As the colder days draw in you may be wondering what you can do to strengthen your immune system and reduce the risk of winter bugs. Dietary changes and specific supplements are a common place to start with this, so let’s examine whether there are evidence-based nutritional choices that support the immune system.

'Boosting’ the immune system

Although it’s so common to hear that XYZ will boost your immune system, this isn’t scientifically accurate.

Our immune system is complex and involves many defence mechanisms and biological interactions.

The immune system can be broken down into two main categories:

1. The innate immune system - immediate, non-specific defence including physical barriers (e.g. skin, nasal hair), secretions (e.g. saliva, stomach acid, tears) and general immune responses by certain white blood cells (e.g. phagocytes, macrophages, neutrophils, basophils, mast cells, natural killer cells and dendritic cells). 

2. The adaptive immune system - A.K.A. “acquired immunity” - this creates a memory of the threat, leading to a more specific and targeted response. Adaptive immunity involves white blood cells called B cells and T cells. One of B cells main jobs is to release proteins called antibodies which identify and attack invading substances. T cells can attack foreign cells, infected cells and cancer cells. 

So it isn’t clear which parts of the immune system would be ‘boosted’ as this could lead to unnecessary inflammation or undesirable symptoms such as fever or excessive mucus etc.

Although we may not be able to (or want to) actually ‘boost’ our immune system, we can support the normal functioning of our immune system.

Vitamin C

Loading up on vitamin C intake is one of the most popular home-remedies for the common cold, so let’s start with this.

This vitamin supports a number of cellular functions in both the innate and adaptive immune system, such as: reducing oxidative stress in the skin, reducing tissue damage related to inflammation and supporting the ability of white blood cells like neutrophils to kill invaders like bacteria (1). Therefore it’s unsurprising that vitamin C deficiency is linked with worsened immunity and increased risk of infection (1, 2).

There isn’t currently good evidence to support the use of vitamin C supplements for reducing colds and viruses in the general public (2, 3). But there have been some positive findings in specific groups that experience heightened physical stress, like athletes or those in the military (2, 3).

However, regularly supplementing with vitamin C may reduce the duration of colds (rather than the overall risk of catching a cold), but more research is needed to confirm this (3). So considering that supplementing with oral vitamin C is generally low risk, it may be worthwhile for some people to experiment with this on an individual basis to see whether this appears to help with shortening the length of colds.

A few recent studies have looked at the impact of vitamin C on the COVID-19 virus, but to date the outcomes haven’t been very positive - with the specific exception of  treating  COVID-19-induced sepsis (4).

Sources of vitamin C: peppers, kiwis, oranges, grapefruit, strawberries, tomatoes and broccoli.

Note - risk related to use of high dose vitamin C supplements include: nausea, diarrhoea, kidney stones and reduced muscle recovery in athletes.

Vitamin D

Vitamin D plays a number of roles  related to the immune system, including involvement in the production of specific antimicrobial proteins (e.g. β-defensin) and promoting macrophage development (5). Interestingly, some immune cells actually produce vitamin D as well (5).

A number of studies have found that taking vitamin D supplements is associated with a lower risk of colds (6).

Low vitamin D levels have also been found to be linked with a higher risk of COVID-19 and a more severe presentation (7, 8).

Sources of vitamin D: oily fish like salmon, sardines and trout, fortified cereals, dairy and plant milks, supplements and sunshine (on the skin).

Note - Vitamin D deficiency is common in Northern countries like the UK and Ireland, due to our limited sunshine and difficulty obtaining this in our diet. So we are recommended to consider supplementing with 10mcg of vitamin D, especially during the winter months of October to March (9). High doses of vitamin D may increase the risk of kidney stones, hardened blood vessels and an irregular heart beat over time, so the NHS advises against consuming more than 100mg per day (9).

Vitamin D

This vitamin helps to strengthen our immune barrier, as it promotes mucous secretion and is involved in epithelium formation (a protective lining which surrounds all organs) (5). Vitamin A is also involved in producing certain antibodies (e.g. Immunoglobulin A), the development of T cells and the functioning of other immune cells including natural killer cells, macrophages, B-cells  (5).

Studies have found that vitamin A deficiency is associated with increased severity of infections and a higher death rate, especially in developing countries (10). A recent study also found that low vitamin A levels were linked with recurrent respiratory infections in children (11).

Sources of vitamin A: 

• Retinol (active form of vitamin A): liver, paté, milk, yogurt, cheese, egg, oily fish (like salmon and herring) and fortified foods.

• Beta-carotene (can be converted into retinol by the body) are found in a variety of plants, such as: carrots, sweet potatoes, papaya, spinach and kale. 

