Vitamin D Deficiency Symptoms: Signs, Testing, Treatment, and What It Means for Athletes

Key Takeaways: Vitamin D Deficiency Symptoms

The best-documented vitamin D deficiency symptoms are aching bones and muscles, muscle weakness and, in older adults, more sway and more frequent falls. Aching bones and muscles are associated with osteomalacia, a mineralization defect of the skeleton linked to vitamin D deficiency.[1]
In my clinical experience, low vitamin D is often asymptomatic and turns up as an incidental finding. Because fatigue is usually multifactorial, I find it is rarely possible to show that a patient’s tiredness comes specifically from low vitamin D.
Fatigue can improve when a clear deficiency is treated. In a 120-person randomized trial, a single 100,000 IU dose reduced self-rated fatigue more than placebo.[2] Low mood is different: 2,000 IU a day for a median of 5.3 years did not prevent depression in 18,353 adults.[3]
Low vitamin D is common. In a pooled global analysis, close to half of people had a serum 25(OH)D below 50 nmol/L, and that prevalence was 1.7 times higher in winter–spring than in summer–autumn.[4]
The right test is serum 25(OH)D.[1] The 2024 Endocrine Society guideline advises against routine testing of healthy adults, but it explicitly does not apply to people with an established reason for testing.[5]
In my practice, a suspected stress fracture, whether in a young athlete or an older patient, is a good reason to add a vitamin D measurement to the workup.
For athletes, the clearest signal is bone. Lower 25(OH)D was linked to stress fractures in Finnish military recruits,[6] and calcium plus vitamin D lowered stress fracture incidence in female Navy recruits.[7] In a meta-analysis of trials in athletes, supplementation did not significantly improve maximal strength or power.[8]
For athletes, my advice is the same as for everyone else: follow the Finnish Current Care Guideline, which sets a recommended daily intake of 10–20 µg (400–800 IU) depending on age,[9] correct a real deficiency, and avoid megadoses and attempts to optimize blood levels or performance.

Introduction: Vitamin D Deficiency Symptoms

Search for vitamin D deficiency symptoms and you will find long lists: tiredness, hair loss, low mood, frequent colds, back pain, poor sleep. The problem with most of those lists is that they mix a few well-documented signs with many weak associations, and they rarely say which is which.

Low vitamin D is not rare. A pooled analysis of 308 studies from 81 countries, covering nearly 8 million people in total, reported that approximately half had a serum 25(OH)D below 50 nmol/L, while 15.7% were below 30 nmol/L.[4] With numbers like that, a low result is common, but it does not automatically explain whatever symptom prompted the test.

As both a physician and a former international competitor in Taekwon-Do and kickboxing, I approach vitamin D from two directions: how the blood test should be interpreted, and what deficiency means for training and bone health. This article covers which vitamin D deficiency symptoms have real evidence behind them, who is at risk, when testing makes sense, how deficiency is treated, and what all of this means for athletes. For a deeper look at performance, target levels and supplementation in sport, see my companion article on vitamin D in athletes.

How Common Is Vitamin D Deficiency?

The global analysis above also shows how much prevalence depends on where and when you measure. In the European region, 53.0% of participants were below 50 nmol/L and 18.0% were below 30 nmol/L.[4] At the northernmost latitudes analysed (60–80°N, which includes Finland), 57.4% were below 50 nmol/L.[4]

Season matters as much as geography. In studies that reported it, the share of people below 50 nmol/L was 1.7 times higher in winter–spring than in summer–autumn.[4] Women were affected somewhat more often than men: 53.3% versus 45.3% below 50 nmol/L.[4]

Season and latitude matter for a physiological reason. One athlete-focused review estimates that ultraviolet B exposure of the skin accounts for 80–90% of circulating 25(OH)D, with diet contributing 10–20%.[10] Above and below latitudes of roughly 33°, skin production of vitamin D3 is very low or absent during most of the winter.[1] This is worth keeping in mind when interpreting vitamin D deficiency symptoms in winter, the season when low results are most common.

In my view, vitamin D deficiency is a well-recognized issue in Finland, which lies far enough north for winter sunlight to be a real limitation. In my clinical experience, I see low vitamin D in patients quite often, particularly during the winter season.

