This story reports on a large meta-analysis of observational studies looking at the association between sedentary behavior (sitting and TV-watching), exercise, and risk of dying prematurely.
Its main take-home message is “it takes about 60 to 75 minutes of ‘moderate intensity’ exercise to undo the damage of sitting for at least eight hours a day.” Although that’s a faithful account of how the study researchers frame their results, we think it’s likely to leave readers with a misleading impression.
Despite what the researchers may claim, this type of study can’t prove that exercise can “undo the damage of sitting” — all it can do is show associations between these behaviors and risk of death. There are many factors, other than exercise and sedentary behavior, that affect one’s risk of death (e.g. diet, illnesses, socioeconomic factors), and the story didn’t explain that researchers can’t account for all of those factors in a study like this. Suggesting that exercise “reverses” the damage of sitting implies a certainty to the study’s conclusions that simply doesn’t exist.
Every day, readers are bombarded with health news messages from observational studies such as this one relating to diet, exercise, and other lifestyle issues. Because this type of research is not definitive, it is liable to be contradicted by the next study that comes along, which in turn generates a new set of headlines that is in conflict with what readers just heard from the last study. This generates confusion among the public and mistrust of both scientists and journalists who can’t seem to get their stories straight. We offer tips for reporting on observational studies in our toolkit.
There’s no direct cost to walking an hour a day, so we won’t ding the story for omitting discussion of cost. However, there is clearly a time cost involved, and many people may struggle to find the time for exercise if it means less time for paying work or caregiving. Others, especially older adults, may have medical problems that preclude the exercise. We wish the story had mentioned these factors.
The story provides no numbers. It doesn’t tell us how much risk of death is increased by sitting 8 hours a day, or how much it is reduced by exercise. It tells us that “Not exercising and sitting all day is as dangerous as being obese or smoking,” but how dangerous is that?
We can’t think of any harms from engaging in a brisk hour-long walk. Injury is always a possibility, but that’s apparent enough that we don’t think the story was obligated to discuss this.
The story inappropriately uses cause-and-effect language in the headline and elsewhere that overstates what this study can tell us (e.g. “To reverse damage of sitting, take a brisk, hour-long walk”). It should have said that walking, sitting, etc. are “associated with” or “linked to” health effects, but not that walking can “undo” or “reverse” damage.
In the study itself, the researchers provide a long list of limitations that suggest a lack of certainty regarding the strength of these conclusions. But the story did not report on any of them. For example, the researchers state,”we attempted to minimise bias from reverse causation (ie, illness causing individuals to become sedentary) by including apparently healthy participants; however, we cannot fully rule this bias out.” What this means is that the results could be skewed by individuals who had undiagnosed, subclinical disease that caused them to become less likely to exercise. While the assumption is that lack of exercise makes people sick, perhaps the reverse is true in some cases, which would obviously affect results from a study such as this one.
The story gets credit here for quoting the author of an accompanying editorial. But we’d point out that it probably wouldn’t have been hard to find someone who would have looked at the study more critically and offered some comment on its limitations.
The story is basically a comparison of the competing effects of exercise vs. sedentary behavior.
The story addresses this as an issue with this line:
Andersen said that some cultures make it easier than others to squeeze in an hour of exercise every day, noting that in Denmark and much of Scandinavia, about half of all people either cycle or walk to work.
Although the story nods to previous research, noting that “Previous studies have found that prolonged sitting can raise the chances of heart disease, various cancers and an earlier death,” it doesn’t explain that this study is meant to address a specific issue that has generated conflicting results in previous research. Here’s how the researchers put it:
Two other meta-analyses had examined the associations of sitting time with non-communicable disease incidence and mortality. One of these concluded that prolonged sitting time was associated with increased risks of deleterious health outcomes regardless of physical activity level, whereas the other concluded that physical activity (no details on the amount of activity were provided) seemed to attenuate the increased risk of all-cause mortality due to high sitting.
In other words, this newer study was attempting to answer this question: Given the conflicting results from older research, what role does physical activity play, if any, in counteracting prolonged daily sitting?
Although the story quotes from a news release, it acknowledges the source of the quotation and also includes an interview with the author of the accompanying commentary. It’s apparent that there was some independent reporting done here.