Waiting for Detailed Guidelines on Sedentary Behavior? Be Patient.

The American Heart Association has released an important Scientific Statement on sedentary behavior in adults.¹ The definition of sedentary behavior used is “waking behavior with an energy expenditure of 1.5 metabolic equivalents (METs) or less while in a sitting or reclining position.” For the most part, this is sitting behavior. There is strong evidence that moderate-to-vigorous aerobic physical activity (MVPA) has substantial health benefits in relatively small amounts—in the range of at least 150 minutes per week of moderate-intensity physical activity,2 which can be achieved throughout the week in blocks of 10 to 15 minutes or more. Some people aim for 30 minutes each day. U.S. public health guidelines have recommended regular MVPA for the past 20 years. But adults are awake for, say, 16 hours each day. The issue addressed by this Statement is essentially whether guidelines should also make recommendations on how to divide the remaining 15 and one half hours between sedentary behavior and light-intensity activities.

The main message of the Statement is that “there currently is insufficient evidence upon which to base specific public health recommendations” regarding the amounts and types of sedentary behavior that reduce risk of cardiovascular morbidity and mortality.¹ In particular, the Statement notes that there is a lack of randomized controlled trials of interventions to reduce sedentary behavior, and, hence, such trials are needed. The fact that the writing group goes on to say “However, we know virtually nothing about the cardiovascular benefits of…engaging in light activities”¹ suggests to me that we need to be patient. The research agenda for sedentary behavior is large. Of the many gaps in knowledge about sedentary behavior, in my opinion, three issues raised in the paper are particularly important. The first issue is whether the health effects of sedentary behavior are independent of MVPA. One recent meta-analysis found that among those with high levels of aerobic physical activity, sedentary time was not associated with all-cause mortality.³ Another study suggested that in adults with high (estimated) cardiorespiratory fitness, sedentary time may not influence cardiovascular risk factors.⁴ It does make intuitive sense to me that at some point, a person can engage in enough MVPA or have sufficient fitness that reducing sedentary behavior doesn’t provide meaningful additional health benefits, but we shall see.

The second issue is related to the first: Are physiologic mechanisms mediating the health effects of sedentary behavior (at least in part) distinct from the mechanisms producing the health benefits of MVPA? If not, in adults who engage in little or no MVPA, higher levels of light-intensity activity would have some health benefits caused by the same mechanisms that produce benefits of MVPA. That is, sedentary behavior research could simply be studying the low end of the same dose-response relationship that accounts for the health benefits of MVPA. But as noted in the Statement, there is some evidence that the physiology of low-intensity activity differs in some respects from the physiology of high intensity activity.

Yet even if the mechanisms of light-intensity physical activity and MVPA are not distinct, reducing sedentary behavior could be important to the extent mechanisms have a short-duration. U.S. adults do not do engage in much MVPA. Only about 50% of U.S. adults meet the public health guideline for aerobic activity—the equivalent of walking about 23 minutes/day or more.⁵ It will be a challenge for adults to do bouts of MVPA frequently and intermittently during the day so as to repeatedly activate a short-duration physiologic effect. Frequent, intermittent light-intensity bouts are likely more feasible. These bouts are common in adults. It’s possible that some health benefits may occur from spreading existing bouts out across the day, even without major reductions in total sedentary behavior. A trial mentioned in the Statement addressed a short-duration effect by studying 2-minute bouts of light-intensity and moderate-intensity walking every 20 minutes over 7 hours in overweight/obese adults.6 Consistent with a short-duration effect of aerobic activity on blood glucose, both light- and moderate-intensity walking increased insulin sensitivity.

The third issue is the feasibility of substantial reductions in sedentary behavior in adults. Self-reported sedentary time varies widely among adults, as indicated by studies that compare less than 4 hours/day with more than 11 hours/day.³ When measured by accelerometer in NHANES, sedentary time also varied among adults, with the lowest quartile of sedentary time averaging about 6 hours a day and the highest quartile averaging about 12 hours/day.⁷ It’s possible that relatively large changes in sitting time are required for substantial health benefits. The Scientific Statement mentions comparing the health benefits of promoting physical activity to reducing sitting time by of 3 to 6 hours per day. A cited meta-analysis found a mean intervention effect of a 91-minute reduction in seven studies,⁸ suggesting that early trials have achieved reductions more in the range of 1 to 2 hours. Of course, newer technologies may be able to significantly enhance the feasibility of reducing sedentary behavior. Much remains to be learned about effective approaches to reducing sedentary behavior in a variety of settings and subgroups of adults.

It should be fascinating to observe the field of sedentary behavior research movement going forward, as I believe we are experiencing a paradigm change in physical activity research. A major part of this change is the well-recognized shift from self-reported measures of activity to wearable devices that provide objective measures. Another part of the paradigm shift is research on continuous measures of intensity provided by wearable devices. Perhaps someday we will have an equation that estimates health benefits of activity based upon continuous measures of energy expenditure and aerobic intensity summed across a week. In other words, we should be able to improve on cut-point systems that use accelerometer data to classify movement into the four intensity categories (sedentary, light, moderate, and vigorous).

Related to issuing guidelines, I think the paradigm change includes a shift from one-size-fits-all recommendations to tailored recommendations. We are early in this shift, but the 2008 Physical Activity Guidelines for Americans had tailored recommendations for a few subgroups.2 For example, older adults at risk of falls should do balance training (typically a light-intensity activity). That is, we already have national guidelines that recommend light-intensity activity for some subgroups. My guess is that new guidelines on sedentary behavior will be incremental, with recommendations for additional subgroups as sufficient randomized trial data become available in these groups.

But for now, we need to step up our efforts to promote MVPA and strength training. According to the Healthy People 2020 website, in 2014 only about 20% of adults met public health guidelines for both MVPA and strength training.⁵ We have a plan for how to promote physical activity, The National Physical Activity Plan, which was updated in spring of 2016.9 The American Heart Association is supporting this plan with initiatives to promote active, healthy lifestyles.

Our Surgeon General, Dr. Vivek H. Murthy, recently issued a call to action on promoting walking and walkable communities.¹⁰ To quote Dr. Murthy, “Walking for better health may seem simplistic, but sometimes the most important things we can do are the easiest and the most obvious. It’s time to step it up, America! The journey to better health begins with a single step.”