A new study of over 1 million people finds that doing at least one hour of physical activity per day, such as brisk walking or cycling for pleasure, may eliminate the increased risk of death associated with sitting for 8h a day.

Physical inactivity is linked to an increased risk of heart disease, diabetes and some cancers and is associated with more than 5 million deaths per year and, as the first global economic analysis of physical inactivity shows, costs the world economy over US$67.5 billion per year in health care costs and lost productivity.

The findings come from a new four-paper Series published in The Lancet and launched in London ahead of the Summer Olympic Games. The authors of the series warn there has been too little progress in tackling the global pandemic of physical inactivity since the 2012 Olympics, with a quarter of adults worldwide still failing to meet current recommendations on physical activity.

Researchers analyzed data from over 1 million people from 16 studies. The research team wanted to see how many hours of daily physical activity would be required to eliminate the association between prolonged sitting time and increased risk of death. Examples of physical activity were brisk walking at 5.6 km/h or cycling for pleasure at 16 km/h.

The researchers classified individuals into four equally sized groups according to how active they were - less than 5 minutes a day for the least active, up to 60-75 minutes a day for the most active.

People who sat for 8 hours a day but were physically active had a much lower risk of death compared to people who sat for fewer hours a day, but were not physically active. This suggests that physical activity is particularly important, no matter how many hours a day are spent sitting. In fact, the increased risk of death associated with sitting for 8 hours a day was eliminated for people who did a minimum of 1-hour physical activity per day. The greatest risk of death was for people who sat for long periods of time and were inactive.

WHO guidelines recommend that adults should do at least 150 minutes of physical activity per week, which is much lower than the 60-75 minutes per day identified in this analysis. The study also warns of the progress that remains to be made in increasing levels of physical activity since only about 25% of people in the analysis did an hour or more physical activity per day.

"There has been a lot of concern about the health risks associated with today's more sedentary lifestyles," says lead author Professor Ulf Ekelund, the Norwegian School of Sports Sciences, Norway and the University of Cambridge, UK "Our message is a positive one: it is possible to reduce - or even eliminate - these risks if we are active enough, even without having to take up sports or go to the gym."

He adds: "For many people who commute to work and have office-based jobs, there is no way to escape sitting for prolonged periods of time. For these people in particular, we cannot stress enough the importance of getting exercise, whether it's getting out for a walk at lunchtime, going for a run in the morning or cycling to work. An hour of physical activity per day is the ideal, but if this is unmanageable, then at least doing some exercise each day can help reduce the risk."

The research team also looked at time spent watching TV per day – a specific type of sedentary behavior – in a subgroup of approximately half a million people. They found similar results: sitting watching TV for over 3 hours per day was associated with an increased risk of death in all activity groups, except the most active. The authors stress that the association is likely not because of a causal link between watching TV and an increased risk of death, but simply that watching TV is a specific type of sedentary behavior. The increased risk of death associated with sitting watching TV for many hours a day was slightly greater than the increased risk of death associated with total sitting time. The authors say that this could be due to a number of factors – for instance, long hours watching TV may be a marker of a more unhealthy lifestyle in general including being less likely to take exercise. Also, because people usually watch TV in the evenings after dinner which might affect their metabolism, or because people may be more likely to snack while watching TV.

The authors warn that the study mainly included data from people aged over 45 years old from the USA, Western Europe and Australia, so may not apply to other populations.