Vitamin D Deficiency Symptoms: What the Evidence Supports

Not every proposed vitamin D deficiency symptom has the same quality of evidence. The sections below separate the classic, well-established signs from the ones where trials have given mixed or negative answers.

Bone and Muscle Aches

In adults, vitamin D deficiency disturbs calcium, phosphorus and bone metabolism, and the resulting mineralization defect in the skeleton is known as osteomalacia.[1] The Endocrine Society’s 2011 guideline notes that in adults this defect often goes undetected, and describes osteomalacia as being associated with isolated or generalized aches and pains in the bones and muscles.[1] In young children, the corresponding picture is rickets, with its characteristic skeletal deformities.[1]

Deficiency severe enough to cause these problems is something I rarely see in clinical practice. I have practically never encountered rickets clinically in Finland, which suggests to me that it is not common here.

Muscle Weakness and Falls

The same guideline states that vitamin D deficiency also causes muscle weakness, and that older adults with deficiency show more body sway and fall more often, which in turn raises fracture risk.[1]

Fatigue

Fatigue is the vitamin D deficiency symptom most people know, and it has at least one double-blind randomized trial behind it. In a double-blind trial, 120 otherwise healthy adults (mean age 29, 53% women) who reported fatigue and had a 25(OH)D below 20 µg/L received either a single oral dose of 100,000 IU of vitamin D or placebo.[2] After 4 weeks, the Fatigue Assessment Scale score improved by 3.3 ± 5.3 points in the vitamin D group versus 0.8 ± 5.3 points with placebo (P = 0.01), and 72% versus 50% reported less fatigue.[2]

That result applies to people with fatigue and a clearly low level. It does not mean every tired person with a borderline result will feel better on vitamin D.

In my practice, fatigue is often the very reason patients want their vitamin D measured, which makes the link hard to judge. Fatigue is typically multifactorial, and in my clinical experience it is very difficult to show that a patient’s tiredness is caused specifically by low vitamin D. When a low level is found, whether it actually affects the person’s energy or performance usually remains an open question, because cause and effect are close to impossible to establish in an individual patient. In my experience, vitamin D deficiency tends to be an unlikely explanation for fatigue on its own.

A recent case from my own practice illustrates the problem. The patient was exhausted, and the blood tests showed both a low vitamin D level and a low iron level. With two possible contributors present at the same time, it remained unclear which finding explained what. I still recommended a vitamin D supplement alongside iron supplementation.

If tiredness is the main complaint, the wider differential in fatigue in athletes is a better starting point than any single lab value.

Low Mood and Depression

Low mood appears on almost every symptom list, but a large randomized test of the idea was negative. In the VITAL-DEP study, an ancillary study of the VITAL trial, 18,353 adults aged 50 and older took either 2,000 IU of vitamin D3 a day or placebo for a median of 5.3 years.[3] Risk of depression or clinically relevant depressive symptoms did not differ (hazard ratio 0.97, 95% CI 0.87–1.09), and mood scores did not change meaningfully between groups.[3]

The trial tested prevention in older adults, so it cannot answer every question about mood. It does mean that, in this large trial, daily vitamin D did not prevent depression or clinically relevant depressive symptoms.

In my experience, seasonal timing makes this especially confusing in Finland. During kaamos, the dark winter months, my patients often report exhaustion and low mood, and with less sunlight their vitamin D levels also tend to be lower. Because both change at the same time of year, it is often unclear what is causing what.

Frequent Respiratory Infections

Getting sick often is another commonly listed sign. Here the evidence is modest and has weakened over time. A 2021 meta-analysis of randomized trials found that vitamin D reduced the odds of at least one acute respiratory infection compared with placebo (odds ratio 0.92, 95% CI 0.86–0.99; 37 trials, 46,166 participants).[11] In its subgroup analyses, a statistically significant reduction was seen with daily doses equivalent to 400–1,000 IU (odds ratio 0.70, 95% CI 0.55–0.89) but not with monthly or less frequent bolus doses (odds ratio 0.98, 95% CI 0.93–1.03), and no subgroup defined by baseline 25(OH)D showed a statistically significant effect on its own.[11] Subgroup results like these do not by themselves prove that one dosing regimen works better than another.

