What is the immune system?
The immune system refers to the complex collection of body tissues, cells, chemicals and processes that function synergistically to help us resist and recover from exposure to pathogens including bacteria, viruses, fungi, parasites, cancer cells and toxins (Marshall et al. 2018). How well we protect ourselves from daily exposure to pathogens and recover from illness is largely attributable to how well our immune system is functioning (Hechtman 2012).
The consequences of a malfunctioning immune system can include inflammation, autoimmunity, immunodeficiency and hypersensitivity which can lead to the development of illness and disease (Marshall et al. 2018). The ability of our immune system to defend and respond is influenced by many factors, including our genes, lifestyle, nutrition, environment, age, and prior infections (Hemilä 2017). It is however, possible to strengthen our immune system to improve its ability to perform surveillance and protection, recovery and return to homeostasis.
One of the most prevalent pathogenic burdens to our immune system are respiratory viruses of which the Rhinovirus (the common cold) and Influenza (the flu) are considered to be some of the most common infectious diseases to afflict humans (Lee et al. 2012; Tamerius et al. 2011). During the colder months of the year we typically see a seasonal spike in these viral infections, which makes the preceding months an ideal time to strengthen our immune system to reduce the frequency and severity of these infections (Tamerius et al. 2011). In this article, a number of simple nutritional and lifestyle strategies will be discussed which have proven benefits for supporting the immune system function and our defence against colds and flus.
Some nutrients have been shown to be helpful for reducing our susceptibility to pathogen invasion so it is important that the body has optimal levels prior to illness/infection (a preventative strategy). This means that the nutrients must be consistently consumed as part of the diet or in some cases supplemented prophylactically. Other nutrients have been shown to be especially valuable during an illness/infection to help the body to fight off the pathogen, allowing the body to recover faster (a treatment strategy).
Because these nutrients work synergistically in the body, it is usually unhelpful to focus on any single nutrient (unless there is a known deficiency or higher biological requirement). That said, some nutrients have been extensively studied over decades for their important activities within the immune system and as such, are widely accepted to be of significant value in promoting immunity. The three key nutrients that will be discussed are Vitamin D, Vitamin C and Zinc. These have been selected because of their well-established abilities to modulate both the innate and adaptive immune system (Rondanelli et al. 2018).
The three major immune nutrients
Despite being commonly referred to as a fat-soluble vitamin, Vitamin D is in fact a pro-hormone which is largely produced by skin in response to ultra violet B (UVB) radiation (sunlight exposure). It then undergoes biological activation in the body with involvement from the liver and kidneys (Morris 2005). Perhaps most well-known for its role in regulating calcium metabolism to help maintain strong healthy bones to reduce the risk of osteoporosis. Vitamin D also holds other important roles in the body such as promoting muscle strength, skin health and fertility (Cancer Council Australia 2016).
Deficiency (levels less than 50nmol/L when tested) has been linked increased risk for certain diseases affecting the heart, lungs, brain and immune system (Cancer Council Australia). Vitamin D has been recognised to have abilities in regulating gene expression, cell growth and differentiation and as such has been researched for its abilities to favourably alter the risk, growth and invasiveness of certain cancers including those of the prostate, breast and colon (Morris 2005). A 2014 review of over 290 scientific studies in humans saw the majority concur that a moderate-strong link (an inverse relationship) exists between vitamin D concentrations and cardiovascular disease, inflammation, disordered glucose metabolism, weight gain, infectious diseases, multiple sclerosis, mood disorders, impaired cognitive and physical functioning and all-cause mortality (Autier et al 2013).
Increasing awareness of the broad biological activities of Vitamin D has seen it emerge as a crucial nutrient for immune function, with its real strengths laying with its major potential as a preventative strategy, coupled with its possible benefits as a treatment strategy for viral infections including influenza (Gruber-Bzura 2018).
A connection has been established between low serum Vitamin D concentrations (the amount circulating in the bloodstream) and higher susceptibility to acute respiratory infections (Martineau et al. 2017). This does not necessarily equate to a causative correlation, however it does suggest that maintaining optimal Vitamin D levels acts as a defence strategy.
