Vitamin D Deficiency: Symptoms, Causes, Treatment

Written by Dr. Simon Khela MBChB MRCGP, GMC Registered Doctor

Last reviewed: 16-07-2026

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A patient came to see me in February a few years ago, convinced she had chronic fatigue syndrome. She'd been exhausted for months, her muscles ached in a way she couldn't quite explain, and she'd started wondering whether something serious was wrong. Her blood test told a simpler story. Her vitamin D level was 18 nmol/L, well into the deficient range, and within two months of starting treatment, she felt more like herself again.

This story captures something important about vitamin D deficiency. It's one of the most common findings I see on routine blood tests, particularly through the autumn and winter, and it's also one of the most underestimated. Patients tend to assume tiredness or aches and pains must have a more complicated explanation, when a straightforward, well-understood, and highly treatable vitamin deficiency is often sitting right there in the results.

This article explains what vitamin D deficiency actually is, why it happens, how it's diagnosed, and what genuinely works to correct it, based on current UK clinical guidance and what I see in practice.

Quick Answer: What Is Vitamin D Deficiency?

Vitamin D deficiency means the level of vitamin D in your blood is too low for your body to function properly, particularly for maintaining healthy bones, muscles and immune function. In the UK, it's generally defined as a blood level below 25 nmol/L, with levels between 25 and 50 nmol/L considered insufficient for some people, depending on individual circumstances.

The most common causes and symptoms include:

• Limited sun exposure (vitamin D is produced in the skin through sunlight)
• A diet low in vitamin D-rich foods
• Darker skin, which produces vitamin D less efficiently from sunlight
• Certain health conditions or medications that affect absorption
• Symptoms including fatigue, bone and muscle pain, and low mood
• In more prolonged or severe cases, bone softening conditions such as osteomalacia in adults or rickets in children

Treatment usually involves vitamin D supplements, taken either daily as tablets or, in cases of significant deficiency, sometimes as a higher-dose loading regime under medical guidance. For most people, correcting a deficiency is straightforward, safe, and produces a noticeable improvement in symptoms within weeks to a few months.

What Does Vitamin D Actually Do?

Vitamin D isn't really a vitamin in the traditional sense. It behaves more like a hormone, and the body can produce most of what it needs itself, through a reaction that occurs in the skin when it's exposed to ultraviolet B (UVB) light from the sun. It plays a central role in regulating calcium and phosphate levels in the blood, which is essential for building and maintaining strong bones and teeth.

It also has a role in muscle function and, increasingly, research points to its involvement in immune regulation, covered in more detail in vitamin D and immune function, which is why deficiency has been linked, though not conclusively, to a range of broader health effects.

In the UK, because of our latitude and weather, the skin can only make vitamin D from sunlight between roughly late March and the end of September, and even then, adequate exposure isn't guaranteed for everyone. This is one of the main reasons vitamin D deficiency is so common here compared with countries closer to the equator, and why everyone in the UK is generally advised to consider a daily supplement during autumn and winter months, regardless of age or health status.

Reasons for Vitamin D Deficiency

Vitamin D deficiency rarely has one single cause. In my experience, it's usually a combination of the following:

Limited sun exposure and darker skin tone

Limited sun exposure is the single biggest factor for most people in the UK. Working indoors, wearing sun protection (appropriately, for skin cancer prevention), spending most of the year in a country with limited winter daylight, and simply not spending much time outdoors all reduce the skin's vitamin D production, and this is particularly common in office workers who leave for work before sunrise and return after sunset for much of the year.

People with darker skin have more melanin, which reduces the skin's efficiency at producing vitamin D from sunlight. This means people of South Asian, African, Caribbean, and Middle Eastern descent living in the UK are at meaningfully higher risk of deficiency, and this group should be considered for supplementation year-round rather than just in winter.

Diet, age and body weight

• Diet: Very few foods naturally contain significant amounts of vitamin D, so a diet low in oily fish, eggs and fortified foods, whether by preference, allergy, or a vegan or vegetarian diet, increases risk, since most natural dietary sources are animal-based.
• Age: As skin ages, it becomes less efficient at producing vitamin D from sunlight. Older adults, particularly those who spend more time indoors or in residential care, are at significantly higher risk.
• Body weight: Vitamin D is fat-soluble, and in people with a higher body fat percentage, it can be sequestered in fat tissue rather than remaining available in the bloodstream, meaning higher supplement doses are often needed to reach the same blood level.

Malabsorption and certain health conditions

Conditions affecting the gut, such as coeliac disease, Crohn's disease, or previous bowel surgery, can reduce the absorption of vitamin D from food and supplements. Chronic kidney or liver disease also affects the body's ability to convert vitamin D into its active form. Certain medications, including some anti-epileptic drugs and long-term steroid use, can also interfere with vitamin D metabolism.

