June 2017 Issue
Dynamics of Diabetes: Is Sitting the New Smoking?
By Carrie Dennett, MPH, RDN, CD
Today's Dietitian
Vol. 19, No. 6, P. 16
Here's what the research says about the effects of prolonged sitting on diabetes risk.
Claims akin to "sitting is the new smoking" have had a growing presence in both the research literature and the popular media, spurring an increase in standing desks and smartphone reminders to "get up." Indeed, research shows that the average person spends more than one-half of their waking hours engaged in activities that involve prolonged sitting, including watching TV and using the computer,1 and that excessive sedentary time impairs glucose and lipid metabolism and raises the risk of developing type 2 diabetes.2-4
In light of this, the American Medical Association has a policy on the health risks of prolonged sitting5; the American Institute for Cancer Research recommends decreasing TV time6; the American Heart Association suggests that we "sit less, move more"7; and the American Diabetes Association encourages moving every 30 minutes.8 But a few recent studies have questioned whether long periods of sitting are universally bad for cardiometabolic health.
Devil in the Details
A study recently published in British Journal of Sports Medicine suggests that not all sitting is created equal. A 13-year follow up of participants in the Whitehall II cohort study found that total time spent sitting—and time spent sitting in front of the TV, specifically—wasn't associated with increased incidence of diabetes once BMI was taken into account, while other types of sitting weren't associated with incident diabetes at all.9 A review published in February in the International Journal of Clinical Practice concluded that while current research shows that reducing sedentary behavior may have modest benefits for metabolic health, the effect is too small to warrant making "large-scale and potentially costly clinical and public health recommendations."10
There's not much research on sitting and diabetes or overall metabolic risk, and what research there is doesn't tend to factor in body fat as a confounder and varies greatly in study design.1,9 Some studies use self-reported data; some use objective measures like accelerometers. Some use healthy subjects, while others use subjects who have already developed cardiometabolic disease. There's also variation in weight-based inclusion criteria. In addition, the effects of sedentary time on diabetes risk may depend on individual genotypes.2
Another limitation is how sedentary time is defined. Because TV viewing is the most "prevalent and pervasive" sedentary behavior,11 it's often used as a proxy for sitting time, even though TV viewing is a poor predictor of total sedentary time2 and is correlated with lower socioeconomic status, which is itself associated with increased chronic disease risk.9 There are in fact many reasons we sit, including desk jobs, commuting, and non-TV sedentary hobbies (eg, reading, knitting).
Sitting in front of the TV appears to be distinct from sitting at a desk job, not just because desk jobs are associated with higher socioeconomic status but also because daytime sitting is more likely to be broken up.4 Much TV viewing occurs in the evening after dinner, and prolonged postmeal sedentary time has been shown to have unhealthful effects on glucose and lipid metabolism.3 TV viewing also may be accompanied by snacking, and the nutritional composition of those snacks may not support health.4 However, one meta-analysis that looked at the cardiometabolic effects of prolonged TV viewing did, even after factoring in diet, find that as TV viewing increased so did risk of diabetes, cardiovascular disease (CVD), and all-cause mortality.11
Does Sitting Undo Exercise?
The standard physical activity recommendation for prevention of chronic diseases is 150 minutes per week in bouts of 10 minutes or more.2 But is physical activity enough to reduce disease risk and poor health? Some studies suggest that the adverse effects of prolonged sitting time could potentially undo the health effects of physical activity,2 a message that has made its way into mainstream media. But could this message be counterproductive (eg, "I have to sit all day for my job, so why bother going to the gym after work if all that sitting negates it?")?
It could, says Emannuel Stamatakis, PhD, MSc, BSc, coauthor of the British Journal of Sports Medicine article and an associate professor and senior research fellow at the University of Sydney. "A good session of exercise repeated a few times across the week can have tremendous cardiovascular, metabolic, and mental health benefits even in the presence of high amounts of sitting," he says. "Of course, that doesn't make excessive sitting OK; it's a good idea to strike a good balance between exercise, rest, and incidental movement across the day. On the days one cannot exercise, it's always a good idea to walk a bit more and sit less."
Glen Duncan, PhD, RCEP, chair of the Nutrition & Exercise Physiology Program at Washington State University, points out that we need to take control of those periods of time when we can replace sitting with activity. "We live in an obesogenic environment whereby physical activity has been systematically phased out of our lives," Duncan says. "Most jobs are sedentary, and passive automobile transportation encompasses the vast majority of our trips, so this is the norm rather than the exception."
Glucose Control
Research suggests that breaking up sitting time with even brief interruptions may help reduce risk of cardiometabolic diseases and improve postmeal glucose and lipid metabolism compared with uninterrupted sitting.12,13 A small randomized crossover study of 24 adults with type 2 diabetes and BMIs above the normal range found that interrupting prolonged sitting with three minutes of light-intensity walking or simple resistance exercises every 30 minutes improves postmeal metabolic responses14 and that the improvements in glucose levels persisted through the night.15
Is there significant value in reducing sitting time in the absence of actual exercise, especially given the established benefits of exercise for cardiometabolic health? Yes and no, Duncan says. "The general idea behind reducing sitting/sedentary time can be summarized by the old adage 'something is better than nothing,' so getting up and moving around some is better than staying seated and not moving around at all," he says. "However, simply reducing overall sitting/sedentary time without replacing it with moderate to vigorous levels of physical activity will not likely reduce CVD risk factors and mortality."