A recent large-scale review of evidence indicated that supplementation with Vitamin D on a regular basis (daily or weekly doses) delivered the strongest protective benefits for reducing the risk of respiratory infections. The greatest benefits were observed in those who were deficient or had lower serum vitamin D levels, however people with higher vitamin levels also benefited. Reasons for this have been attributed to the following mechanisms (Martineau et al. 2017; Gruber-Bzura 2018; Vargas Buonfiglio et al. 2017):
- Localised actions – Important biological functions taking place on the surface of the airways and by the cells that line the respiratory tract, including the transformation of inactive Vitamin D to its active form and the induction of antimicrobial peptides.
- Systemic actions – Preventing overactivity inflammatory processes brought on by infection. This is important because the release of pro-inflammatory cytokines appears to correlate with illness severity.
Cold and flu viruses are most prevalent during winter and spring which also correlates with when Vitamin D levels tend to be at their lowest in people, which suggests that an opportune time to test and optimise Vitamin D status would precede this to allow time for levels to increase in the body. The preference is to aim for body concentration levels of 100nmol/L when tested (Papadimitrious 2017).
Despite the risk for toxicity being rare, supplementation should always be overseen by a clinician as contraindications with certain conditions do exist and doses for supplementation doses and duration are calculated with consideration given to individual factors including known conditions, sun exposure, skin colour, age, body mass index and dietary intake (Hechtman 2012; Papadimitrious 2017).
Vitamin D deficiency may display no overt symptoms and those who may be at risk of deficiency include (Cancer Council Australia 2016; Morris 2005):
- People who have very dark skin (due to the presence of melanin, the pigment in the skin which filters out UVB light)
- Those with little-to-no regular exposure to sunlight (eg; due to hospitalisation or being house-bound, indoor occupations and hobbies or practice sun avoidance behaviours for other reasons)
- People who conceal their skin with clothing when outdoors (eg; during winter months, for religious/cultural purposes or wear protective clothing for outdoor occupations)
- The very young (exclusively breast-fed babies when the mother has a deficiency)
- Aging/elderly populations (the body becomes less efficient in producing vitamin D as the skin thins – however this can be mitigated by maintaining an outdoor lifestyle during older age)
- Those whose level of exposure to UVB radiation is reduced by seasonal changes or the geographical location (latitude) in which they live
- People with certain health conditions or who take medications that impair the body’s ability to produce vitamin D
It is thought that more than 80% of serum Vitamin D levels can be attributed to sunshine exposure and the remaining 20% (or less) to dietary intake (Cancer Council Australia 2016; Papadimitrious 2017). Food sources of Vitamin D are egg yolks, sun-exposed mushrooms, fortified beverages, wild fish including mackerel, herring, salmon and sardines.
Vitamin C (ascorbic acid) is a water soluble vitamin that is essential for human health and fortunately tends to be prevalent in a healthy diet. Interestingly, humans are one of the only species who cannot manufacture vitamin C endogenously (from in the body). With a plethora of facilitating roles in promoting immune function, this extensively studied nutrient appears to have the ability to both prevent and treat respiratory and systemic infections, with key functions said to be (Carr & Maggini 2017):
- Enhancing the epithelial barrier against pathogens (this is the surface layer of cells that lines skin, organs, vessels etc which is the body’s first line of defence)
- Heightening the proliferation of immune cells (B and T cells) and antibodies
- Concentrating in immune cells (including neutrophils) where it is used in processes that destroy microbes. This is important because neutrophils rapidly increase when infection is detected.
- Modulating production of inflammatory molecules and protects healthy tissues against oxidative damage.