Pregnancy and breastfeeding

Vitamin D requirements increase during pregnancy and breastfeeding, partly to support the baby's bone development, and deficiency in these groups deserves particular attention, covered in more detail below.

Vitamin D Deficiency Symptoms in Women

Vitamin D deficiency symptoms overlap significantly between men and women, but there are a few areas where I see women raise specific concerns more often in practice.

Fatigue, rather than bone pain, is often the symptom that brings women to see a GP. This is particularly true in women who are also managing perimenopausal symptoms, where tiredness, low mood and joint aches can already be present and easily attributed to hormonal changes alone, when vitamin D deficiency may be contributing as well or entirely.

Common symptoms I see reported include:

• Persistent fatigue, not relieved by rest
• Bone pain, particularly in the lower back, hips, and legs
• Muscle weakness or aches, sometimes described as a generalised, hard-to-pinpoint discomfort
• Low mood, though the evidence for a direct causal link between vitamin D and depression is still developing
• Hair thinning, in some cases, though this is a less consistent finding and rarely the only symptom
• Frequent infections, linked to vitamin D's role in immune function

Women who are pregnant, breastfeeding, wear substantial skin-covering clothing for religious or cultural reasons, or who have darker skin are at particularly increased risk, and this is worth raising proactively with a GP rather than waiting for symptoms to become severe.

Vitamin D Deficiency in Pregnancy

Vitamin D plays a specific and important role during pregnancy. It supports the baby's developing bones and teeth, and maternal deficiency has been associated in research with a higher risk of certain pregnancy complications, though the evidence base continues to develop.

All pregnant women in the UK are generally advised to take a daily vitamin D supplement of 10 micrograms (400 IU), available through the Healthy Start scheme for those who are eligible. This applies regardless of season, since pregnancy increases vitamin D demand beyond what sun exposure and diet alone can typically provide.

Women who are pregnant and have additional risk factors, including darker skin, limited sun exposure, or a pre-pregnancy history of deficiency, may need a higher dose, which is worth discussing directly with a midwife or GP. Anyone planning a pregnancy should also consider having their vitamin D level checked beforehand, alongside other routine assessments available through a health check-up, so any deficiency can be corrected before conception where possible.

How to Test for Vitamin D Deficiency

The only reliable way to diagnose vitamin D deficiency is a blood test measuring 25-hydroxyvitamin D, often written as 25(OH)D, which reflects the body's stored vitamin D levels over the preceding weeks.

Interpreting Your Results

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When Testing Is and Isn't Recommended

Routine population-wide vitamin D testing isn't generally recommended for people without symptoms or risk factors, partly because deficiency is so common and mild-to-moderate insufficiency in an otherwise well person often doesn't need urgent correction beyond standard supplementation. Testing tends to be prioritised for people with symptoms suggestive of deficiency, known risk factors, bone health concerns, or certain underlying conditions, and can be arranged fairly quickly and privately for anyone with persistent, unexplained fatigue or bone and muscle symptoms, rather than starting supplements blindly without knowing the actual baseline level.

Vitamin D Deficiency Treatment

Treatment depends on how deficient someone is and their individual risk factors, but broadly falls into two approaches.

Standard maintenance dose

For people with a mild insufficiency or simply looking to prevent deficiency, a daily supplement of 10 micrograms (400 IU) is generally sufficient, in line with UK government guidance for the general population during autumn and winter, and year-round for higher-risk groups.

Higher-dose correction (loading) regime

For confirmed deficiency, particularly with symptoms or a very low blood level, a GP may recommend a higher-dose "loading" course, often a much larger dose taken weekly for several weeks, followed by a return to a standard daily maintenance dose. In some cases this is given as a vitamin D injection rather than oral tablets, particularly where absorption is a concern. This approach corrects the deficiency more quickly than a standard daily dose alone and is a common, well-established approach in UK general practice.

Safety Considerations for Higher-Dose Treatment

Higher-dose treatment should always be guided by a clinician rather than self-administered, partly because the right dose depends on how deficient someone is, and partly because, while rare, vitamin D toxicity is possible with excessive, prolonged high-dose supplementation, and can cause elevated calcium levels with symptoms including nausea, weakness, and in severe cases, kidney problems. Most patients notice an improvement in fatigue and muscle aches within four to twelve weeks of starting appropriate treatment, though bone pain related to more established deficiency can take longer to resolve fully.

