Links between seasons, sunshine and immune-mediated diseases

Immune-mediated diseases arise when there is aberrant activity in the body’s immune system, such that it directs an immune response against its own cells and tissues, causing damage and disease. Prominent examples of immune-mediated diseases include rheumatoid arthritis (RA), Crohn’s disease (CD), ulcerative colitis (UC), systemic lupus erythematosus (SLE) and multiple sclerosis (MS). These conditions affect 5-10% of the population in the developed world. This percentage has been increasing and the economic and health burdens associated with these conditions are significant.

These are complex diseases, which mean that there is no single cause but rather a combination of genetic and environmental factors. As the overall incidence of immune-mediated diseases is increasing, this suggests that environmental and lifestyle factors may play a central role.

It is already known  from various studies that there is an association between the month of birth and the risk of immune-mediated disease, but the reasons behind this are unclear. However, one factor that links these conditions is vitamin D deficiency, a common cause of which is inadequate exposure to sunlight, since ultraviolet B (UVB) rays in sunlight convert cholesterol in the skin into vitamin D.

In an attempt to reconcile these associations, Professor George Ebers and colleagues performed a large case control study which has been published in BMC Medicine. They used data from Scotland and England to compare the monthly distribution of births of those affected by immune-mediated diseases to the general population. The research group first confirmed that month of birth affects the prevalence of immune-mediated disease, and then predicted the UVB exposure and gestational vitamin D status at different time frames during pregnancy for each month of birth. Intriguingly, they discovered an inverse relationship between the risk of immune-mediated disease and third trimester vitamin D status, which itself correlated with the predicted UVB exposure. This suggests that vitamin D status during gestation may be an environmental factor that may influence risk of immune-mediated disease.

Vitamin D, sometimes called the ‘sunshine vitamin’, has been hitting the headlines recently, as there is increasing evidence that indicates low levels of sunlight exposure, which in turn leads to vitamin D deficiency, is strongly associated with MS development. As a result, international experts are now calling for food in Scotland to be fortified with vitamin D in an attempt to reduce the large numbers of people who develop MS in sunshine deprived regions. This recent study adds further weight to the body of evidence pointing towards the importance of vitamin D in immune-mediated diseases.

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