faster correction; follow medical guidance
- Elderly adults: 800–1,000 IU daily is specifically recommended by NICE for adults over 65 and those at high risk of falls. For age-specific guidance, see our full post on vitamin D for older adults.
- Teenagers: 400–1,000 IU daily during winter months is appropriate for adolescents with limited sun exposure
Vitamin D3 (cholecalciferol) is more effective than D2 (ergocalciferol) at raising and maintaining serum levels. Choose D3 wherever possible.
Should you take vitamin K2 with vitamin D?
Vitamin D increases calcium absorption from the gut. That’s the intended mechanism, but it raises a practical question: where does that calcium go? Vitamin K2, specifically the MK-7 form, activates proteins called osteocalcin and matrix Gla-protein (MGP) that direct calcium to bones and teeth rather than to soft tissue and arteries.
Research published in Thrombosis and Haemostasis (2015) found that K2 MK-7 supplementation significantly reduced arterial stiffness in postmenopausal women with existing arterial calcification. The D3 and K2 pairing has sound biochemical rationale: D3 drives calcium absorption, K2 directs where it is deposited.
If you’re taking vitamin D at 1,000 IU or above on a long-term basis, adding K2 MK-7 is a practical step. Our Vitamin D3 K2 with Zinc, Boron and MCT Oil combines both in an MCT oil base, which improves absorption of fat-soluble vitamins.
Frequently asked questions
What are the main symptoms of vitamin D deficiency?
The most common symptoms include fatigue, bone pain, muscle aches and weakness, low mood, and frequent colds or infections. Many people with mild deficiency have no obvious symptoms. A blood test (serum 25(OH)D) is the only reliable way to confirm deficiency.
How common is vitamin D deficiency in the UK?
Around one in five UK adults has low vitamin D levels at some point during the year, according to SACN. Prevalence is highest in late winter and spring. People with darker skin, older adults, and those who spend little time outdoors are at greatest risk.
Can vitamin D deficiency cause muscle and joint pain?
Yes. Vitamin D receptors are present in skeletal muscle, and deficiency impairs muscle function, protein synthesis, and calcium handling. Research consistently links low vitamin D to widespread muscle aches, proximal weakness, and increased pain sensitivity. Joint pain can also worsen when vitamin D is low, as the vitamin helps regulate inflammatory pathways. Correcting deficiency through supplementation has been shown to reduce pain scores, particularly in people with the lowest baseline levels.
Can vitamin D deficiency cause hair loss?
Vitamin D receptors are present in hair follicles and play a role in the growth cycle. Low vitamin D has been associated with telogen effluvium and alopecia areata in several studies. Correcting deficiency may help, but hair loss has many causes and vitamin D is not always the primary factor.
Can vitamin D deficiency cause fatigue?
Yes. Fatigue is one of the most frequently reported symptoms. Vitamin D influences mitochondrial function and its deficiency is associated with reduced energy levels. However, a blood test is needed to confirm whether low vitamin D is contributing.
Does magnesium affect vitamin D levels?
Yes, significantly. Magnesium is required for both enzymatic conversion steps that activate vitamin D in the body. Without adequate magnesium, vitamin D cannot be converted to its active form even if serum D levels appear normal on a blood test. Addressing both deficiencies together is more effective than addressing either alone.
Can you have vitamin D and B12 deficiency at the same time?
Yes, and it is fairly common. Both deficiencies share the same high-risk groups: older adults, people with gut absorption issues, and those with limited dietary variety. Both can cause fatigue and low mood. If testing for one, it is worth checking the other at the same time.
Can you get vitamin D from sunlight in the UK?
Between April and September, yes. Around 10–20 minutes of midday sun exposure to uncovered skin is enough for most lighter-skinned adults. From October through March, UK sunlight lacks sufficient UVB energy to trigger vitamin D synthesis, making supplements the only reliable source during these months.
Should you take vitamin K2 with vitamin D?
Yes, particularly at doses of 1,000 IU or above. Vitamin D increases calcium absorption; K2 MK-7 directs that calcium to bones rather than soft tissue. Taking both together addresses the full calcium utilisation pathway rather than just absorption.
References
- Scientific Advisory Committee on Nutrition (SACN). Vitamin D and Health Report. Public Health England, 2016.
- Uwitonze AM, Razzaque MS. Role of Magnesium in Vitamin D Activation and Function. The Journal of the American Osteopathic Association. 2018;118(3):181–189. doi:10.7556/jaoa.2018.037
- Almohanna HM, et al. The role of vitamins and minerals in hair loss: a review. Dermatology and Therapy. 2019;9(1):51–70. doi:10.1007/s13555-018-0278-6
- Bivona G, et al. The Immunological Implication of the New Vitamin D Metabolism. Central European Journal of Immunology. 2018;43(3):331–334.
- Knapen MH, et al. Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporosis International. 2013;24(9):2499–2507.
- Vermeer C, et al. Beyond deficiency: potential benefits of increased intakes of vitamin K for bone and vascular health. European Journal of Nutrition. 2004;43(6):325–335.
- National Institute for Health and Care Excellence (NICE). Vitamin D: supplement use in specific population groups. PH56, 2014.
- Yong WC, Sanguankeo A, Upala S. Effect of vitamin D supplementation in chronic widespread pain: a systematic review and meta-analysis. Clinical Rheumatology. 2017;36(12):2825–2833. doi:10.1007/s10067-017-3754-y
- Helde-Frankling M, Björkhem-Bergman L. Vitamin D in Pain Management. International Journal of Molecular Sciences. 2017;18(10):2170. doi:10.3390/ijms18102170
This article is for informational purposes only and does not constitute medical advice. If you are experiencing symptoms of vitamin D deficiency, speak with your GP. Do not start high-dose supplementation without medical guidance.