An updated meta-analysis published in 2025 changed the picture. With 40 trials and more than 60,000 participants, the overall reduction in risk was small and no longer statistically significant, with a 95% confidence interval that reached no effect, and the pre-specified subgroup analyses did not detect statistically significant evidence of effect modification by age, baseline vitamin D status, dose or dosing frequency.[12] Frequent colds are therefore not a reliable vitamin D deficiency symptom, and the current trial evidence does not show that vitamin D supplements reliably prevent respiratory infections.

In my experience, Finland also has a pronounced cold and flu season that runs from autumn into spring. During those months, patients often wonder whether their repeated infections could be related to low vitamin D, and in my experience it is a question clinicians consider as well. In practice, I tend to recommend vitamin D supplementation to patients who are getting sick frequently and whose vitamin D is found to be low, with dosing based on the Finnish Current Care Guideline.[9]

Why Vitamin D Deficiency Symptoms Are Easy to Miss

Taken together, the strongest evidence describes what happens after deficiency has been present long enough to affect bone and muscle. The complaints people most often attribute to milder deficiency, such as tiredness and vague aches, are non-specific and can have many other causes. That is why a blood test, ordered for the right reason, matters more than a symptom checklist.

Patients often ask me how vitamin D deficiency actually shows itself, and there is not always a straightforward answer. In my clinical experience, low vitamin D is frequently asymptomatic and turns up as an incidental finding on a blood test, which often comes as a surprise to the patient.

Who Is Most at Risk of Vitamin D Deficiency?

The 2011 Endocrine Society guideline, since replaced by the 2024 guideline discussed under testing below, recommended screening people at risk of deficiency rather than the whole population, and listed the groups it considered candidates for 25(OH)D measurement.[1] These included:

People with rickets, osteomalacia or osteoporosis, and older adults with a history of falls or non-traumatic fractures
Malabsorption conditions such as inflammatory bowel disease, Crohn’s disease, cystic fibrosis, bariatric surgery and radiation enteritis
Chronic kidney disease, liver failure and hyperparathyroidism
Obesity
Pregnancy and breastfeeding
Certain medications, including antiseizure drugs, glucocorticoids, some antifungals, cholestyramine and some HIV medications
Granuloma-forming disorders such as sarcoidosis and tuberculosis, and some lymphomas
African-American and Hispanic children and adults

Beyond that list, factors that limit vitamin D production in the skin also matter, including winter at higher latitudes, sunscreen, covering clothing, darker skin pigmentation and ageing.[1] This is because most circulating vitamin D comes from UVB exposure of the skin.[10]

In my practice, a combined calcium and vitamin D preparation is a common sight on the medication lists of older patients, especially postmenopausal women with a tendency toward osteoporosis. In my experience, these supplements are often started after one or more low-energy fractures. Wrist fractures are among the most typical, and from time to time I assess, reduce and cast them in the emergency setting. Hip fractures are more serious: they usually require acute surgery, and in our practice they are triaged in the most urgent category because of their high mortality. For these reasons, calcium plus vitamin D is in my experience a widely recognized need for older people at risk of osteoporosis. That applies to men as well, but women in particular seem to be at risk.

Testing for Vitamin D Deficiency

The test to ask for is serum 25-hydroxyvitamin D, 25(OH)D, measured with a reliable assay. The 2011 guideline specifically recommends against using 1,25-dihydroxyvitamin D for this purpose.[1] If your result is reported in ng/mL rather than nmol/L, note that the guideline pairs 20 ng/mL with 50 nmol/L.[1]

Thresholds are where recommendations have shifted most. The 2011 guideline defined deficiency as 25(OH)D below 20 ng/mL (50 nmol/L) and insufficiency as 21–29 ng/mL (52.5–72.5 nmol/L).[1] The 2024 Endocrine Society guideline went further in the other direction: it no longer endorses 30 ng/mL (75 nmol/L) as a target, and no longer endorses any specific 25(OH)D level as the definition of sufficiency.[5]

