These findings provide initial experimental confirmation of hypotheses generated by epidemiologic observational studies on the deleterious health consequences of prolonged sedentary time (too much sitting as distinct from too little exercise). Of particular importance is the potential for reducing cardiovascular disease risk by briefly breaking up prolonged periods of sitting with activity of at least light intensity. Brief interruptions to sitting led to significant reductions in postprandial glucose and insulin, irrespective of the activity intensity. The 24–30% lowering in the plasma glucose iAUC and the 23% lowering in the insulin iAUC seen after the activity break conditions, at the very least, is comparable in magnitude to an acute bout of moderate-intensity aerobic or resistance exercise in overweight/obese individuals (32).
Our findings are the first to document, among overweight/obese adults, an elevated postprandial glucose and insulin response during an acute bout of prolonged sitting, relative to sitting with brief activity interruptions. Among young nonobese adults, significant reductions in whole-body insulin sensitivity have been observed after 1 day of prolonged sitting (24). The magnitudes of changes observed in postprandial glucose and insulin during the uninterrupted sitting condition in our study are consistent (∼23–30%) with the changes in metabolic outcomes reported after 1 day of prolonged sitting (24). The lowering of postprandial glucose and insulin suggests both increased insulin sensitivity and reduced insulin secretion, the latter effect consistent with preservation of pancreatic β-cell function. Intervention study findings demonstrating that blunting of postprandial spikes in glucose improves inflammation and endothelial function (33) and reduces carotid intima-media thickness (11,33) suggest that the blunted glucose response seen in our study through the inclusion of three brief (2-min) activity breaks per hour during prolonged sitting may ameliorate such consequences of postprandial hyperglycemia (9,18).
Although larger reductions in postprandial glycemia have yielded clinical benefits to surrogate cardiovascular end points, including carotid intima-media thickness (8), smaller differentials in the postprandial glucose seen in response to pharmaceutical agents, generally comparable in magnitude to those seen in the activity break conditions, have been linked to reductions in oxidative stress, circulating adhesion molecules, and endothelial function in patients with and without type 2 diabetes (6,8,33). Additional research using measures of inflammation and endothelial function will be necessary to further elucidate the influence of frequent breaks from sitting and whether the exaggerated postprandial glucose response observed after 5 h of uninterrupted sitting is more pronounced when imposed over multiple days. Potential long-term consequences, both beneficial and deleterious, of different sitting-time patterns are needed to strengthen the case for relevant public health and clinical initiatives.
These findings support the hypothesis that brief interruptions to sedentary time with a minimum of light-intensity physical activity can attenuate acute postprandial plasma glucose and serum insulin response during prolonged sitting. Importantly, the brevity of the interruptions to sitting (2 min) indicates that such breaks would not count toward the minimum amount of aerobic activity necessary for substantial health benefits within current physical activity guidelines because at least 10-min episodes of activity are stipulated (34). However, consistent with studies of the effects of continuous exercise on glucose metabolism (35), the sum of total activity time over a 5-h period was 28 min. A logical next step would be to build on these findings to design a study that would provide a head-to-head comparison of a single continuous exercise bout to the breaking up of prolonged sitting protocol used in this study.
The moderate-intensity activity break condition was relatively strenuous for some of our study participants. Our original protocol called for a consistent 6.4 km/h for all participants, but we found it necessary to slightly reduce the speed for some so that they could complete the prescribed 2-min moderate-intensity bouts without discomfort. Thus, the higher-intensity breaks condition was close to the practical maximal feasible intensity for these overweight/obese individuals and likely close to the tolerable upper limit of activity for expected physiologic benefit.
Our trial does have some potential limitations. First, it examined acute effects of a 1-day exposure to prolonged uninterrupted versus interrupted sitting; thus, implications cannot be extrapolated to long-term exposures. Second, it examined activity bouts of fixed frequency and length. Systematic variations in length and frequency of activity bouts and possible moderating effects of factors such as sex and adiposity status should be examined.
Our experimental protocol involved brief and regular interruptions (2 min of activity for every 20 min sitting) to prolonged sitting time. Interrupting sedentary time in this way could be feasible in domestic and workplace settings where adults sit for prolonged periods (36). However, further experimental evidence that can define dose-response relationships is required. For example, evidence from animal studies (37) suggests that it may be important primarily to interrupt the contractile inactivity in postural muscles that takes place during prolonged sitting. From body weight–dependent equations (38), it is estimated that treadmill walking at 3.2 km/h in people weighing 90.5 kg demands an energy expenditure of only 18.8 kJ/min, yet most forms of nonexercise activity that naturally interrupt sitting would be expected to yield even less energy expenditure. Although the body weight–dependent equations do not cover the 5.6–6.4 km/h walking speed used in this trial, it would be realistic to expect that the energy expenditure for the moderate walking bouts would be expected to exceed the 25.6 kJ/min estimated for walking at 4.8 km/h. Simply regularly standing up for a short period may have beneficial metabolic effects. Estimating the caloric expenditure of such changes would be informative; the increased substrate utilization that is required to meet differences in energy demands when transitioning from sitting to standing may be an important mechanism (37,38). However, whether simple brief standing, as opposed to longer activity bouts, will be protective for metabolic health, remains to be determined.
Given the high prevalence of overweight and obesity among those of low socioeconomic status living in affluent populations (39) and the average “greying” of demographic profiles, our study participants are representative of large numbers at risk for developing type 2 diabetes and subsequent cardiovascular complications. Pragmatic trials with large numbers of participants would be highly informative (40), such as demonstrating metabolic health effects of reducing and breaking up sitting time in the workplace over sustained periods of time (i.e., months or years) among representative groups of working adults.
No comments:
Post a Comment