Vitamin D deficiency is one of the most common nutrient deficiencies in the world. A 2023 pooled analysis in Frontiers in Nutrition, covering data from 7.9 million people, found that nearly 48 percent of the global population has levels below the threshold for sufficiency. Vitamin D deficiency affects roughly 1 billion people worldwide, according to a 2025 review in StatPearls. Yet most people with low vitamin D have no dramatic symptoms at all. The signs are often subtle — fatigue, low mood, aching bones — and easy to mistake for something else entirely.
Understanding what vitamin D actually does, what causes deficiency, and how to correct it safely helps you take an active role in a problem that often goes unnoticed for years. This article explains the early warning signs, the risk factors that make deficiency more likely, how doctors diagnose it, and what treatment actually looks like.
For more on how persistent low energy connects to underlying nutrient and health issues, read our guide on Fatigue.
What Vitamin D Actually Does in Your Body
Vitamin D is not a typical vitamin. Your body can produce it itself, through a reaction that happens in your skin when it’s exposed to sunlight. Vitamin D functions more like a hormone than a nutrient once it’s activated. It regulates how much calcium and phosphate your body absorbs from food, which is essential for strong bones and teeth. It also plays a role in muscle function, immune regulation, and mood.
Without enough vitamin D, your body cannot absorb calcium efficiently. Bones can gradually weaken, even if your diet contains enough calcium itself. In children, severe deficiency causes rickets — a condition involving soft, weakened bones. In adults, the equivalent condition is called osteomalacia. Milder, more common deficiency rarely causes anything this dramatic. It tends to show up instead as a cluster of vague, easy-to-dismiss symptoms.
Early Signs of Vitamin D Deficiency
Most people with vitamin D deficiency have no obvious symptoms in the early stages. When signs do appear, they build gradually and are frequently attributed to stress, ageing, or simply being busy.
- Persistent fatigue: tiredness that doesn’t improve with rest is one of the most commonly reported early signs of low vitamin D.
- Bone and back pain: a dull, generalised ache — particularly in the lower back — can signal that bones aren’t mineralising properly.
- Muscle weakness or aches: vitamin D plays a direct role in muscle function. Weakness, particularly in the legs, is a well-documented sign of deficiency, especially in older adults.
- Low mood: several studies have found an association between low vitamin D and depressive symptoms, though researchers are still working out how much of this link is causal.
- Slow wound healing: vitamin D plays a role in the skin’s ability to repair itself after injury.
- Hair loss: severe deficiency has been linked to hair thinning and loss in some studies, though this connection is less consistent than the others.
- Getting sick often: vitamin D supports immune function. Frequent infections, particularly respiratory ones, can be a signal worth checking.
None of these symptoms are specific to vitamin D deficiency on their own. They overlap heavily with normal tiredness, other nutrient deficiencies, and everyday stress. This overlap is exactly why testing — not symptom-spotting alone — is the reliable way to confirm a diagnosis.
Who Is Most at Risk of Vitamin D Deficiency?
Certain groups face significantly higher risk of vitamin D deficiency because of biology, lifestyle, or geography.
- People who spend little time outdoors: since sunlight triggers vitamin D production in the skin, people who work indoors, are housebound, or live in institutional care face elevated risk.
- People with darker skin: higher melanin levels reduce the skin’s ability to produce vitamin D from sunlight, meaning people with darker skin often need considerably more sun exposure to generate the same amount.
- People living at higher latitudes: during autumn and winter in many temperate regions, the sun sits too low in the sky for skin to produce meaningful vitamin D, regardless of time spent outside.
- Older adults: ageing skin produces less vitamin D from the same sun exposure, and older adults are also more likely to spend limited time outdoors.
- People with obesity: vitamin D is fat-soluble and can become sequestered in fat tissue, reducing the amount that circulates and remains available to the body.
- People who cover most of their skin: clothing that covers the arms, legs, and face for cultural, religious, or occupational reasons limits the skin surface available for vitamin D production.
- People with certain digestive conditions: conditions such as Crohn’s disease, coeliac disease, and cystic fibrosis can impair the absorption of fat-soluble vitamins, including vitamin D, from food.
- Exclusively breastfed infants: breast milk alone typically doesn’t provide enough vitamin D, which is why supplementation is commonly recommended for breastfed babies.
How Vitamin D Deficiency Is Diagnosed
A simple blood test measures 25-hydroxyvitamin D, the form of vitamin D that circulates in your bloodstream and best reflects your overall status. Results are typically reported in nanomoles per litre (nmol/L) or nanograms per millilitre (ng/mL).
