One of the themes that has emerged from the past few years of studies on the epidemiology of activity versus inactivity is the suggestion that time spent sitting is harmful regardless of whether or not an individual exercises. This relationship is extracted from statistical studies across populations, and as is usual in these matters the conclusion is disputed. In general, it is a good idea to give little weight to any one such epidemiological study and look instead for the consensus across many studies. That a sedentary lifestyle is bad for health and that regular moderate exercise is good for health is not in dispute, but arguments take place over the interpretation of population data for more subtle aspects of the relationship between these two things. This latest research should be added to the existing stack and considered in that context:

Ever since a study back in 1953 discovered that London bus drivers were at greater risk of heart disease compared to bus conductors, scientists have found increasing evidence that lack of physical activity is a major risk factor for several diseases and for risk of early death. Recent estimates suggest that more than 5 million people die globally each year as a result of failing to meet recommended daily activity levels. Studies in high-income countries have suggested that adults spend the majority of their waking hours sitting down. Current physical activity guidelines recommend that adults undertake at least 150 minutes of moderate intensity exercise per week.

In a recent analysis that draws together a number of existing studies, an international team of researchers asked the question: if an individual is active enough, can this reduce, or even eliminate, the increased risk of early death associated with sitting down? In total the researchers analysed 16 studies, which included data from more than one million men and women. The team grouped individuals into four quartiles depending on their level of moderate intensity physical activity, ranging from less than 5 minutes per day in the bottom group to over 60 minutes in the top. Moderate intensity exercise was defined as equating to walking at 3.5 miles/hour or cycling at 10 miles/hour, for example. The researchers found that 60 to 75 minutes of moderate intensity exercise per day were sufficient to eliminate the increased risk of early death associated with sitting for over eight hours per day. However, as many as three out of four people in the study failed to reach this level of daily activity.

The greatest risk of early death was for those individuals who were physically inactive, regardless of the amount of time sitting - they were between 28% and 59% more likely to die early compared with those who were in the most active quartile - a similar risk to that associated with smoking and obesity. In other words, lack of physical activity is a greater health risk than prolonged sitting. "There has been a lot of concern about the health risks associated with today's more sedentary lifestyles. 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."