Key Nutrients for Immunity & Viral Resilience

Vitamin D

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.