Scientists finally figured out why you rarely get sick in the summer
Turns out our genes change with the seasons, just like the weather.
During the winter months, our bodies pump up the levels of many of the genes linked with inflammation, triggering the tell-tale signs of swelling and discomfort that our bodies use to protect us from colds and the flu.
In the summer, on the other hand, an altogether different set of genes get more highly expressed, including some that help regulate our blood sugar, potentially curbing cravings and helping us burn off excess fat.
A quarter of our DNA shifts with the seasons
All this is the finding, at least, of a study published May 12 that finds that roughly 25% of all the chunks of DNA that code for various behaviors and traits in our bodies, otherwise known as genes, shift significantly with the seasons.
Many parts of our immune system, which kicks into action to fend off an infection or cold, shift too.
"I wasn't expecting to find that many," Chris Wallace, a researcher at the Diabetes and Inflammation Laboratory at Cambridge University and an author of the new paper, told Business Insider.
The researchers combed through data from previous studies looking at people's DNA until they had information on roughly 1,000 people living in six different countries: Australia, Germany, the US, the UK, Iceland, and Gambia, a small West African country between Senegal and Guinea-Bisseau.
This way, they could get a look at people's genes and how they changed (if they did at all) over time and according to their location and exposure to sunlight.
Our changing gene expression likely helps us fend off illness
In Europe they found the expression of inflammatory genes got ramped up during the winter months. Here's a chart showing the difference in gene expression during the winter and summer months from a sample of German children whose data was gathered in 2013:
But in Gambia, where there is virtually no winter, these inflammatory genes followed a totally different pattern: They were amplified in the rainy months, when mosquitoes are virtually everywhere and the risk of malaria is the highest.
Previous research has found similar seasonal changes in various components of the immune system. A study from last year, for example, found gene expression in red blood cells shifted with the seasons. This study, however, is one of the first of its kind to look at the whole picture, looking also at white blood cells and genes that play key roles in our immune response.
Wallace and her team think this finding might shed some light on how evolution affected the way we respond to potential sources of infection or illness.
"It could be that our inflammatory response developed as a means of fighting things that are more abundant at certain times," said Wallace.
So perhaps we've evolved that immune response in response to those threats of certain seasons - activating those inflammation-associated genes - and making us more likely to feel sick in the winter, but also making our immune system more active to fight off threats.