Low Vitamin D Increases Risk of Respiratory Infections: What Current Research Shows

Vitamin D has long been associated with bone health, but growing research suggests it may also play an important role in how the body responds to respiratory infections. In recent years, large population studies have begun to explore whether vitamin D status is linked to the risk of developing more severe respiratory illness — including infections serious enough to require hospital care.

One of the largest analyses to date, led by researchers from the University of Surrey using data from the UK Biobank, found that people with very low vitamin D levels were significantly more likely to be hospitalised with respiratory tract infections such as bronchitis and pneumonia. These findings, published in peer-reviewed journals including The American Journal of Clinical Nutrition, have renewed interest in vitamin D as a potential marker of vulnerability to serious respiratory illness.

Importantly, this research does not suggest that vitamin D alone prevents infections or replaces other health measures. Instead, it highlights how severe vitamin D deficiency may be associated with increased risk, particularly in older adults and those already more vulnerable to illness.

What the research found

Much of the recent discussion around vitamin D and respiratory infections comes from a large population-based analysis using data from the UK Biobank, one of the world’s most extensive long-term health studies. Researchers analysed health and blood test data from 36,258 UK adults, examining whether vitamin D status was associated with hospitalisation for respiratory tract infections (RTIs).

The study focused on blood levels of 25-hydroxyvitamin D, the standard marker used to assess vitamin D status. Participants were followed over time using linked hospital records to identify admissions related to respiratory infections such as bronchitis and pneumonia.

Key findings from the analysis

Several important patterns emerged:

• Severe vitamin D deficiency was linked to higher risk
  Participants with very low vitamin D levels (below 15 nmol/L) had an approximately 33% higher risk of being hospitalised for a respiratory tract infection compared with those who had sufficient levels (75 nmol/L or higher).
  

• Higher vitamin D levels were associated with lower risk
  For every 10 nmol/L increase in vitamin D, the risk of hospitalisation due to respiratory infection decreased by around 4%.
  

• The strongest association occurred at very low levels
  Mild to moderate vitamin D deficiency did not show a statistically significant increase in hospitalisation risk. The elevated risk was concentrated among those with severe deficiency, suggesting a possible threshold effect rather than a simple linear relationship.
  

• Findings were consistent across ethnic groups
  Although vitamin D deficiency was more common in some ethnic groups, the association between low vitamin D levels and respiratory infection hospitalisation did not differ significantly by ethnicity, indicating a broadly similar risk pattern across the population.

These findings were supported using multiple statistical approaches, strengthening confidence in the observed associations. However, the authors also emphasised that this was an observational study, meaning it identifies relationships rather than proving cause and effect.

What counts as “low” vitamin D?

Vitamin D deficiency often develops gradually, and many people are unaware their levels are low until symptoms appear — or a blood test reveals it. Because the signs can be subtle or overlap with other health issues, low vitamin D is frequently under-recognised.

Common signs and symptoms of low vitamin D

Symptoms can vary between individuals, but commonly reported signs include:

• Ongoing fatigue or low energy
  
• Frequent illnesses or infections
  
• Muscle aches, weakness, or cramps
  
• Bone or joint pain
  
• Low mood or mood changes
  
• Slower recovery from illness

In more severe cases, prolonged deficiency may affect bone strength and muscle function, particularly in older adults.

Read related article: Symptoms of Vitamin D Deficiency: How to Test for It

How vitamin D levels are measured

Vitamin D status is assessed through a blood test measuring 25-hydroxyvitamin D, reported in nanomoles per litre (nmol/L). While thresholds may vary slightly between guidelines, according to clinical guidance from the Royal Children’s Hospital Melbourne, levels are generally classified as follows:

• Severe deficiency: below 12.5 nmol/L
  
• Moderate deficiency: 12.5–29 nmol/L
  
• Mild deficiency: 30–49 nmol/L
  
• Sufficient: 50 nmol/L or higher
  
• Elevated: 250 nmol/L or higher

Vitamin D toxicity is considered rare and is typically associated with very high levels (above 250 nmol/L), particularly when accompanied by elevated calcium levels and suppression of parathyroid hormone.

Read related article: Can You Have Too Much Vitamin D? What You Need to Know About Safe Supplementing

Why vitamin D plays a role in respiratory health

Vitamin D does more than support bone health. It also plays a role in how the immune system functions — particularly in the body’s response to respiratory infections.

Supports immune defence in the airways

Vitamin D helps regulate both innate and adaptive immune responses. In simple terms, this means it supports the immune system’s ability to:

• Recognise and respond to invading pathogens
  
• Produce antimicrobial peptides that help limit viral and bacterial growth
  
• Maintain the integrity of the respiratory lining, which acts as a first line of defence in the airways

These mechanisms are especially relevant in the lungs and upper respiratory tract, where infections commonly take hold.

