To summarize the study for you, they looked at 17,013 patients who participated in the 1981 CFS and they asked these people how often they sat on most days of the week. Participants were given 5 choices: 1) almost none of the time 2) 1/4 of the time 3) 1/2 of the time 4) 3/4 of the time and 5) all of the time. Baseline characteristics were taken for these people from 1981 and included smoking status (yes, no, previous), alcohol consumption (broken down into 5 categories), BMI (only actually measured on a subset of the population), and an exercise recall from the past 12 months. Participants were then classified either as leisure time active (who did moderate exercise 30 mins for 5 days a week) or not active (less than 30 mins of exercise 5 days a week).
Now let us get to the meat of the study. For the outcome which is the most important, they looked at an end point of death (all cause mortality) in both men and women using multivariate adjusted hazard ratio. They used the group who sat almost none of the time as a hazard ratio = 1. For the following groups the hazard ratio is (1/4 time sitting) 1 (95%CI 0.86-1.18), (1/2 time sitting) 1.11 (0.94 - 1.30), (3/4 time sitting) 1.36 (1.14 - 1.63), (all of the time sitting) 1.54 (1.25 - 1.91). OK, from these results I would argue that sitting almost none of the time, sitting 1/4 of the time and even sitting 1/2 of the time results in no considerable change in all cause mortality. There does appear to be an increase in the hazard ratio for sitting 3/4 of the time or all of the time. So, who cares?
The interesting question to ask is about relevance. What if I am an endurance road cyclist? Does sitting on my bike training for 4-5 hours a day no longer benefit my health because I am in a seated position the entire time? Conversely, what if I watch 12 hours of TV a day, but I am standing the entire time I am doing it, does that undo the harm? I would argue this study just re-enforces what is already known - exercise! A news station interviewed a woman who was so surprised/scared by this study she went out and bought herself a special trendmill that fits her laptop so she can type on her laptop while walking at a ridiculously slow pace. Now the question arises of whether running in the morning for 30-40 mins and taking walking breaks during the day would be more effective then walking very slowly for 5 hours. This study does not address any of these questions. It also doesn't discuss sitting duration. If I sit 5 hours a day but take 15 min stand breaks every 15 mins is that better than sitting less, 3 hours, but for 3 straight hours? I feel this study just points out the obvious that if you are sitting "most of the day" you would not be able to do a sufficient amount of exercise. The study argues there is a "dose-dependent relationship between sitting and all cause mortality" I would argue that for the reference group and the first two groups which sit the least they are all comparable. It is only when you get into the extreme groups (ie sitting 3/4 of the time or all the time) you really experience an increased risk of death from all causes. However people in the two extreme groups tend to be older and exercise less. The study argues, "the results of this study provide evidence to support the suggestion that recommendations to limit sedentary time may be important for public health". This is already known and already in place. The current guidelines recommend 5 days a week of at least 30 mins of moderate exercise. Therefore recommending a person should be less sedentary is the same as recommending people exercise more, just a different way of saying it!
My conclusion: the media is blowing this paper slightly out of proportion. Nothing in this study is new, novel, or surprising. People should get out and be more active (and being active does not involve sitting!).
