Winter Immunity: The Nutrients That Really Work

You may notice that as soon as all the kids are back in school, everyone is coming down with something. Or, you might find that every year when you finish work for Christmas, you all come down with the flu. Why is this happening? And what can we do about it?

When the weather turns colder, shorter daylight hours can leave many of us feeling completely run down. You may find yourself catching more colds, feeling more tired, or noticing that your gut feels more unsettled - all of which can affect our immune resilience. 

The good news is that there are things we can do with our food that can strengthen our body’s defences. This blog explores why immunity tends to decline in winter and which nutrients have the strongest scientific backing for supporting immune health.

Why Immunity Declines in Winter

There are several reasons why winter puts extra pressure on your immune system.

Reduced sunlight and vitamin D synthesis
From October to March in the UK, the sun’s rays are too weak for the skin to produce enough vitamin D. The process depends on ultraviolet B (UVB) light converting a cholesterol compound in the skin (7-dehydrocholesterol) into vitamin D3. So, winter is a time of year when the sun is low in the sky and people are spending more time indoors, therefore production effectively shuts down.

Research shows that average vitamin D levels fall steadily through winter and early spring. This is a particular issue for people with darker skin tones, because higher levels of melanin reduce the skin’s ability to synthesise vitamin D from sunlight. People of South Asian, African, or Caribbean heritage are therefore at higher risk of deficiency in the UK climate, and maintaining good status often requires dietary supplementation. Older adults, people who cover their skin for cultural or medical reasons, or those with gut absorption issues are also more likely to have low vitamin D.

More time indoors
As temperatures drop, we spend more time inside in close proximity to others. 

This makes it easier for respiratory viruses to spread, especially when windows are closed and air circulation is poor. Cold, dry air may also help viruses survive longer in the environment.

Lifestyle and digestive changes
Shorter days can disrupt sleep and mood, both of which directly influence gut motility and digestive secretions through the gut–brain axis. Reduced sunlight lowers serotonin levels, and since serotonin helps regulate intestinal movement, this can slow digestion and may lead to bloating or constipation.

People often move less and drink less water in winter, which reduces the mechanical stimulation of the gut and slows peristalsis. Colder weather can also shift food choices toward heavier, starchier meals and fewer fresh fruits and vegetables, reducing fibre intake and microbial diversity.

Meanwhile, chronic low-grade stress – common during darker months – can activate the sympathetic nervous system thereby suppressing the “rest and digest” parasympathetic response. This leads to lower stomach acid and digestive enzyme secretion, which can reduce nutrient absorption. Together, these factors make digestion feel slower and less efficient, particularly in those already prone to gut symptoms.

And, because around 70% of immune cells are located in or around the gut wall, any disruption to digestion and gut barrier function can weaken immune regulation, leaving the body more vulnerable to infection and inflammation.

Key Nutrients for A Resilient Immune System

But it’s not all bad news! There are several things that we can do to support our immune system over these cold, wintry months. Here are the most evidence-based:

Vitamin D and Immune Defence

Vitamin D acts more like a hormone than a vitamin, influencing over 200 genes, many of which are involved in immune regulation. 

Immune cells carry vitamin D receptors, allowing vitamin D to influence how they behave. It helps maintain the integrity of epithelial barriers (such as the linings of the lungs and the gut), encourages the production of antimicrobial peptides that directly fight pathogens, and supports a balanced immune response - strong enough to defend, but not so strong that it triggers excess inflammation.

Why winter matters
During the darker months, vitamin D synthesis from sunlight is negligible across the UK. Dietary sources are limited, and deficiency is common in people who spend little time outdoors or have darker skin. Low levels have been linked with increased susceptibility to respiratory infections and slower recovery.

While research findings are mixed, studies consistently show that those who start winter with low vitamin D tend to benefit most from supplementation. The evidence suggests that steady, daily doses are more effective than occasional high doses. In the UK, public health guidance recommends everyone take a 10 µg (400 IU) daily vitamin D supplement between October and March, with some people needing higher amounts under clinical supervision.

Zinc

Zinc is critical for the development and activity of immune cells. It helps regulate inflammation, supports wound healing, and is essential for maintaining the integrity of the skin and mucous membranes - our first physical barriers to infection.

Even mild zinc deficiency can impair immune function. Several randomised controlled trials have shown that zinc lozenges, when taken within 24 hours of the onset of cold symptoms, can shorten duration by a few days. The effect depends on the formulation and dose: lozenges that provide more than 75 mg elemental zinc per day, taken for short periods, appear most effective.

Good dietary sources include shellfish, beef, lamb, cheese, pumpkin seeds, and wholegrains. Because high intakes can interfere with copper absorption, long-term supplementation should not be done without supervision.

Vitamin C

Vitamin C is one of the body’s main antioxidants and plays an essential role in immune defence. It supports the function of white blood cells, enhances the activity of natural killer cells, and protects immune cells from oxidative stress during infection. It also helps maintain the integrity of those epithelial barriers, including the gut and lungs.

While daily supplementation with vitamin C does not seem to prevent colds in the general population, it can modestly reduce their duration and severity. People under physical stress, such as athletes or those with higher oxidative load, may benefit more. 

Regular intake of vitamin C-rich foods - citrus fruits, kiwi, berries, peppers, and cruciferous vegetables - remains the best approach for most people.

Selenium

Selenium is a trace mineral needed to produce selenoproteins, enzymes that protect immune cells from oxidative damage. It helps control inflammation, supports the production of antibodies, and assists in the clearance of pathogens.

In regions with low selenium soil levels, such as parts of Europe, sub-optimal intake may be more common. Deficiency has been associated with reduced immune responses and more severe infections. 

Most people can maintain good levels by eating selenium-rich foods such as Brazil nuts (just one or two per day), eggs, tuna, sardines, and wholegrains.

Probiotic foods and the Gut–Immune Axis

Around two-thirds of immune activity originates in or around the gut. The gut microbiota plays a vital role in training and regulating immune responses. When the balance of gut bacteria is disrupted, for example, after antibiotics or periods of high stress, the immune system can become either sluggish or over-reactive.

Several meta-analyses have found that specific probiotic strains can reduce the risk or duration of upper respiratory tract infections, particularly in children and older adults. Benefits are strain-specific and depend on regular intake. 

Fermented foods such as live yoghurt, kefir, sauerkraut, and kimchi can contribute to microbial diversity. Always check with a nutritionist before embarking on probiotic supplementation, as it is important to pick the right strain.

Everyday Food Sources of Immune Nutrients

You can support immune resilience through simple, whole-food choices:

• Vitamin D: Oily fish such as salmon, sardines, and mackerel; eggs; fortified foods; and mushrooms exposed to UV light.
• Zinc: Shellfish, red meat, cheese, pumpkin and sesame seeds, and wholegrains.
• Vitamin C: Citrus fruits, kiwi, berries, peppers, broccoli, Brussels sprouts.
• Selenium: Brazil nuts, eggs, tuna, sardines, and wholegrain cereals.
• Probiotics: Live yoghurt, kefir, sauerkraut, kimchi, and other fermented vegetables.

For those of us with gut symptoms, make sure to introduce higher-fibre or fermented foods gradually and support digestion with mindful eating and adequate hydration.

If you find that you’re frequently run down in winter or struggle with fatigue and gut symptoms, it may be time to review your nutrient intake and gut–immune balance.  Book a free 30-minute call with me, and we can talk through options to support your energy, gut health, and immune strength through the colder months.