Taking 10,000 steps a day is often suggested as a desirable exercise goal for people who wish to improve their health.
But a new study of postal workers in Scotland suggests that this number could be too conservative and that, to best protect our hearts, many of us might want to start moving quite a bit more.
It has been almost 70 years since the publication of the London Transit Workers Study, a famous work in which researchers tracked the heart health of London bus drivers and conductors. They found that the conductors, who walked up and down bus aisles throughout the working day, were substantially less likely to develop or die from heart disease than the drivers, who sat almost constantly while at work.
This study was one of the first to persuasively show that being physically active could lower someone's risk of heart disease, while being sedentary had the opposite effect.
But precisely how much exercise might be needed in order to avoid heart disease has remained very much in question. The threshold of 10,000 daily steps, incorporated as a goal into many activity monitors today, has not been scientifically validated as a way to lessen disease risk.
So for the new study, which was published last month in The International Journal of Obesity, researchers at the University of Warwick in England and other institutions decided not only to refer back to but also advance and expand upon the results of that foundational Transit Workers Study, by examining another group of employees whose work routine involve mostly walking or sitting.
Number of steps researchers are now recommending to lower the risk of cardiac diseases, compared to their previous recommendation of 10,000 steps.
Number of hours of brisk walking university researchers say is needed to hit the 15,000 step count.
They turned to postal workers in Glasgow, Scotland. The Glaswegian mail carriers generally cover their routes on foot, not by driving, and spend many hours each day walking. However, the mail service's office workers remain seated at their desks during the bulk of the workday.
The researchers began by recruiting 111 of the postal-service workers, both men and women, and most between the ages of 40 and 60. They measured the body mass index (BMI) of volunteers, as well as their waist sizes, blood sugar levels and cholesterol profiles.
Then they had each volunteer wear a sophisticated activity tracker for a week, while at work and at home and during the weekend. Afterwards, the researchers determined how many waking hours each day the volunteers had spent seated or on foot. They also calculated how many steps each person had taken each day.
The variations were considerable. Some of the office workers sat for more than 15 hours each day between work and home, while most of the mail carriers barely sat at all during working hours.
These differences were echoed in the volunteers' risk factors for heart disease, the researchers found. Those workers who sat for most of each day tended to have much larger waistlines, higher BMIs and worse blood sugar control and cholesterol profiles than those who frequently stood and moved, even after scientists allowed for age, family history, late-night shift work (which is known to affect heart health) and other factors.
The risks were magnified at the extremes. For every hour beyond five that workers sat each day, the researchers found, they added to their likelihood of developing heart disease, based on their cumulative risk factors.
Meanwhile, almost any amount of standing and walking reduced a worker's chances of having a large waistline and other risk factors for heart disease.
But the greatest benefits came from the most exaggerated amounts of activity. Those mail carriers who walked for more than three hours a day, covering at least 15,000 steps, which is about seven miles (11.2km), generally had normal body mass indexes, waistlines and metabolic profiles. Together, these factors meant that they had, effectively, no heightened risk for cardiac disease.
Of course, this study provides a single, limited snapshot of people's health and lives. The researchers did not follow their volunteers for decades to see who actually developed heart disease. This kind of study also cannot prove that walking or sitting caused the differences in people's risks factors for heart disease, only that there were associations between activity and risks.
But the findings do imply that there are good reasons to get up from our desk chairs and move, even more than many of us may already be trying to do, said Dr William Tigbe, a physician and public health researcher at the University of Warwick who led the study.
"It takes effort," he said, but we can accumulate 15,000 steps a day by walking briskly for two hours, he said.
"This can be done in bits," he added, perhaps with a 30-minute walk before work, another at lunch, and multiple 10-minute bouts throughout the day.
"Our metabolism is not well suited to sitting down all the time," he concluded.