How Vitamin D Supports Immunity During Cold and Flu Season

When the days get darker and illnesses are more likely to spread during autumn and winter, many people look for ways to increase their body’s defences. 

This is where vitamin D comes in, playing a vital role in protecting your body from colds, flu, and other infections. It ultimately increases your vitamin D levels to support and maintain your immune system throughout the low-sun months, where many struggle to get enough sunlight.     

In this guide, we will explore the role of vitamin D in building healthy immune systems and the best sources available. 

How Do Immune Systems Fight Off Illnesses?  

Your immune system acts as a natural defence against germs such as bacteria and viruses. It is made up of two key systems: innate, which delivers an immediate response, and adaptive, which seeks to learn, recognise, and fight off current and future illnesses. These systems produce immune cells, including white blood cells, helping to neutralise and combat infections.   

The Role of Vitamin D and Immune Systems 

There is often a misconception that vitamin D supplements are only used to build bone and muscle strength; however, vitamin D is essential to boosting your immune system.     

A strong vitamin D immune system response provides your body with the right nutrients to keep your defences healthy, boosting your innate and adaptive response systems. They activate immune cells such as macrophages, stimulating antimicrobial proteins and increasing the physical barrier of the respiratory tract.    

As people experience less exposure to sunlight in autumn and winter, they can be more susceptible to respiratory infections. As reported in Science Daily, a study with 19,000 adults showed that over a third (36%) of individuals have low vitamin D levels, and there are worries that not enough Britons are getting the appropriate amount needed. A recent study by Medichecks (Dulcie Phipps, 2025) has also shown that younger people are more at risk of low vitamin D levels due to indoor lifestyles, poor diets, location, and lack of awareness.  

Sourcing alternative solutions can help to increase defences. A good diet of vitamin D rich foods, like oily fish and fortified foods, can build your immune system, while supplements help to provide a steady supply of vitamin D throughout the winter months. 

How Effective is Vitamin D for Immunity?  

Several scientific studies have demonstrated the effectiveness of vitamin D for colds, flu, and other illnesses. For example, a study in the British Medical Journal (A R Martineau, 2017) found that vitamin D supplementation can protect against acute respiratory tract infections. 

Elsewhere, in another study published in Nutrition Journal (2024), it was discovered that supplementation may slightly prevent acute respiratory infections, when doses of the correct amount are taken in summer, autumn, and winter.    

These studies prove that vitamin D is a practical means of preventing illness during the low-sun months; however, it should not be considered a cure. According to the NHS (2020), 10 mcg is the recommended daily dosage of vitamin D.     

The Best Type of Vitamin D for Immune System Support 

When selecting your vitamin D supplement, it’s important to understand the difference between vitamin D2 and D3. While both can increase vitamin D levels, D3 is the form your body naturally produces when exposed to sunlight.    

It is widely believed that D3 is the best type of vitamin D to strengthen your immune system, as it is highly effective in maintaining the correct levels in the blood. In addition, it can work well alongside vitamins A, B6, B12, C, and E.    

Protect Your Immune System this Winter  

At Sunvit D3, we believe that it’s better to be proactive in building your immune system before the height of winter. It’s important to note that vitamin D is not a cure for colds and flu, and you can still be prone to getting ill when taking supplements. However, it does provide a proven way to help prevent illness.  

Alongside exercise, exposure to sunlight, sufficient rest, and health foods, supplements can help to maintain your vitamin D levels to reduce your risk of seasonal illness.  

We provide a range of vitamin D3 supplements to improve immune function and overall health. Our high-quality supplements are available in capsule and tablet form, and we offer a range of options, from daily doses to high-strength formulas.   

Discover more on our vitamin D3 page.