Finland’s national Current Care Guideline (Käypä hoito) on osteoporosis uses a practical scale: a serum 25(OH)D below 25 nmol/L indicates severe deficiency, below 50 nmol/L indicates deficiency, 50–75 nmol/L is generally considered sufficient, and levels above 120 nmol/L are not recommended.[9] For patients with osteoporosis, it sets a target of 75–120 nmol/L.[9]

The 2024 guideline also suggests against routine 25(OH)D testing in the general adult population and in healthy adults.[5] That advice is easy to misread. The guideline states that its recommendations do not apply to people with established indications for vitamin D testing or treatment, or to conditions that substantially change vitamin D physiology, such as malabsorption.[5] At the same time, it specifically suggests against routine testing during pregnancy, in adults with obesity and in adults with dark complexion,[5] groups that overlap with the 2011 list above.[1] The 2024 guideline states that it replaces the 2011 guideline,[5] so the 2011 risk list is best read as background on who is more likely to be deficient, not as a list of current indications for routine testing.

In my own practice, testing often looks different. In occupational health, vitamin D is included in many of the basic laboratory panels I use, because, in my understanding, low levels are common in a country with a long dark season. Not every panel includes it, but as a result I measure vitamin D fairly often, and the result is quite often below our laboratory’s lower reference limit of 50 nmol/L.

For athletes deciding which markers belong in a routine panel, my guide to which blood tests athletes actually need puts vitamin D in context with iron, thyroid and other markers.

Treating Vitamin D Deficiency

Once testing confirms a low level, the practical questions in my view are about dose and duration rather than whether to treat. In my clinical practice, I recommend vitamin D supplementation to patients whose vitamin D is found to be low, following Finland’s national Current Care Guideline (Käypä hoito).[9]

The Finnish guideline gives a recommended total daily vitamin D intake of 10 µg (400 IU) for adults up to age 74 and 20 µg (800 IU) for those aged 75 and older, who are advised to take a 20 µg supplement year-round.[9] These are intake recommendations, not a treatment regimen for every confirmed deficiency. For patients with osteoporosis, the guideline bases the vitamin D dose on a measured 25(OH)D level,[9] and a daily supplement of 20 µg is usually enough to raise 25(OH)D above 50 nmol/L, and reaching the target level can be confirmed with a repeat measurement after 6–12 weeks if needed.[9] If the level rises above 120 nmol/L, the dose is too high.[9] The guideline places the upper limit of safe intake at 100 µg (4,000 IU) a day.[9]

Internationally, for adults with confirmed deficiency, the 2011 Endocrine Society guideline suggested 50,000 IU of vitamin D2 or D3 once a week for 8 weeks, or the equivalent of 6,000 IU daily, followed by maintenance of 1,500–2,000 IU a day.[1] As an approximate figure, the same guideline states that each 100 IU a day raises serum 25(OH)D by less than about 1 ng/mL.[1] That regimen was designed to reach a 25(OH)D above 30 ng/mL,[1] a target the 2024 guideline no longer endorses.[5]

For generally healthy people without deficiency, the 2024 guideline points back to the dietary reference intakes, which are 600 IU a day up to age 70 and 800 IU a day above age 70.[5]

Safety deserves the same attention as dosing:

Upper limit: the 2011 guideline sets 4,000 IU a day as the tolerable upper limit for everyone over 8 years, not to be exceeded without medical supervision. It notes that up to 10,000 IU a day may be needed to correct deficiency in adults, which is a supervised treatment dose, not a maintenance dose.[1]
Toxicity: vitamin D toxicity is rare, and the guideline places concern at blood levels above 150 ng/mL.[1]
Large single doses: in a trial of 2,256 women aged 70 or older, a yearly oral dose of 500,000 IU increased falls (rate ratio 1.15, 95% CI 1.02–1.30) and fractures (rate ratio 1.26, 95% CI 1.00–1.59) compared with placebo.[13] The Finnish Current Care Guideline likewise notes that large single doses, such as 12,500 µg (500,000 IU) once a year, increase the incidence of fractures, and that large doses given daily or less often, such as 1,500 µg (60,000 IU) once a month, have been linked to a higher tendency to fall.[9] Both point toward regular moderate doses rather than occasional very large ones.

