Vitamin D deficiency: 5 warning signs to watch for | Clinical evidence and practical context

Vitamin D deficiency: 5 warning signs to watch for begins with a central clinical limitation: symptoms can be subtle, nonspecific, or absent. Persistent bone pain, muscle weakness, fatigue, recurrent illness, and fractures may warrant medical assessment, but no single symptom confirms low vitamin D. Blood testing and evaluation of other possible causes are important before treatment decisions are made. 1

Why vitamin D matters for bones and muscles

Vitamin D helps the body absorb calcium, supporting normal bone mineralisation and skeletal strength. When deficiency becomes severe or prolonged, adults may develop osteomalacia, a softening of the bones, while children may develop rickets, which can affect bone growth and motor development. The Merck Manual identifies bone and muscle weakness and pain as central effects of inadequate vitamin D. 1

Vitamin D also has a role in muscle movement and nerve signalling, although research on broader effects, including mood, immunity, and chronic disease prevention, remains mixed for several conditions. Large observational studies can show associations without proving that low vitamin D directly caused an illness. Consequently, symptoms should be interpreted alongside medical history, diet, sun exposure, medications, and laboratory findings rather than treated as a standalone diagnosis. 2

Warning sign 1: Persistent bone pain

A deep, persistent ache in the lower back, hips, legs, ribs, or pelvis can occur when inadequate vitamin D contributes to poor bone mineralisation. Penn Medicine lists dull aching pain in the hips, lower back, legs, and ribs among common osteomalacia symptoms. This discomfort may develop gradually and can be mistaken for ordinary aging, an injury, arthritis, or another musculoskeletal condition. 10

Bone pain is more concerning when it occurs with tenderness, difficulty walking, or a fracture after little or no injury. Prolonged deficiency can weaken bones and increase fracture risk through osteomalacia, while osteoporosis involves reduced bone density and has different underlying mechanisms. A clinician may consider a 25-hydroxyvitamin D blood test and, when appropriate, imaging or assessment of calcium and phosphate status. 10

Warning sign 2: Muscle weakness or cramps

Muscle weakness is a recognised possible manifestation of vitamin D deficiency, particularly in the thighs and shoulders. Practical clues include difficulty climbing stairs, rising from a chair, walking, or maintaining balance. Merck describes muscle weakness as an important consequence, and Johns Hopkins notes that weakness can occur with bone pain. These changes may be gradual rather than sudden. 19

Frequent muscle aches, cramps, twitching, or spasms can also appear when vitamin D deficiency is accompanied by low calcium or other mineral disturbances. However, these symptoms have many potential explanations, including medication effects, neurological disorders, dehydration, and other nutritional problems. New, severe, or rapidly worsening weakness requires clinical evaluation, especially when it affects walking, breathing, swallowing, or everyday independence. 10

Warning sign 3: Ongoing tiredness

Persistent fatigue or low energy is often reported in connection with low vitamin D, but it is one of the least specific warning signs. Mayo Clinic includes fatigue among possible symptoms or effects associated with deficiency, while healthdirect notes that many people have no symptoms at all. Sleep problems, anemia, thyroid disease, infection, depression, and chronic illness can produce similar tiredness. 74

Fatigue becomes more relevant when it appears alongside bone pain, muscle weakness, limited sunlight exposure, or a condition that reduces vitamin D absorption. It should not be used to justify high-dose supplementation without assessment, because excessive vitamin D can cause harmful calcium elevations. A clinician can decide whether testing is appropriate and whether another explanation better accounts for the symptom pattern. 8

Warning sign 4: Repeated illnesses or infections

Low vitamin D status has been associated with immune function and respiratory illness, but recurrent infections are not specific enough to diagnose deficiency. A 2026 analysis of more than 36,000 adults found that participants with the lowest vitamin D levels were 33% more likely to be hospitalised for respiratory infections than those with the highest levels over 15 or more years. This was an observational association, not proof of causation. 3

Repeated colds, flu-like illnesses, bronchitis, or other infections should therefore prompt attention to the overall clinical picture rather than a single-nutrient explanation. Immune disorders, exposure patterns, smoking, respiratory disease, and age may also affect infection frequency. Vitamin D testing may be considered when recurrent illness occurs with other suggestive symptoms or recognised risk factors, but routine testing of every asymptomatic person is not generally recommended. 36

Warning sign 5: Fragility fractures or childhood bone changes

A fracture caused by minimal force can signal weakened bone and warrants medical assessment. In adults, severe vitamin D deficiency may cause osteomalacia, making bones softer and more vulnerable to bending or breaking. In children, rickets may present with delayed growth, bone deformities, bone pain, or delayed motor development, such as being slow to sit or crawl. 16

Childhood signs can include a soft skull or abnormal bone growth, while adults may experience difficulty walking or a waddling gait. These findings have important alternative causes, including calcium or phosphate disorders, malabsorption, kidney disease, and inherited conditions. A suspected deficiency should be investigated with appropriate clinical examination, laboratory testing, and sometimes X-rays rather than inferred from appearance alone. 110

Who faces higher risk and how deficiency is confirmed

Risk increases with limited sunlight exposure, darker skin, covered skin, an indoor or housebound lifestyle, malabsorption conditions, chronic liver or kidney disease, bariatric surgery, obesity, and certain enzyme-inducing medications. In the United Kingdom, government guidance identifies several at-risk groups who may need vitamin D throughout the year, while many adults and children over four are advised to consider 10 micrograms daily during autumn and winter. 5

The standard assessment is a serum 25-hydroxyvitamin D test, which reflects vitamin D stores. The active form, 1,25-dihydroxyvitamin D, is not routinely used to assess stores because it can remain normal or be misleading. A result commonly interpreted as deficient is below 20 ng/mL, although thresholds and treatment decisions can vary by guideline, medical condition, and laboratory. 69

What to do with a possible warning sign

Symptoms alone cannot establish vitamin D deficiency because fatigue, pain, weakness, mood changes, and repeated illness overlap with many conditions. Assessment is particularly reasonable when several signs occur together, when a person belongs to a recognised high-risk group, or when there is osteoporosis, a fragility fracture, or evidence of malabsorption. Clinical review can determine whether testing and further investigation are justified. 46

Vitamin D supplements can correct deficiency, but dosing should account for age, medical history, kidney function, calcium status, pregnancy, and existing supplements. High-dose products are not automatically safer or more effective, and prolonged excessive intake can produce toxicity. The evidence supports identifying genuine deficiency and addressing its cause, while avoiding claims that supplementation prevents or treats every condition linked with low vitamin D. 12

Sources

1. Merck Manual Consumer Version
2. UPMC HealthBeat
3. Harvard Health Publishing
4. NHS
5. GOV.UK
6. National Institutes of Health Office of Dietary Supplements
7. Mayo Clinic
8. Cleveland Clinic
9. Johns Hopkins Medicine
10. Penn Medicine