A new gene study suggests there may be an increase in inflammatory chemicals during winter, which can protect against infection but could also make the body more vulnerable to other chronic diseases.
The study looked at gene expression in 23,000 genes in blood samples taken from 1,315 children and adults in different months throughout the year in a range of different countries, including the UK, the United States, Australia, Germany, Iceland and the Gambia.
The aim was to see if there was seasonal variation in the gene expression of inflammatory proteins and receptors such as interleukin-6 (IL-6) and C-reactive protein and the number of each type of white cell in the blood. They also looked at gene expression in fat tissue samples to see whether it was only cells in the immune system showed variation in gene expression with the seasons.
In the first group of children and adults from Germany, the researchers found almost a quarter of all genes (23%, about 5,000 genes) showed seasonal variation in the white blood cells assessed. Some genes were more active in the summer and others in winter. When looking at all of the population groups they tested, 147 genes were found to show the same seasonal variation in the blood samples taken from the children and adults whether from the UK/Ireland, Australia and the US. Again, some genes were more active in the summer and others in the winter. The genes included one encoding protein, which controls the production of anti-inflammatory proteins and was found to be more active in the summer months. Other genes involved in promoting inflammation were more active in the winter. Seasonal genes from the samples of Icelandic people did not show the same pattern.
The numbers of different types of white blood cells from the UK samples also showed seasonal variation. Lymphocytes, which mostly fight viral infections, were highest in October and lowest in March. Eosinophils, which have many immune functions, including allergic reactions, were highest in the summer.
There were also seasonal patterns in the numbers of different types of white blood cell from people in the Gambia, but these were different from those in the UK. All white cell types increased during the rainy season. The researchers also found some genes showed seasonal variation in their activity in fat cells.
The authors concluded that this seasonal change in the immune system could, for example, contribute to the worsening of some autoimmune disorders during the winter, such as rheumatoid arthritis. The immune system, however, is extremely complex, and different genes showed different seasonal expression patterns. There were also important discrepancies in expression patterns in different parts of the world. An analysis of this research by Bazian cautions against simplifying the interpretation of these findings and says that the study should not be used to prove cause and effect because it is also likely these seasonal changes could at least in part be a response to changes in infections and allergens, such as pollen in the summer.