Scientists have shown that there are seasonal changes in how our immune system functions.
In a study published in the journal Nature Communications, blood and tissue samples from more than 16,000 people worldwide were analysed. The activity of 5,136 out of 22,822 genes was found to vary depending on the season - some genes were more active in the winter and others the summer. Seasonal differences in the types of cells found in the blood were also observed.
'It helps explain why so many diseases, from heart disease to mental illness, are much worse in the winter months', said Professor John Todd, director of the JDRF/Wellcome Trust Diabetes and Inflammation Laboratory in Cambridge, who led the research.
The researchers were particularly interested in genes involved in immunology and inflammation. For those living in the northern hemisphere these genes were most active in the coldest winter months of December to February; whereas for those living in the southern hemisphere, these were more active in June to August.
A different pattern was observed for those living close the equator. Here, pro-inflammatory genes were most active during the rainy season, which coincides with the time when infectious diseases such as malaria are most prevalent. In contrast, much less variation was observed in Iceland, where it is cold all year around.
Inflammation plays an important part in a range of diseases, including type-1 diabetes, which Professor Todd has studied.
'In the UK, we see a rise in new cases of type-1 diabetes in January, February and March,' said Professor Todd. 'Our results suggest that part of the reason for this is heightened inflammation and that gene activity is involved.'
At present scientists are only able to speculate about the mechanisms that may be responsible for this seasonal variation, and those in the field have emphasised that it will be important to think about other factors, such as nutrition and stress, when considering the inflammatory response.
'Another dimension that could be as important are our gut microbes, which also change between seasons and could be driving these changes because of seasonal changes in diet,' said Professor Tim Spector, a genetic epidemiologist at King's College London.