In practice, vitamin D overdose is something I see extremely rarely, at least in my practice in Finland. During my 10-year career, I have never personally seen hypercalcemia caused by vitamin D, although such cases may well appear in endocrinology settings. That said, calcium is not part of most of the basic laboratory panels I use in occupational health, so hypercalcemia from excess vitamin D could often go unnoticed unless it caused symptoms. In my experience, overdose rarely becomes the focus of clinical assessment, which fits the 2011 Endocrine Society guideline’s view that toxicity is rare.[1]

In my experience, patients often know that a vitamin D overdose is uncommon, and that as a fat-soluble vitamin it does not have to be taken every day. As a result, many tend to take it in larger, less frequent doses. That is not something I can recommend, because larger doses are not necessarily always completely safe. Missing a day here and there is hardly a problem, but in my clinical practice I do not recommend large single doses, as they are not entirely harmless. The trial in older women described above, in which a yearly 500,000 IU dose increased falls and fractures, is one example of why.

Treatment plans should also consider calcium intake, since the strongest athletic bone trial discussed below combined the two. My article on calcium in athletes covers that side.

What Vitamin D Deficiency Symptoms Mean for Athletes

Everything above applies to athletes too, but the athlete-specific questions tend to center on three situations: bone stress injuries, illness during winter training blocks, and unexplained drops in training quality. The companion article on vitamin D in athletes linked in the introduction goes deeper into performance outcomes and supplementation protocols; this section focuses on what symptoms and test results mean in practice.

How Common Is Low Vitamin D in Elite Athletes?

A 2022 systematic review of vitamin D status in elite athletes (51 studies, 5,456 athletes) found that 30% of adult athletes (95% CI 22–39%) and 39% of adolescent athletes (95% CI 25–55%) had a 25(OH)D of 50 nmol/L or lower.[14] The authors considered these figures conservative because relatively few studies were done in winter.[14] Being fit does not guarantee good vitamin D status, so vitamin D deficiency symptoms in an athlete may deserve attention even in a young, active person.

Stress Fractures: The Clearest Athletic Signal

The clearest bone evidence comes from military training rather than from sport itself. A Finnish prospective study enrolled 800 healthy military recruits (mean age 19), of whom 756 completed the 90-day follow-up; 22 developed a stress fracture. A 25(OH)D below the group median of 75.8 nmol/L was associated with a 3.6-fold higher odds of stress fracture (95% CI 1.2–11.1).[6]

The intervention evidence points the same way. In a double-blind randomized trial of 5,201 female US Navy recruits, 2,000 mg of calcium plus 800 IU of vitamin D a day during basic training lowered stress fracture incidence from 6.6% to 5.3% in the intention-to-treat analysis, about 20% fewer fractures.[7] Because calcium and vitamin D were given together, that trial cannot isolate the effect of vitamin D alone.

Stress fractures are something I see from time to time, in my experience particularly in young athletes, although young people also tend to be more physically active on average than the sedentary population I see in clinic. When a young athlete presents with unusual foot, shin or back pain, or other bone pain, in my experience it is usually investigated further, often with MRI if plain X-rays are negative, as they often are early on,[15] and where MRI is available, we try to avoid unnecessary additional imaging in young people. Stress fractures also occur in adults, particularly when there is unusual loading behind them. In runners, for example, I see stress fractures of the foot, and sometimes a stress fracture of the shin is suspected.

When a stress fracture is suspected in a young or an older patient, I think it is usually worth adding a vitamin D measurement to the basic workup, especially in athletes. Low vitamin D is, after all, something that can be corrected easily with supplementation.

My article on stress fractures in athletes covers how bone stress injuries are recognized and diagnosed.

Frequent Colds During Winter Training

In the general population, the 2025 meta-analysis no longer found a statistically significant protective effect of vitamin D against respiratory infections.[12] Athlete-specific trials are small. In one, 25 male collegiate taekwondo athletes who were all insufficient at baseline (mean 31.3 nmol/L) took 5,000 IU of vitamin D3 a day or placebo during four weeks of winter training.[16] In the vitamin D group, 25(OH)D rose from 28.7 to 100.1 nmol/L, and upper respiratory symptom scores were lower than with placebo.[16] Only 17 athletes returned the symptom questionnaires, however, and salivary IgA did not differ between groups, so this is a promising signal rather than firm proof.[16]

If an athlete has recurrent respiratory symptoms, vitamin D status may be considered alongside other clinical factors, particularly when risk factors for deficiency are present. For decisions about training through an illness, see exercise when sick.