- Deficient: below 30 nmol/L (12 ng/mL)
- Insufficient: 30 to 50 nmol/L (12 to 20 ng/mL)
- Sufficient: above 50 nmol/L (20 ng/mL) for most people
Routine screening isn’t recommended for everyone. Most guidelines suggest testing specifically for people with risk factors, symptoms that could indicate deficiency, or a condition affecting bone or calcium metabolism. If you fall into one of the higher-risk groups above, it’s worth asking your GP whether testing is appropriate for you.
How to Treat Vitamin D Deficiency
Treatment for vitamin D deficiency depends on how low your levels are and what’s driving the deficiency in the first place.
Supplementation
For confirmed deficiency, doctors commonly prescribe a short course of high-dose vitamin D to correct levels quickly, followed by a lower daily maintenance dose. In the UK, public health guidance recommends that most adults consider a daily supplement of 10 micrograms (400 IU) during autumn and winter, when sunlight alone isn’t enough. People in higher-risk groups are often advised to take this dose year-round.
Vitamin D absorbs best when taken with a meal that contains some fat, since it’s a fat-soluble vitamin. Very high doses should only be taken under medical supervision — excessive vitamin D can, in rare cases, lead to dangerously high calcium levels in the blood.
Sensible sun exposure
Short periods of sun exposure on bare skin — arms, legs, or face, for roughly 10 to 15 minutes around the middle of the day — can help most people produce meaningful vitamin D during the months when the sun is strong enough. This needs to be balanced carefully against skin cancer risk, and it isn’t a reliable year-round strategy in many climates.
Dietary sources
Few foods naturally contain significant vitamin D, which is part of why deficiency is so common. Useful sources include:
- oily fish such as salmon, mackerel, and sardines
- egg yolks
- fortified foods, including some milk, plant milks, and breakfast cereals
- red meat and liver, in smaller amounts
- mushrooms exposed to UV light during growing
Because dietary sources alone rarely provide enough, supplementation remains the most reliable way to correct or prevent deficiency for most people — particularly during the darker months of the year.
Vitamin D and Bone or Joint Symptoms
One of the most overlooked connections is between low vitamin D and ongoing joint or bone discomfort. Because vitamin D governs calcium absorption, prolonged deficiency can contribute to bone pain, increased fracture risk, and a general ache that’s easy to write off as normal ageing. If you’re dealing with persistent joint or bone discomfort without a clear cause, it’s worth discussing vitamin D testing with your doctor alongside other possibilities. Our guide on Joint Pain and Stiffness covers the wider range of causes worth ruling out.
When to See a Doctor
Speak to your GP if you experience:
- persistent fatigue that doesn’t improve with rest
- unexplained bone or muscle pain
- frequent infections, particularly respiratory ones
- you fall into a higher-risk group listed above and haven’t been tested
- you’re already taking a supplement but symptoms haven’t improved after several months
The NHS guide to vitamin D provides further detail on recommended daily amounts, safe supplementation, and who is most at risk.
Key Takeaways
- Vitamin D deficiency affects close to 1 billion people worldwide, with nearly half the global population falling below the threshold for sufficiency, according to recent pooled research.
- Most people with low vitamin D have no dramatic symptoms. Fatigue, bone or muscle aches, low mood, and frequent infections are the most common early signs.
- Risk increases significantly for people who spend little time outdoors, have darker skin, live at higher latitudes, are older, or have certain digestive conditions.
- A simple blood test measuring 25-hydroxyvitamin D confirms diagnosis — symptoms alone aren’t a reliable way to tell.
- Treatment usually combines a correction dose of supplements, ongoing maintenance supplementation during darker months, and sensible sun exposure where appropriate.
- Few foods naturally contain much vitamin D, which is why supplementation is the most reliable way to prevent or correct deficiency for most people.
Final Thoughts
Vitamin D deficiency is easy to overlook precisely because its symptoms feel so ordinary. Tiredness, a stiff back, catching every cold that goes around the office — none of it screams “nutrient deficiency.” That’s exactly why it goes undiagnosed in so many people for so long.
The fix, in most cases, is genuinely simple. A blood test tells you where you stand. A daily supplement, taken consistently, corrects the vast majority of cases without complication. If any of the signs above sound familiar — especially if you fall into one of the higher-risk groups — a conversation with your GP costs very little and could explain a lot.
Know your levels. Don’t guess.
Medical Disclaimer: The information on this page is provided for educational and informational purposes only and is not intended as medical advice. It should not replace consultation with a qualified healthcare professional. For full details, please read our Disclaimer.