A deficiency in Vitamin C has been associated with impaired immunity leading to greater susceptibility to pathogenic infection, therefore maintaining adequate body levels is considered very important (Carr & Maggini 2017). Some common factors known to deplete body stores include (Carr & Maggini 2017):
- Insufficient dietary intake (eg; overreliance on processed foods)
- Exposure to air pollution and cigarette smoke (including passive exposure)
- During an episode of illness/acute infection
- Increased metabolic requirements due to inflammatory or oxidative diseases (such as Diabetes)
- Age (the very young or very old population groups)
The strongest benefits of prophylactic supplemented Vitamin C are experienced by physically active people whereby the number of acquired colds has been shown to be halved as a result (Hemilä 2017). As a preventative strategy, the results have been less remarkable amongst the other less active population groups, however even modest daily doses (as little as 200mg) appears to be effective as a treatment strategy in reducing the severity and duration of a cold and importantly, reducing the risk of developing more serious complications such as pneumonia (Carr & Maggini 2017; Hemilä 2017).
As a therapy to treat an acute infection, significantly higher doses are required to compensate for the rapid depletion of Vitamin C that occurs due to the inflammatory response and metabolic demands associated with active viral infection (Carr & Maggini 2017; Gonzalez et al. 2018). Some scientific studies have shown doses between 6000mg – 8000mg to result in shorter cold duration by 17-19% respectively when supplemented as soon as possible after the onset of symptoms (Hemilä 2017). Supplementation is always best directed by a clinician and tailored to an individual’s requirements as adverse effects can occur from inappropriate doses.
Food sources of vitamin C include green and orange/red coloured organic fresh fruits and vegetables such as broccoli, cabbage, snow peas, kiwi fruit, parsley, guava, orange, pawpaw, tomato and strawberries.
Zinc is an essential mineral with a number of important functions in the body associated with immunity due to its antimicrobial and antioxidant actions and necessity for tissue repair (wound-healing). It also has many other important biological functions beyond the immune system, for example zinc is a critical cofactor in the production of neurotransmitters and for activation of the body’s toxic metal scavenging system (metallothionein promotion).
The immune system has proven to be particularly sensitive to changes in levels of zinc (Skrainowska & Bobrowska-Korczak 2019). With an inability for the body to store zinc in reserve, a deficiency can manifest relatively quickly and unfortunately deficiency is common (Skrainowska & Bobrowska-Korczak 2019). This is often due to disease-mediated factors or the impact of aging, diet and lifestyle factors (Rondanelli et al. 2018) and consequently, the body’s immune defences are weakened and susceptibility to infection increases (Skrainowska & Bobrowska-Korczak 2019).
Zinc has been extensively studied its roles in prevention and treatment of viral infections, with efficacy established for significantly decreasing the risk of colds and frequency of other respiratory tract infections including pneumonia and particularly amongst children and the elderly, which are typically the most vulnerable populations (Rondanelli et al. 2018). Its major roles are believed to include:
- Helping to maintain the integrity of both the external and internal physical barriers to pathogens (specifically skin and mucosal membranes). The latter lines the respiratory tract which is the site that inhaled pathogens adheres to.
- Directly exerting antiviral effects to interfere with viral replication during a cold episode.
- Acting as an essential catalyst for enzymes involved in immune function.
- Playing a key role in the regulation of inflammatory cytokines and oxidative stress.
Large-scale reviews of the scientific evidence have demonstrated zinc may reduce the duration of colds by approximately 33% and it is recommended that supplement commences within 24 hours of the onset of symptoms (Rondanelli et al. 2018).
Food sources of zinc include oysters, red meats, poultry, shellfish, liver, nuts, seeds (sesame, sunflower, pepitas), tahini, oats, tomato (sundried and paste) and mushrooms.
While nutrient sufficiency is integral component for a healthy immune system, the impact of lifestyle factors and behaviours should not be underestimated. Consideration should be given to the following factors that have the ability to affect the ability of the immune system to perform surveillance and protection and recovery:
- Maintain regular and appropriate physical activity or movement
- Manage stress and emotions well.
- Maintain a sound mindset. Maintain good social network.
- Get regular sunlight exposure (even during the winter months)
- Maintain a healthy body weight
- Wash hand regularly and avoid unwell/contagious people where possible
- Rest as appropriate and ensure adequate restorative sleep
- Do not smoke tobacco and avoid passive smoking
- Curb unhelpful dietary habits (especially consuming excessive sugar or alcohol)
- Manage other existing health conditions
- Maintain a hygienic home and work environment
- Most importantly, be consistent with your healthy immune-boosting behaviours
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