Baku, May 14, AZERTAC

The human immune system may be stronger in the summer, explaining why we are more likely to fall ill in winter, new research suggests.

A Cambridge University study which tested 23,000 genes from men and women living all round the globe found that almost one in four genes behaved differently according to the season.

Crucially, a gene associated with suppressing infections was found to be more active in summer.

Previous research has shown that a host of health conditions - including heart attacks and strokes - are more common in winter.

The new study is the first to show that this may be down to seasonal changes in our immune system.

It helps explain why people are more likely to suffer certain conditions - including diabetes, arthritis and heart disease - in winter months while people tend to be healthier in summer.

Professor Mike Turner, head of infection and immunobiology at the Wellcome Trust, said: "This is an excellent study which provides real evidence supporting the popular belief that we tend to be healthier in the summer.

"Seasonal variation to this extent is a fascinating find - the activity of many of our genes, as well as the composition of our blood and fat tissue, varies depending on the seasons.

"Although we are still unclear of the mechanism that governs this variation, one possible outcome is that treatment for certain diseases could be more effective if tailored to the seasons."

Researchers studied samples from people living in both the northern and southern hemispheres, including the UK, USA, Iceland, Australia and The Gambia.

They found that the activity of almost a quarter of genes tested differed according to the time of year, with some more active in winter and some in summer.

One particular gene, known as ARNTL, which has been associated with suppressing infections in mice was more active in summer, suggesting it may have a similar effect in humans.

It is not known exactly what causes these variations, but researchers say it may be down to environmental cues like daylight and ambient temperature.

Our internal body clock - known as our circadian rhythm - is in part coordinated by changes in daylight, which explains why people in jobs that do not fit with the daily cycle, such as factory shift workers or crews on long haul flights, can be affected by poorer health, researchers added.