OBJECTIVE Observational studies show breaking up prolonged sitting has beneficial associations with cardiometabolic risk markers, but intervention studies are required to investigate causality. We examined the acute effects on postprandial glucose and insulin levels of uninterrupted sitting compared with sitting interrupted by brief bouts of light- or moderate-intensity walking.
RESEARCH DESIGN AND METHODS Overweight/obese adults (n = 19), aged 45–65 years, were recruited for a randomized three-period, three-treatment acute crossover trial: 1) uninterrupted sitting; 2) seated with 2-min bouts of light-intensity walking every 20 min; and 3) seated with 2-min bouts of moderate-intensity walking every 20 min. A standardized test drink was provided after an initial 2-h period of uninterrupted sitting. The positive incremental area under curves (iAUC) for glucose and insulin (mean [95% CI]) for the 5 h after the test drink (75 g glucose, 50 g fat) were calculated for the respective treatments.
RESULTS The glucose iAUC (mmol/L) ⋅ h after both activity-break conditions was reduced (light: 5.2 [4.1–6.6]; moderate: 4.9 [3.8–6.1]; both P < 0.01) compared with uninterrupted sitting (6.9 [5.5–8.7]). Insulin iAUC (pmol/L) ⋅ h was also reduced with both activity-break conditions (light: 633.6 [552.4–727.1]; moderate: 637.6 [555.5–731.9], P < 0.0001) compared with uninterrupted sitting (828.6 [722.0–950.9]).
CONCLUSIONS Interrupting sitting time with short bouts of light- or moderate-intensity walking lowers postprandial glucose and insulin levels in overweight/obese adults. This may improve glucose metabolism and potentially be an important public health and clinical intervention strategy for reducing cardiovascular risk.
Prolonged sitting time is associated with premature cardiovascular and all-cause mortality, independent of leisure-time physical activity and adiposity (1,2). Findings from the 2003–2004 and 2005–2006 U.S. National Health and Nutrition Examination Survey (NHANES) and the 2004–2005 Australian Diabetes, Obesity and Lifestyle (AusDiab) study showed that objectively measured total sedentary time was detrimentally associated with cardiometabolic risk markers (3,4). In contrast, frequent interruptions (breaks) to sedentary time (defined as the transition from sedentary to an active state for ≥1 min) were beneficially associated. In both populations, the mean break duration was approximately 4 min and was characterized by light-intensity activity (approximately 500 accelerometer counts/min). Furthermore, these relationships persisted after accounting for moderate-to-vigorous activity, suggesting that frequent short breaks in sedentary time may impart unique benefit. A next step for the science of sedentary behavior is to identify the metabolic underpinnings of these deleterious and beneficial relationships.
Regular ingestion of high-calorie meals rich in processed carbohydrates and saturated fat can lead to transient exaggerated postprandial spikes in glucose and lipids, which promote oxidative stress that triggers a biochemical inflammatory cascade, endothelial dysfunction, and sympathetic hyperactivity (5–9). These postprandial excursions, when repeated multiple times each day, can create a milieu conducive for the development of atherosclerosis and cardiovascular disease (9,10). Postprandial hyperglycemia or glucose variability has been associated with indices of atherosclerotic progression, including carotid intima-media thickening (11,12) and coronary artery calcium (13) as well as development or progression of retinopathy (14,15), cardiovascular events (16), and death (17) in patients with type 2 diabetes. Guidelines for cardiovascular health include minimizing the magnitude of postprandial hyperglycemia (18).
Postprandial glucose levels and insulin sensitivity are beneficially influenced by regular moderate-intensity exercise training (19–21). Recent epidemiologic findings have also shown 2-h plasma glucose to be beneficially associated with objectively measured light-intensity activity (22). Light-intensity activity may be effective for reducing postprandial glucose, and corroborative experimental evidence exists. In middle-aged women, 15- and 40-min bouts of light-intensity activity both led to a reduction in the acute blood glucose response to a carbohydrate-rich meal relative to 2-h seated rest (23). Compared with sitting, light nonexercise activities of intensities between 1.1 and 2.7 metabolic equivalents can improve insulin action in young men and women (24). In addition, 20 min of light exercise (40% of maximal power output) performed 45 min after the ingestion of a standardized pre-exercise carbohydrate load in young men resulted in similar glucose and insulin responses to that of moderate and vigorous exercise (65 and 80%) (25). This evidence, together with observational study findings on breaking up sedentary time (22), points to the need for further experimental studies. Specifically, there is the need to examine the effects of frequent brief interruptions to prolonged sitting involving light-intensity or moderate-intensity activity (as distinct from continuous ambulatory activity) on postprandial glucose and insulin. This may have important implications for settings where continuous ambulatory activity may not be feasible or practical, such as workplaces.
We examined the acute effects of uninterrupted sitting on postprandial plasma glucose and serum insulin, compared with sitting interrupted by short 2-min bouts of light- or moderate-intensity walking in overweight middle-aged adults. We hypothesized that postprandial blood glucose control during sitting would be improved by brief intermittent bouts of activity, irrespective of whether these were of light or moderate intensity.
RESEARCH DESIGN AND METHODS
Study overview
This randomized, three-period, three-treatment crossover trial was approved by the Alfred Hospital Human Ethics Committee and was in accordance with the Declaration of Helsinki. Participants provided signed, written informed consent. The study is registered as a clinical trial with the Australian New Zealand Clinical Trials Registry (ACTRN12609000656235).
Participants attended three separate visits to the laboratory to complete each of the trial conditions in a randomized order: 1) uninterrupted sitting; 2) sitting interrupted by light-intensity activity breaks; and 3) sitting interrupted by moderate-intensity activity breaks. Because an acute bout of physical activity may enhance insulin sensitivity for up to 72 h (26), we used a minimum wash-out of 6 days between each condition to eliminate potential carryover effects of the activity conditions.
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