This piece was written by one of our contributors; dietitian – Maeve Hanan.
It’s that time of year when many people will be looking for ways to ‘boost their immune system’ in the hopes of avoiding coughs and colds.
Let’s take a look at the evidence as to whether our diet can have a positive impact on our immune system.
The Immune System
Our body uses the immune system to defend itself from harmful invaders (A.K.A. pathogens) like viruses, parasites, bacteria and fungi. Ultimately, this helps to reduce our risk of infection and disease.
This complex system is made up of two main parts:
- The innate immune system – this triggers an immediate, non-specific response to pathogens. This response includes physical barriers (such as our skin and eyelashes), secretions (such as mucous, stomach acid, saliva, sweat and tears) and general immune responses by certain white blood cells (such as phagocytes, macrophages, neutrophils, basophils, mast cells, natural killer cells and dendritic cells).
- The adaptive immune system – this is also referred to as “acquired immunity” as it creates a memory of the threat, leading to a more specific and targeted immune response. This uses white blood cells called B cells and T cells which are created in our bone marrow. B cells secrete proteins called antibodies which identify and attack invading substances. B cells also secrete cell signalling molecules called cytokines which aid communication between cells in immune responses. T cells can attack foreign cells, cancer cells and cells which are infected with a virus.
Is it Possible to ‘Boost our Immune System’?
This concept is overly simplistic and not very scientific.
Our immune system is complex and involves numerous different defense mechanisms and interactions. So it isn’t clear what an optimal ‘boosted immune system’ would look like in terms of white blood cell levels etc. Furthermore, the body has a clever system for destroying excess immune cells (called apoptosis). This helps the body to maintain the right level of immune cells to promote immunity, while also reducing the risk of excess inflammation.
For example, if it was possible to boost the innate immune system (which it isn’t), this would result in an unpleasant inflammatory response such as creating mucous or becoming feverish – Trust us, this is not something you want!
In fact the only way the immune system can be ‘boosted’ is by speeding up the response of the adaptive immune system using a vaccination. This works by introducing a harmless amount of a pathogen into our system, so that the body can create antibodies. Therefore, the adaptive immune response occurs more efficiently the next time it comes into contact with that pathogen. For more information about the importance of vaccines check out this overview of vaccines and this article about the flu vaccine.
However, although we may not be able to ‘boost’ our immune system, can support the normal functioning of our immune system.
Let’s look at some dietary factors that play a role in our immune health.
Increasing vitamin C intake is one of the most popular home-remedies for the common cold. This antioxidant is found in many types of fruit and vegetables, including: peppers, kiwis, oranges and broccoli.
A high quality review from 2013 found that regular use of vitamin C supplements didn’t reduce the risk of developing a cold in the general public (1). However, consuming 1-2g of vitamin C per day was found to reduce the length and severity of a cold, particularly for those who experience short periods of extreme physical stress (such as professional athletes) (1). This effect was only seen when taken on a regular basis before the symptoms of a cold develop. The authors highlight that further research is needed to see whether it is useful to start taking vitamin C supplements once a cold has already developed. Check out our article on this here.
NOTE! Risk related to use of high dose vitamin C supplements include: nausea, diarrhoea, kidney stones and reduced muscle recovery in athletes.
Vitamin A deficiency is associated with a weakened immune system and a higher risk of infection. 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) (2). Vitamin A is also needed for the development of macrophages – a type of white blood cells that engulf harmful substances in the body. Furthermore, animal studies have found that vitamin A deficiency impairs T cell and antibody function (3). More human research is needed to investigate how vitamin A impacts our immune system.
Vitamin A is found in two main forms in our diet:
- Retinol is found in animal sources such as milk and liver
- Carotenoids like beta-carotene are found in a variety of plants, such as: carrots, sweet potatoes, papaya, spinach and kale
However, we need to be cautious about the use of vitamin A supplements. These need to be strictly avoided by pregnant women, as excess vitamin A can lead to birth defects. Other risks associated with high dose or regular use of vitamin A supplements includes liver damage and weakening of the bones. The CARET study from 2004 found that smokers who used vitamin A supplements had a higher risk of death from heart disease and lung cancer (4).
NOTE! Vitamin A supplements can also interact with other medications (such as Acne medications) so please speak to your GP or pharmacist before taking any new supplements.
Due to the association between vitamin D deficiency and autoimmune disorders, it is thought that vitamin D is needed for a healthy immune system (5, 6). This may be related to the role of vitamin D in T cell function, but more research is needed in this area (6).
Vitamin D deficiency is common in Northern countries like the UK and Ireland, due to our limited sunshine and difficult obtaining this in our diet. Therefore, we are all recommended to consider supplementing with 10mcg of vitamin D, especially during the winter months of October to March (5).
High doses of vitamin D may increase the risk of kidney stones, hardened blood vessels and an irregular heart beat (5). Therefore, it is recommended to limit vitamin D intake from supplements to 25mcg per day.
