Generally healthy humans do not get colds or other upper respiratory tract infections in the summer. However, as autumn proceeds cold season begins and infections of the upper respiratory tract become common. Many explanations have been put forward to explain this phenomenon but none have really ever been able to explain the occurrence of cold primarily in the winter. More recently the link between vitamin D and immune function has been elucidated in the nutritional literature. In particular, vitamin D appears to be involved in the regulation of innate and adaptive immune systems in humans. Low circulating levels of the accepted biomarker for vitamin D status, 25-hydroxyvitamin D, have been shown to increase the risk of infection. Therefore as the strength of the sunlight decreases in autumn, circulating levels of 25-hydroxyvitamin D also decrease and unless dietary intakes of vitamin D are adjusted upwards to compensate, susceptibility to infection increases significantly.
An association between vitamin D and protection from infection has been observed in the nutritional literature. For example, in one study1, researchers measured the 25-hydroxyvitamin D plasma levels of patients admitted to a number of hospitals in the United States and then observed the rates of hospital acquired bloodstream infections. The results showed that those patients with 25-hydroxyvitamin D plasma levels below 30 ng/mL had a higher odds of developing a hospital acquired bloodstream infection compared to those with 25-hydroxyvitamin D levels above 30 ng/mL. Likewise those patients with 25-hydroxyvitamin D levels below 10 ng/mL had a greater odds of developing a hospital acquired bloodstream infection compared to those with levels above 10 ng/mL. Therefore those patients with the highest 25-hydroxyvitamin D levels upon admittance to hospital had the lowest odds of developing a hospital acquired bloodstream infection.
Vitamin D is not a true vitamin, but really a steroid hormone. Vitamin D receptors are expressed in the nucleus of immune cells such as T cells, dendritic cells and activated B cells. Studies have shown that low levels of 25-hydroxyvitamin D are associated with depressed macrophage phagocytosis, chemotaxis and proinflammatory cytokine production. Vitamin D is also required for the production of antimicrobial peptides such as cathelicidin and β-defensins. Low levels of 25-hydroxyvitamin D can also inhibit the correct function of the interferon-γ-dependent T cell response to infection. These biochemical activities of vitamin D can explain why poor vitamin D status is associated with an increased risk of infection. They also explain why the low incidence of the sun in the winter is associated with an increase in the prevalence of upper respiratory tract infections. The role played by vitamin D in maintaining a proficient immune system has been overlooked until recently, but is now becoming more understood.
Dr Robert Barrington’s Nutritional Recommendation: Maintaining vitamin D levels above 100 ng/mL (40 nmol/L) is recommended for optimal health. In the summer, this is easily achieved by maintaining exposure to the sun. In the winter supplements of around 2000 IU per day of vitamin D3 are recommended for adults to compensate for the weak sun.
RdB