Another study coauthored by Stamatakis suggests that whether sedentary time increases cardiometabolic risk may depend on the type of activities that are displaced. Breaking up sedentary time with brief bouts of light activity may have cardiometabolic benefit only for people who already have diabetes or other chronic disease. Healthy adults with low risk factors may need to replace some sedentary time with moderate-to-vigorous activity to improve cardiometabolic health.16
"Reducing sitting is certainly very important for people who do not exercise," Stamatakis says, adding that not exercising is "a very bad situation," but that sitting for too long makes it worse.
Studies on the health effects of sitting that do account for physical activity find that any adverse effects of sitting decrease as physical activity increases.1 A systematic review that included data from more than 1 million individuals found that high levels of moderate-intensity physical activity (60 to 75 minutes/day) eliminated the increased risk of all-cause mortality that's associated with high sitting time but only lessened the risks from prolonged TV viewing (more than five hours per day).3
Another study that used accelerometers to measure activity in a population with known risk factors for diabetes found that as sedentary time increased, physical activity decreased, and biomarkers for cardiometabolic disease worsened, although the authors state that due to the cross-sectional design of the study, the direction of causality is unclear.17
Advising Patients
When looking at reduction of sedentary time vs increasing exercise time, where should dietitians focus their behavior change efforts?
"Our society is so sedentary that it makes the most sense to first target sitting/sedentary time and then work on the activity/exercise piece," Duncan says. "It's just not practical to think that a population that is overwhelmingly sedentary will be able to 'switch' to achieving activity goals."
Stamatakis says that reducing sitting and increasing light activity can be valuable "second-line treatment" for individuals who struggle with starting or maintaining an exercise program. "The effects of a balanced exercise program that involves some physical exertion and huffing and puffing cannot be reproduced by reducing sitting by any amount," he says. "But many people will be resistant to exercise, and the practitioners will need to employ an alternative strategy."
— Carrie Dennett, MPH, RDN, CD, is the nutrition columnist for The Seattle Times and speaks frequently on nutrition-related topics. She also provides nutrition counseling via the Menu for Change program in Seattle.
References
1. Biswas A, Oh PI, Faulkner GE, et al. Sedentary time and its association with risk for disease incidence, mortality, and hospitalization in adults: a systematic review and meta-analysis. Ann Intern Med. 2015;162(2):123-132.
2. Wilmot EG, Edwardson CL, Achana FA, et al. Sedentary time in adults and the association with diabetes, cardiovascular disease and death: systematic review and meta-analysis. Diabetologia. 2012;55(11):2895-2905.
3. Ekelund E, Steene-Johannessen J, Brown WJ, et al. Does physical activity attenuate, or even eliminate, the detrimental association of sitting time with mortality? A harmonised meta-analysis of data from more than 1 million men and women. Lancet. 2016;388(10051):1302-1310.
4. Benatti FB, Ried-Larsen M. The effects of breaking up prolonged sitting time: a review of experimental studies. Med Sci Sports Exerc. 2015;47(10):2053-2061.
5. AMA declares prolonged sitting a serious health risk. MyLifeStages website. https://www.mylifestages.org/blog/healthy-living-blog/ama-declares-prolonged-sitting-a-serious-health-risk. Published June 20, 2013.
6. Reduce your cancer risk: physical activity. American Institute for Cancer Research website. http://www.aicr.org/reduce-your-cancer-risk/physical-activity/reduce_physical_getting_started.html. Accessed April 8, 2017.
7. Young DR, Hivert MF, Alhassan S, et al. Sedentary behavior and cardiovascular morbidity and mortality: a science advisory from the American Heart Association. Circulation. 2016;134(13):e262-e279.
8. Colberg SR, Sigal RJ, Yardley JE, et al. Physical activity/exercise and diabetes: a position statement of the American Diabetes Association. Diabetes Care. 2016;39(11):2065-2079.
9. Stamatakis E, Pulsford RM, Brunner EJ, et al. Sitting behaviour is not associated with incident diabetes over 13 years: the Whitehall II cohort study. Br J Sports Med. 2017;51(10):818-823.
10. Chin SH, Kahathuduwa C, Binks M. Is sedentary behaviour unhealthy and if so, does reducing it improve this? Int J Clin Pract. 2017;71(2):e12925.
11. Grøntved A, Hu FB. Television viewing and risk of type 2 diabetes, cardiovascular disease, and all-cause mortality: a meta-analysis. JAMA. 2011;305(23):2448-2455.
12. Henson J, Davies MJ, Bodicoat DH, et al. Breaking up prolonged sitting with standing or walking attenuates the postprandial metabolic response in postmenopausal women: a randomized acute study. Diabetes Care. 2016;39(1):130-138.
13. Dunstan DW, Kingwell BA, Larsen R, et al. Breaking up prolonged sitting reduces postprandial glucose and insulin responses. Diabetes Care. 2012;35(5):976-983.
14. Dempsey PC, Larsen RN, Sethi P, et al. Benefits for type 2 diabetes of interrupting prolonged sitting with brief bouts of light walking or simple resistance activities. Diabetes Care. 2016;39(6):964-972.
15. Dempsey PC, Blankenship JM, Larsen RN, et al. Interrupting prolonged sitting in type 2 diabetes: nocturnal persistence of improved glycaemic control. Diabetologia. 2017;60(3):499-507.
16. Hamer M, Stamatakis E, Steptoe A. Effects of substituting sedentary time with physical activity on metabolic risk. Med Sci Sports Exerc. 2014;46(10):1946-1950.
17. Henson J, Yates T, Biddle SJ, et al. Associations of objectively measured sedentary behaviour and physical activity with markers of cardiometabolic health. Diabetologia. 2013;56(5):1012-1020.