Winter immune boost may actually cause deaths, study suggests
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Version 0 of 1. The increase in deaths during winter months is typically put down to icy weather, bouts of flu and the more sluggish lifestyles we adopt as the days draw in. Now scientists have suggested that our own immune systems may be to blame for the annual peak in conditions from heart attacks to diabetes and schizophrenia. A study has revealed that the immune system has a seasonal cycle, in which its activity is boosted during the winter and relaxes during the summer. The winter increase in immune defences ought to help stave off infections, such as flu, but also raises the risk of harmful inflammation in the body, effectively lowering the threshold for heart attacks, stroke, diabetes and even some psychiatric conditions. Prof John Todd, a geneticist at the University of Cambridge, said: “Until now the seasonal peaks in cardiovascular deaths, multiple sclerosis, Seasonal Affective Disorder (SAD) and schizophrenia have been well known but the cause has been a mystery.” The latest work suggests that our own immune systems could be “tipping us over the edge” into illness - or even death. In the future, doctors may be able to reduce this peak, by prescribing medicines, such as statins, diabetic drugs and anti-depressants on a seasonal basis, the scientists suggested. In England and Wales, the winter peak in cardiovascular deaths accounts for an additional 20,000 fatalities each year, and people are most likely to be diagnosed with diabetes between November and February. In the latest work, scientists analysed blood samples from more than 16,000 people living in both the northern and southern hemispheres, in countries including Britain, the US, Iceland, Australia and the Gambia. By looking at proteins expressed in particular cells or tissues, they were able to show that thousands of genes operate on a seasonal basis, and that opposing patterns of activity were seen in the northern and southern hemispheres. One gene, known as ARNTL, that was seen to be more active in the summer, has previously been shown in mice to suppress inflammation, the body’s response to infection. Many other immune genes followed a similar trend, the study found. In the past, our seasonal immune cycle may have given us a powerful evolutionary advantage in the winter months, when the main cause of fatalities would have been disease and hunger. “You have to take yourself back to prehistoric humans who suffered cold and starvation in the winter,” said Todd. “It would’ve been a really perilous situation.” In these conditions, even a slight boost to the readiness of the immune system could improve survival chances. However, the inflammatory molecules that flood the body during an immune response have a downside – they can attack the body’s own tissues causing conditions from diabetes to heart attack and stroke. More recent studies have shown compelling links between inflammatory markers and mental health, from schizophrenia, severe depression and Alzheimer’s disease. In the modern world, the disadvantages of the winter peak in immune activity may have come to outweigh the benefits, the researchers suggest. Drugs that target the mechanisms behind inflammation could offer a way of helping treat these diseases more effectively during the winter periods. The study also found people from the Gambia had distinct seasonal variation in the numbers of immune cells in the blood that correlated with the rainy season (June-October), when infectious diseases such as malaria are rife. “We know that humans adapt to changing environments,” said Dr Chris Wallace, a co-author, also of the University of Cambridge. “Our paper suggests that human immune systems adapt to show different seasonal variation in equatorial regions with fewer distinct seasons compared to regions at higher and lower latitudes with more pronounced differences between winter and season.” Prof 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 ... One possible outcome is that treatment for certain diseases could be more effective if tailored to the seasons.” Karen Addington, Chief Executive of JDRF in the UK, said: “We have long known there are more diagnoses of type 1 diabetes in winter. This study begins to reveal why.” The findings are published in the journal Nature Communications. |