Helps balance the immune response

An effective immune system needs to respond strongly enough to fight infection — but not so aggressively that it causes excessive inflammation. Vitamin D is thought to help modulate immune activity, supporting a more balanced response.

This balance is important in respiratory infections, where excessive inflammation can contribute to worsening symptoms and complications, particularly in vulnerable individuals.

May influence susceptibility, not exposure

Vitamin D does not prevent exposure to respiratory viruses or bacteria. Instead, adequate levels may influence how the body responds once exposure occurs. This helps explain why research tends to show associations with illness severity and hospitalisation, rather than infection rates alone.

Ways to support healthy vitamin D levels

Maintaining healthy vitamin D levels doesn’t usually rely on a single approach. Instead, it’s best supported through a combination of sun exposure, diet, and — where appropriate — supplementation, tailored to individual circumstances.

1. Sensible sun exposure

Sunlight is the primary natural source of vitamin D for most people. When ultraviolet B (UVB) rays reach the skin, they trigger vitamin D production.

In Australia:

• Short periods of sun exposure can contribute to vitamin D levels, depending on location, season, skin type, and time of day
  
• Vitamin D production is generally lower during winter months, particularly in southern regions
  
• People with darker skin may require longer sun exposure to produce the same amount of vitamin D as those with lighter skin

It’s important to balance vitamin D needs with sun safety guidance, including skin cancer prevention. Sunscreen use, when applied normally, does not eliminate vitamin D production and remains an important protective measure.

2. Dietary sources

Only small amounts of vitamin D are obtained through food, but dietary intake can still contribute to overall levels.

Common sources include:

• Fatty fish such as salmon, sardines, and mackerel
  
• Eggs
  
• Fortified foods, such as some dairy products and breakfast cereals

Diet alone is often insufficient to maintain optimal vitamin D levels, particularly for people with limited sun exposure.

3. Supplementation (where advised)

Vitamin D supplements may be recommended in certain situations, particularly for individuals at higher risk of deficiency, such as:

• Older adults
  
• People who spend most of their time indoors
  
• Individuals with limited sun exposure
  
• People with darker skin tones
  
• Those with medical conditions affecting absorption

Supplementation should be guided by individual needs and healthcare advice, as excessive intake can lead to elevated vitamin D levels over time.

👉 Shop vitamin D supplements at Aussie Pharma Direct.

Why monitoring vitamin D levels can be helpful

Vitamin D deficiency is often described as a silent issue — not because it is rare, but because it frequently develops without clear or specific symptoms. Many people assume their vitamin D levels are adequate based on diet, sun exposure, or general health, yet population data suggests deficiency is common, particularly during winter months and in at-risk groups.

Symptoms aren’t always reliable

Fatigue, low mood, muscle aches, or frequent illness can be linked to many different factors. Because these symptoms are non-specific, they don’t reliably indicate whether vitamin D levels are low — or where those levels sit on the deficiency spectrum.

This is why objective measurement matters.

Levels can vary more than expected

Vitamin D status can change over time due to:

• Seasonal shifts in sun exposure
  
• Changes in work or lifestyle (such as spending more time indoors)
  
• Age-related differences in vitamin D synthesis
  
• Underlying health conditions affecting absorption

Without testing, these changes often go unnoticed.

Accessible options for checking vitamin D status

For people who want greater visibility into their vitamin D levels, at-home vitamin D testing offers a convenient screening option. These tests allow individuals to measure their vitamin D status without the need for an initial clinic visit and can be useful for ongoing monitoring, particularly for carers, families, or those managing long-term health considerations.

At-home testing is not a replacement for medical diagnosis, but it can be a practical first step in understanding vitamin D status and supporting more informed health conversations.

Need to test your vitamin D levels now? Buy the Alltest Vitamin D Rapid DIY Test Kit at Aussie Pharma Direct.

Conclusion

Research continues to highlight an important association between severe vitamin D deficiency and the risk of serious respiratory infections. While vitamin D is not a cure, a treatment, or a guarantee against illness, very low levels may signal increased vulnerability — particularly in older adults and those already at higher risk.

Vitamin D is one piece of a much larger health picture that includes nutrition, lifestyle, underlying conditions, and access to appropriate medical care. Understanding your vitamin D status can help support more informed decisions, especially during seasons when respiratory infections are more common.

For many people, maintaining healthy vitamin D levels is achievable through a combination of sensible sun exposure, diet, and — where appropriate — supplementation. Monitoring levels can add clarity, helping to avoid unnecessary guesswork and supporting constructive conversations with healthcare professionals.

Essentials to Help You Better Understand and Support Your Vitamin D Levels