Strength, Power, and “Feeling Flat”

Many athletes hope vitamin D will make them stronger. A 2023 meta-analysis of 11 randomized trials in 436 athletes found that supplementation raised 25(OH)D but produced no statistically significant improvement in maximal strength or power, including in analyses stratified by baseline 25(OH)D (below 75 nmol/L or 75 nmol/L and above).[8] A review focused on athletes likewise described the effect of vitamin D on muscle function in trained people as debatable, with limited and underpowered evidence.[10]

So if training feels flat, low vitamin D is one possible factor to check, but the trials above give little reason to expect supplementation alone to restore strength or power. The broader differential is covered in why is my performance declining?

A Practical Approach for Athletes

In my clinical experience, athletes sometimes take large amounts of different vitamins in an effort to maximize performance. A belief I often encounter is that water-soluble vitamins can be taken without limit, and that because vitamin D toxicity is rarely seen, vitamin D can be taken in excess as well. I tell athletes that it is not worth taking vitamin D above the recommended amounts either: excess intake is unlikely to bring any benefit, and adverse effects may start to appear with so-called megadoses. In practice, I advise athletes to follow the same Finnish Current Care Guideline recommendations as everyone else.[9] Low vitamin D is genuinely common in elite athletes,[14] but my advice is aimed at correcting a deficiency, not at optimizing the dose, the blood level or performance.

Test when there is a reason. A stress fracture, vitamin D deficiency symptoms such as bone or muscle aching with weakness, or very limited sun exposure can all be reasons to measure 25(OH)D. An athlete-focused review recommends testing at-risk athletes before supplementing rather than supplementing everyone.[10]
Avoid blanket megadoses and bolus regimens. The same review advises against blanket supplementation, megadoses and bolus doses,[10] and the 500,000 IU trial shows why very large single doses deserve caution.[13]
Respect the upper limit. The 2011 Endocrine Society guideline and the Finnish guideline both set the upper limit of safe daily intake at 4,000 IU (100 µg); higher doses belong under medical supervision.[1][9]
Expect bone and health benefits, not a performance boost. Correcting deficiency makes sense, but a meta-analysis of trials in athletes found no statistically significant improvement in maximal strength or power with supplementation, including in analyses stratified by baseline 25(OH)D.[8]
Look at the whole picture. Vitamin D sits alongside calcium, energy availability and iron. My overview of vitamins for athletes covers where vitamin D fits among the other vitamins athletes commonly take.

Conclusion: Vitamin D Deficiency Symptoms

The vitamin D deficiency symptoms with the strongest evidence are bone and muscle aches, muscle weakness and, in older adults, falls, the classic signs described in the 2011 Endocrine Society guideline.[1] Fatigue can improve when a clear deficiency is treated,[2] while vitamin D has not prevented depression in a large trial.[3] Low levels are common, especially in winter and at northern latitudes,[4] which is why a single low number should be interpreted in context rather than taken as the explanation for every symptom.

In my clinical experience, the picture is often less dramatic than symptom lists suggest. Low vitamin D is frequently an incidental finding without symptoms, and in my experience the dark winter months in Finland bring fatigue, low mood and low vitamin D at the same time, so it is often unclear what is causing what. When a deficiency is found, I recommend correcting it in line with the Finnish Current Care Guideline rather than with large single doses.[9]

For athletes, the strongest reason to care is bone: lower 25(OH)D has been linked to stress fractures,[6] and calcium with vitamin D reduced them in military training.[7]

In my practice, a suspected stress fracture is a good reason to measure vitamin D, because a low level is easy to correct. For athletes, as for everyone else, I recommend correcting a real deficiency and leaving megadoses and performance optimization aside.

Vitamin D Deficiency Symptoms: Signs, Testing, Treatment, and What It Means for Athletes

Table of Contents

Key Takeaways: Vitamin D Deficiency Symptoms

Introduction: Vitamin D Deficiency Symptoms

How Common Is Vitamin D Deficiency?