How to Improve Vitamin D Deficiency Through Lifestyle

Alongside supplementation, several practical steps help maintain healthy vitamin D levels:

• Safe sun exposure. Around 10 to 15 minutes of midday sun on the arms and legs, several times a week between late March and September, is generally sufficient for most people with lighter skin, without needing to risk sunburn. People with darker skin generally need longer exposure to produce the same amount.
• Including vitamin D-rich foods regularly, covered in detail below.
• Considering supplementation proactively rather than reactively, particularly for higher-risk groups, rather than waiting until symptoms appear.
• Reviewing medications with a GP or pharmacist if you're on long-term steroids or anti-epileptic medication, since these can affect vitamin D metabolism.
• Addressing underlying gut or absorption conditions, such as coeliac disease, if these haven't already been diagnosed and managed.

Vitamin D Rich Foods

Very few foods contain substantial amounts of vitamin D naturally, which is part of why deficiency is so widespread in the UK. The main dietary sources include:

• Oily fish, such as salmon, mackerel, sardines and herring
• Egg yolks
• Red meat, in smaller amounts
• Liver (though this should be limited in pregnancy due to high vitamin A content)
• Fortified foods, including some breakfast cereals, and fortified plant milks and spreads, which have vitamin D added during production

For people following a vegan or vegetarian diet, dietary vitamin D intake is generally more limited, since most natural sources are animal-based, making a supplement (vitamin D2 or a vegan-friendly D3, derived from lichen) a sensible year-round consideration rather than relying on fortified foods alone.

How Much Vitamin D Per Day Do You Actually Need?

UK Government guidance suggests:

• Everyone in the UK aged 4 and over, including pregnant and breastfeeding women, should consider taking a daily supplement containing 10 micrograms (400 IU) of vitamin D during autumn and winter (October to March)
• Higher-risk groups, including people with darker skin, those who cover most of their skin for cultural or religious reasons, people who are housebound or spend very little time outdoors, and adults over 65, should consider taking this supplement year-round
• Babies and young children have different specific guidance, generally lower doses depending on age and whether they're breastfed or formula-fed, which is worth discussing with a health visitor or GP directly

These are general maintenance doses for the wider population. Someone with a confirmed, significant deficiency will usually need a higher initial dose, guided by their GP, before returning to this standard maintenance level.

Common Myths and Misconceptions

“I get plenty of sun, so I can't be deficient.”

Even regular outdoor time in summer doesn't guarantee adequate levels, particularly with sun cream use, time spent indoors during peak sunlight hours, or darker skin tone. UK sunlight simply isn't strong enough to produce vitamin D at all between October and March, regardless of how much time is spent outside.

“More vitamin D is always better.”

Vitamin D toxicity is rare but real, generally only occurring with excessive, sustained high-dose supplementation well beyond standard maintenance doses. More isn't automatically better once levels are sufficient.

“Vitamin D deficiency only affects bones.”

Bone health is the best-established effect, but deficiency is also linked to muscle weakness, fatigue, and immune function, and research continues into broader associations, though not all proposed links are equally well established.

“You only need to worry about it in winter.”

For most of the general population, winter is when risk is highest, but higher-risk groups, including people with darker skin or limited sun exposure year-round, may be deficient regardless of season.

“Supplements are only necessary if you're already unwell.”

Supplementation is a preventive measure recommended for most of the UK population during winter months, not solely a treatment for existing deficiency.

Benefits and Limitations of Vitamin D Testing and Treatment

Benefits

Testing and correcting vitamin D deficiency is generally safe, well-tolerated, and often produces a noticeable improvement in symptoms such as fatigue and bone or muscle pain, as covered in more detail in this guide to vitamin D treatment options. It's a straightforward blood test and an inexpensive, low-risk treatment for most people.

Limitations

That said, it has limitations worth being honest about. Not everyone with fatigue or aches and pains has vitamin D deficiency as the primary cause, and treating vitamin D levels shouldn't come at the expense of investigating other possible explanations, particularly if symptoms don't improve after correction. Some proposed associations between vitamin D and conditions such as depression, autoimmune disease, or general immunity remain areas of ongoing research rather than settled fact.

When to See a GP

Arranging a vitamin D blood test, alongside a broader clinical assessment, is generally worthwhile if you have:

• Persistent, unexplained fatigue or muscle weakness
• Bone pain, particularly if it's new or worsening
• Known risk factors, including darker skin, limited sun exposure, pregnancy, or a diagnosed malabsorption condition
• Recurrent infections without another clear explanation
• A family history of osteoporosis or osteomalacia, alongside other risk factors

A private GP appointment can arrange the relevant blood tests quickly and interpret the result alongside your wider health picture, rather than treating a single number in isolation. This matters particularly if you're also dealing with other symptoms that could have overlapping causes, since vitamin D deficiency is common enough that it can coexist with, rather than fully explain, other health issues, including a vitamin B12 deficiency, which can produce a very similar picture of fatigue.

Frequently Asked Questions