A complex and fascinating study from the scholarly journal Nature reveals that the expression of several genes involved in the immune response is higher in winter and lower in summer, which reduces the levels of inflammatory molecules and the risk of several serious associated diseases to inflammation. Like what our immune system also takes a few weeks of summer vacation.
The changes in climate that accompany the seasons have a great influence on the life of all living species on the planet. Just think of the seasonal blooming of flowers, the enormous distances traveled by migratory species for their reproduction or even certain extreme variations in metabolism (the hibernation of bears, for example).
In humans, numerous studies have clearly shown that the incidence of several diseases varies enormously according to the seasons, with a peak observed during the winter. We obviously think of certain infectious diseases such as the flu, which strikes hard from December to April, but several other diseases which are not due to viral or bacterial agents also show great seasonal variations. For example, myocardial infarction, stroke, certain autoimmune diseases (type 1 diabetes, rheumatoid arthritis) and even certain mental disorders (depression, schizophrenia) are all more frequent during cold seasons. It therefore seems that certain variations in our physiological functions during the winter could promote the appearance of these serious diseases.
An increase in inflammation associated with an increase in serious illnesses in winter
All diseases that strike preferentially in winter have the common characteristic of being favored by inflammatory conditions, which suggests that the immune system could play a role in these seasonal variations.
This is confirmed by the results of the largest analysis to date of gene expression variations that occur across seasons. Using blood samples taken at different times of the year, scientists were able to measure the expression levels of 22,822 genes present in the white blood cells of 16,000 people living in the northern and southern hemispheres of the globe. The results are quite spectacular: of the 22,822 genes measured, almost a quarter (5,136) varied considerably by season, in particular several genes known to be involved in the immune and inflammatory response.
For example, it has been observed that certain inflammatory molecules (IL-6, CRP) increase in winter and it is likely that the inflammation associated with an increase in the levels of these molecules contributes to the development of several diseases. Studies have shown that high levels of the molecule IL-6 are associated with an increased risk of heart disease, type 1 diabetes and certain psychiatric disorders, all pathologies that preferably occur in winter. This negative effect would be all the more accentuated since the study revealed that certain molecules which act as natural anti-inflammatories are at their lowest level during the winter.
Although the mechanisms responsible for these variations remain poorly understood, there is little doubt that these changes could greatly contribute to the increase in several diseases observed during the cold season.
A memory of our evolution
The increase in immune and inflammatory activity during the winter observed in this study is probably a physiological adaptation intended to fight infectious agents, which are especially active during the cold seasons. It should be remembered that infectious diseases have until very recently represented the main cause of mortality of the human species and it is normal that the immune system has developed during evolution a greater activity during the winter, followed by a well deserved rest during the hot season.
Take advantage of summer to improve your health
This greater reactivity of immunity in cold seasons, however, can become problematic when combined with pro-inflammatory conditions such as overweight, sedentary lifestyle and poor diet. We can therefore take advantage of the immune system’s summer holidays to change our lifestyle by adopting a healthy lifestyle that will minimize this predisposition to inflammation during the coming winter and will at the same time prevent several serious chronic diseases.
Dopico Cetcoll.Widespread seasonal gene expression reveals annual differences in human immunity and physiology. Nat Common; 6:7000.