Does it seem like you get sick more often in the winter?  Most people think that it is a direct result of spending more time crowded inside with other people.  However, it isn’t necessarily that simple.

Doctors and scientists have tracked seasonal trends in illness that have nothing to do with proximity to others. 

Many illnesses, especially autoimmune illnesses, get worse in the winter months, only to improve for seemingly no reason in the summer.  Numerous studies examining the seasonality of illness have confirmed that immunity, inflammation, and other components of health DO have a seasonal rhythm … a rhythm that is caused by differences in gene expression throughout the year.  In fact, about a quarter of the body’s genes are more active during certain parts of the year.  What this means is that in addition to circadian, infradian, and ultradian rhythms, we also experience seasonal rhythms.

It’s known that non-genetic factors can influence certain seasonal rhythms of the body, like being around others in enclosed spaces increasing the chances of infectious disease.  Or the idea that mental illness and conditions like Seasonal Affective Disorder (S.A.D.) are, at least partially, due to decreased vitamin D levels from less exposure to sunlight.

While these definitely contribute to seasonal rhythms in our health, they are not the only cause.  The University of Cambridge published a study on the chronobiological effects of seasonality on the immune system and found that seasonal changes in gene expression affect everything from inflammation, gestational diabetes, and heart disease, to blood and adipose (fat) composition, as well as immune system response from season to season.

Gene Expression is the process where genetic instructions create the proteins that perform particular body functions.  This means that genes create a protein that carries its message around the body.  Different cells are due to different protein expression.

The team at the University of Cambridge took blood and tissue samples from 16,000 people all across areas in the Northern and Southern hemispheres.  They measured the types and amounts of cells in the blood, as well as the protein composition of those cells.  What they found was consistent variations across different populations, in nations as diverse as Gambia and the U.K.

One gene that presented differently in the summer and winter was a gene called ARNTL.  This gene has been the subject of many studies and has been linked to the circadian rhythm, as well as disorders like gestational diabetes, inflammation, and heart disease.  Recently, ARNTL was found to be far more active in the summer, and less so in the winter.  This may explain why diseases involving inflammation, like rheumatoid arthritis and heart disease, usually become worse in the winter.

This connection between seasons and the ARNTL gene (as well as others) presents new treatment possibilities for people with chronic disease.

Another study of seasonality and the immune system examined blood samples from over 329,000 people over a 10-year period.  Researchers found obvious differences in inflammation markers and white blood cell counts in different seasons, as well as at different times of day.  Based on their findings, it appears that people may be more susceptible to certain diseases during less active times (like nighttime) and in the winter months.

Interestingly, these findings did not seem to be connected to vitamin D levels or the environment.

The question is why does the season affect the immune system?  Scientists believe it isn’t coincidental.  For example, people that live in Gambia have higher levels of immune cells in their blood from June to September, when malaria is most common.  Similarly, in winter months, the body produces more inflammatory proteins to better fight off illness-causing microorganisms that are encountered more during winter months.

However, this seasonal rhythm can become maladaptive in modern times, when chronic conditions like Type 1 or juvenile diabetes become worse in winter and cause more problems than the contagious illnesses we associate with the winter months.  It is known that more cases of this autoimmune form of diabetes is diagnosed in the winter, likely due to the increase in inflammation and immune cells in the body during this time of year.

On top of confirming that there is a definite seasonal rhythm in health, this study in chronobiology has led to a variety of new questions.  How can this knowledge of seasonal rhythms be used to treat illnesses with seasonal variation?  What mechanism leads to the variation in immunity and inflammation over the seasons?

Future studies will strive to answer these and other questions about circadian biology and our bodies’ internal clocks.

For more information, read:

https://www.chronobiology.com/seasonality-and-the-immune-system