Note - Vitamin A supplements need to be strictly avoided by pregnant women, as excess vitamin A can lead to birth defects. Other risks associated with excess vitamin A intake include liver damage, weakening of the bones, and in extreme cases, vitamin A toxicity (hypervitaminosis A) (12).

Zince

This mineral plays an important role in maintaining healthy skin and protective lining on organs (5). It also supports antibody production (immunoglobulin G) and the function of a number of immune cells including monocytes, macrophages and natural killer cells (5).

Zinc supplementation has been seen to reduce the risk of pneumonia in children (13). When given alongside medical treatment, it has been seen to reduce the risk of death from severe pneumonia (14).  Zinc may also help to reduce the duration of a cold (15).

Use of zinc supplements have also been associated with improved outcomes for COVID-19 inpatients (16).

Sources of zinc: shellfish, red meat, eggs, nuts, seeds, lentils, chickpeas and whole grains.

Note - Risks related to zinc supplements: high doses may compete for absorption with calcium, iron, copper and magnesium. Excess zinc may cause gut issues, slight nausea, loss of appetite and temporary taste issues (17).

Probiotics

The majority of our immune cells live in our gut, and there’s two-way communication between these immune cells and the gut microbiome (18).

Probiotics are microorganisms like bacteria which have beneficial health effects. Certain probiotics have been seen to positively interact with immune cells in the gut, triggering specific immune functions and helping to maintain balance in the immune system (18).

For example, probiotics have been seen to positively influence the composition of the mucosal barrier in the gut and prevent the growth of harmful invaders (pathogens) (19, 20).

The use of probiotic supplements has been seen to stimulate the human immune system via an increase in antibody (IgA) producing cells and a type of protein in the saliva which plays a pivotal role in the immune system (cytokines) (probiotic strains: lactobacillus casei, lactobacillus lactis, lactobacillus plantarum) (21, 22). Although it looks promising, ongoing human studies are needed to clarify the impact of probiotics on the immune system.

Sources of probiotics: Probiotic supplements (e.g. drinks, capsules, liquid supplements), probiotic yogurts, kefir, sauerkraut, kimchi and kombucha.

Note - Research in this area is still relatively new and there are currently no approved health claims in relation to the impact of probiotics on immunity. Consuming a varied diet also supports gut diversity and health. 

Other nutritional considerations

A number of other nutrients impact the immune system as well, which is why an overall balanced and varied diet is so important when it comes to a strong immune system.

For example (5):

• Protein: plays an important role in wound healing, immune cell functions, producing antibodies, enzymes and more. 

• Fats: vital for cell membrane production which plays a role in cell protection, increasing absorption of vitamins which play a role in immunity (e.g. vitamin A, D, E and K) and essential fatty acids like omega-3 are vital for regulating inflammation. 

• Vitamin E: reduces oxidative stress in the body, supports antibody production and the functioning of a number of immune cells including natural killer cells and T-cells. 

• B-Vitamins: play a variety of roles in the immune system including supporting antibody production and natural killer cell activity. 

• Iron: needed for epithelial tissue formation which is a vital immune barrier, supports T-cell production and the activity of a number of immune cells. 

• Selenium: supports antibody production, natural killer cell activity and T-cell production. 

• Copper: supports T-cell production and white blood cell and natural killer cell function

A weakened immune system is also common in people who don’t consume enough calories to support their activity levels - which can lead to a condition called relative energy deficiency in sports (RED-S) (23).

There is some limited evidence that specific foods and herbs benefit the immune system, such as:

• Garlic: has been seen to help with regulating cytokines and improving T cell and Natural Killer Cell function (24). 

• Echinacea: may support the inflammatory process, however more robust research is needed. (25) 

• Elderberries: some research suggests this may reduce the duration and severity of colds and the duration of the flu; although the current evidence supporting this isn’t robust (26). 

Beyond diet

It’s important to remember that what we eat is only one piece of the puzzle when it comes to a healthy immune system.

Other factors that support the healthy functioning of our immune system include: 

• Genetics

• Medical conditions

• Exposure to contagious illnesses

• Sleep

• Regular exercise  

• Stress levels

• Alcohol, tobacco and drug intake 

• Hand hygiene

Vaccination status e.g. getting COVID-19 and flu vaccines (when eligible)

Conclusion

Although it’s a myth that we can ‘boost our immune system’, we can support its function by having a healthy balanced diet and lifestyle. The best approach when it comes to nutrition is to focus on consuming a balanced and varied diet, avoiding nutritional deficiencies (or seeking support from a Dietitian or Registered Nutritionist if you have any gaps in your diet) and taking your vitamin D supplement during the autumn and winter months.