The activity of some of our genes varies with the seasons throughout the year. The discovery comes from an analysis of blood samples from more than 16,000 people in both hemispheres. The most striking pattern was that 147 genes involved in the immune system made it more reactive or "pro-inflammatory" during winter or rainy seasons, probably to battle the onslaught of cold and flu viruses.
"This could explain why for some people with underlying disease, winter may be the tipping point," says Derek Gilroy of University College London, who was not involved in the research.
He points out, however, that we don't know if the immune system becomes more reactive in winter as a consequence of infections, or if it naturally gears up for action as part of a yearly cycle.
Either way, this change in activity seems to have unwanted knock-on effects. In the UK, for instance, deaths from heart disease go up by 20 per cent in the winter months. Explanations for this include direct effects from the cold, or respiratory infections putting more strain on the heart. But inflammation is increasingly being implicated in heart disease, autoimmune diseases and many other conditions.
The research team stumbled across the latest finding when they were looking at gene activity -- as measured by levels of messenger RNA in blood cells -- in samples from German infants, to try to identify risk factors for type 1 diabetes, which usually develops in childhood.
Many of our genes change their activity over the course of a day, probably to orchestrate our sleep cycle and energy levels. Because blood samples had been taken from the German children every three months for five years, the team wondered if they would find seasonal changes in gene activity.
Babies turned out to be ideal for answering the question, says John Todd of the University of Cambridge, one of the study's authors. "Their lives aren't complicated by shift work or long-haul travel."
About a quarter of the nearly 23,000 genes tested had different activity in winter and summer, with increased immune reactivity in winter the only obvious pattern to emerge.
Head for the sun
The team then looked at the data for thousands of blood samples from other studies. The same pattern occurred in adults from the UK, the US, Ireland, Australia and, to a lesser extent Iceland. "It was a lot of fun discovering something in available data sets," says Todd.
In The Gambia in West Africa, on the other hand, people's immune systems became more reactive during the rainy season, from June to October, which is when infectious diseases peak in that region.
Todd says future studies of gene activity, especially immune system genes, should take into account the time of year.
He thinks the findings also suggest we should watch our health more closely in winter, taking care to eat plenty of fruit and vegetables, for instance. "If you swapped hemispheres every winter you could probably lower this pro-inflammatory status," he says. "Some people do move to sunnier climates in winter and they probably feel better for it."