Zinc deficiency has been found to negatively impact many cells related to the innate and adaptive immune system, such as: neutrophils, natural killer cells, macrophages, B cells and T cells (7).
Dietary sources of zinc include: shellfish, red meat, eggs, nuts, seeds, lentils, chickpeas and whole grains.
Using zinc acetate lozenges daily for up to one week has been found to reduce the severity and duration of a cold, when taken within 24 hours of the first symptoms presenting (8).
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 (9). Zinc gels, nasal sprays and swabs have been seen to lead to permanent loss of smell and taste in some cases, so these are not recommended (10).
Studies have found that 600 – 1200mg aged garlic extract (which is similar to 3-6 cloves of garlic) may help to reduce the risk of developing a cold or infection (11). This is because garlic can increase T cell production and support the ability of white blood cells to fight harmful invaders (12).
Risks related to garlic supplements: interaction with medications such as blood thinners, anticoagulants, certain oral contraceptive pills and certain HIV medication – so consult your doctor before taking garlic supplements if you are on medication. In very high doses, garlic can also be toxic. A less serious side effect can be smelly breath and body odour!
As the majority of our immune cells live in our gut, it is thought that promoting overall gut health can support our immune system. More specifically, consuming probiotics may help to mucosal cell, epithelial cells and T cells (13, 14). Some studies have also found that certain strains of lactobacillus casei and lactobacillus plantarum may reduce the risk of viral infections (14, 15). However, research in this area is still quite new and there are no approved health claims in relation to the impact of probiotics on immunity.
For more information on probiotics check out this article.
The following options may be beneficial for the immune system, however there is less evidence to back these up:
- N-acetylcysteine supplements
Beyond diet, there are many other lifestyle factors that support the healthy functioning of our immune system, such as:
- Getting enough sleep
- Regular exercise
- Managing stress levels
- Avoiding an excess intake of alcohol or drugs
- Not smoking
- Hand hygiene
- Getting the flu vaccine if you are eligible
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).
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. A diet which includes enough energy and a good variety of nutrients (including vitamin C, vitamin A and zinc) is a vital component of this.
Most people who live in the UK and Ireland should consider taking a vitamin D supplement during the winter months for their bone health, and this may have an added benefit for the immune system. In addition, those who are exposed to physical stress or are prone to colds may benefit from regular vitamin C supplements, probiotics or garlic supplements.
(1) Hemilä & Chalker (2013) “Vitamin C for preventing and treating the common cold” [accessed November 2019 via: https://www.cochrane.org/CD000980/ARI_vitamin-c-for-preventing-and-treating-the-common-cold] [accessed November 2019 via: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6162863/]
(2) Van Bennekum et al. (1991) “Mitogen response of B cells, but not T cells, is impaired in adult vitamin A-deficient rats” [accessed November 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/1941260]
(3) Goodman et al, (2004) “The Beta-Carotene and Retinol Efficacy Trial: incidence of lung cancer and cardiovascular disease mortality during 6-year follow-up after stopping beta-carotene and retinol supplements” [accessed November 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/15572756?dopt=Abstract]
(4) SACN (2016) “Vitamin D & Health” [accessed November 2019 via: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/537616/SACN_Vitamin_D_and_Health_report.pdf]
(5) Saul et al. (2019) “1,25-Dihydroxyvitamin D3 Restrains CD4+ T Cell Priming Ability of CD11c+ Dendritic Cells by Upregulating Expression of CD31” [accessed November 2019 via: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6447667/]
(6) Prasad (2008) “Zinc in Human Health: Effect of Zinc on Immune Cells” [accessed November 2019 via: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2277319/]
(7) is Singh & Das (2013) “Zinc for the common cold.” [accessed November 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/23775705]
(8) National Institute of Health “Zinc” [accessed November 2019 via: https://ods.od.nih.gov/factsheets/Zinc-HealthProfessional/#h8]
(9) Hsieh et al. (20161) “Mechanistic studies of the toxicity of zinc gluconate in the olfactory neuronal cell line Odora” [accessed November 2019 via: https://www.sciencedirect.com/science/article/abs/pii/S088723331630087X]
(10) Examine.com “Summary of Garlic” [accessed November 2019 via: https://examine.com/supplements/garlic/]
(11) Nantz et al. (2012) “Supplementation with aged garlic extract improves both NK and γδ-T cell function and reduces the severity of cold and flu symptoms: a randomized, double-blind, placebo-controlled nutrition intervention” [accessed November 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/22280901]
(12) Galdeano et al. (2019) “Beneficial Effects of Probiotic Consumption on the Immune System” [accessed November 2019 via: https://www.ncbi.nlm.nih.gov/pubmed/30673668]
(13) Yan & Polk (2011) “Probiotics and immune health” [accessed November 2019 via: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4006993/]
(14) Kanauchi et al. (2018) “Probiotics and Paraprobiotics in Viral Infection: Clinical Application and Effects on the Innate and Acquired Immune Systems” [accessed November 2019 via: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6006794/]