Sedentary behaviour increases the risk of infection and death: a two-sample Mendelian randomisation study

For decades, scientists have watched people who move their bodies less get sick more often. It is a pattern seen in hospitals and neighborhoods alike: those who sit for long stretches tend to catch more infections, while those who stay active seem to stay healthier. But watching a pattern is not the same as understanding the cause. People who sit a lot might also smoke more, eat differently, or live in conditions that make them sick, making it hard to know if the sitting itself is the culprit or just a companion to other bad habits. To cut through this confusion, researchers have turned to a method that uses our own biology as a natural experiment. This approach, known as Mendelian randomization, looks at the genetic code we inherit from our parents. Because these genetic instructions are set at conception, long before we ever sit down or stand up, they act like a random assignment. If a person carries genetic variants that make them naturally prone to sitting for long periods, and that same person also gets sick more often, it suggests a direct link between the behavior and the illness, rather than just a coincidence caused by lifestyle choices.

A team of researchers from Uppsala University in Sweden decided to use this genetic lens to investigate a specific question: does a life spent sitting down actually increase the risk of bacterial infections and death from those infections? They focused on two sides of the same coin: the time people spend sitting, specifically watching television, and the time they spend moving with moderate to vigorous effort. To get a clear answer, they did not rely on a single group of people or a single way of measuring things. Instead, they combined data from two massive, independent groups of people: one from the United Kingdom and another from Sweden. These groups included hundreds of thousands of individuals whose DNA had been mapped and whose medical histories were recorded in national registers. The researchers looked for specific genetic markers associated with sitting and moving, and then checked if people with those markers were more or less likely to end up in the hospital with pneumonia, urinary tract infections, or other bacterial illnesses, or to die from such causes.

The results painted a clear picture for the time spent sitting. The study found that people with a genetic tendency to spend more leisure time watching television had a significantly higher chance of getting infected. When the researchers combined the data from both countries, they calculated that for every standard increase in this genetic tendency to sit, the odds of getting any kind of infection rose by about 43 percent. The risk was even steeper for the most serious outcomes. The likelihood of dying from an infection increased by 55 percent, and the risk of developing pneumonia jumped by 54 percent. The connection was also visible for urinary tract infections, which were 28 percent more likely in those genetically prone to sitting. These findings held true across both the British and Swedish populations, suggesting that the link is not just a local quirk of one healthcare system but a broader biological reality. The researchers noted that while some of the statistical signals were stronger in one country than the other, the overall direction was consistent: more sitting, more sickness.

On the other side of the equation, the story for physical activity was promising but slightly more complex. The researchers looked at genetic markers that make people more likely to engage in moderate to vigorous physical activity during their free time. In the combined analysis, people with these genetic traits had lower odds of getting infected. Specifically, the risk of getting any infection dropped by 19 percent, and the risk of a urinary tract infection fell by 28 percent. However, the evidence for physical activity preventing pneumonia or death from infection was not as strong or consistent as the evidence against sitting. In some of the separate analyses, the numbers did not reach the level of statistical certainty required to rule out chance. The researchers also had to be careful here because the tools they used to measure physical activity were less precise than those for sitting, and there were hints that some genetic factors might influence infection risk through pathways other than just exercise. This means that while the data points toward a benefit, the certainty is not quite as absolute as it is for the dangers of sitting.

The study also looked closely at whether the results could be explained by other factors, such as the genes influencing infection risk through a different route entirely. They ran several checks to see if the genetic markers were acting like a double agent, affecting the outcome in a way that had nothing to do with sitting or moving. For the sitting data, these checks mostly passed, reinforcing the idea that the sitting itself is the driver. For the physical activity data, some of the checks raised small flags, suggesting that the relationship might be a bit messier and influenced by other biological factors. Despite these nuances, the overall message from the study is robust. The researchers concluded that a life characterized by prolonged sitting appears to genuinely increase susceptibility to bacterial infections and the risk of dying from them, while a genetic inclination toward being active offers some protection.

This work does not claim to have solved the mystery of why infections happen, nor does it offer a simple prescription for every individual. It does, however, provide a strong piece of evidence that moves beyond simple observation. By using the randomness of our genetic inheritance, the study suggests that the act of sitting for long periods is not merely a sign of a less healthy lifestyle but may be a direct cause of increased vulnerability to disease. The findings support the idea that how we move our bodies is a fundamental part of our immune defense, and that reducing the time we spend stationary could be a powerful way to lower the risk of serious infections. While the study focused on people of European descent and middle age, the biological mechanisms suggested here likely apply more broadly, offering a compelling reason to reconsider how much time we spend in a chair.