Abstract
Sedentary behaviour is a risk factor for cardio-metabolic disease. Results from observational studies indicate that regularly interrupting sedentary behaviour with light bouts of physical activity is associated with a lower risk. It has been proposed (from the results of a series of studies conducted in rats) that sedentary behaviour influences cardio-metabolic risk, principally by acting on processes involved in postprandial metabolism, particularly the activity of lipoprotein lipase. Lipoprotein lipase plays an important role in the clearance of triglycerides from the bloodstream after a meal. In addition the results from two recently performed interventions indicate that sedentary behaviour also influences the action of insulin. Therefore, the aim of the current study was to investigate the effects of prolonged sitting, a single 30 min brisk walk followed by prolonged sitting, and sitting interrupted with regular short (1 min and 40 sec) walks on postprandial lipidaemia, insulinaemia and glycaemia.
A randomized crossover, laboratory based study was conducted in Dunedin, New Zealand between February and October 2010. Seventy healthy, normal weight adults (Mean (SD) VO2max 42.9 (10.3) ml·kg·min-1, Body Mass Index 23.6 (4.0) kg·m-2) aged between 18 and 40 y participated in three interventions. The three interventions were: 1) Prolonged Sitting, during which participants sat continuously for 9 h; 2) Physical Activity, in which participants sat for 15 min, walked on the treadmill continuously for 30 min at 60% VO2max and then sat continuously for 8 h and 15 min and; 3) Regular Activity Breaks, in which participants interrupted their sitting every 30 min with a 1 min and 40 sec brisk walk on the treadmill, performing a total of 18 walks over the 9 h period (totalling 30 min of physical activity). In all three interventions a heart rate monitor was worn continuously over the 9 h period, measures of expired air were collected during all walks on the treadmill and resting measurements were collected hourly. Participants were fed a meal replacement beverage at 60, 240 and 420 min. Each meal replacement beverage provided 31.4 ± 6.6 g of fat, 36.8 ± 7.7 g of protein 76.4 ± 16.1 g of CHO and 3105 ± 652 kJ of energy. Blood samples were collected from a venous cannula at baseline and hourly for 9 h, with additional samples collected 30 and 45 min after each feeding. Postprandial measures of heart rate and energy expenditure were calculated as the average of all measurements taken over the 9 h intervention period. Postprandial responses for plasma glucose, insulin and triglyceride were calculated as positive incremental area under the curve (iAUC).
Regular Activity breaks increased mean heart rate by 9 bpm (95% CI 7 to 11; p<0.001) when compared to Prolonged Sitting. Physical Activity also increased mean heart rate by 9 bpm (95% CI 6 to 11; p<0.001) compared to Prolonged Sitting. Mean heart rate did not differ between Physical Activity and Regular Activity Breaks (difference 0 bpm, 95% CI -3 to 1; p=0.684). The mean total estimated energy expenditure for Physical Activity was 1.0 MJ (95% CI 0.8 to 1.2; p<0.001) greater than Prolonged Sitting and 0.2 MJ (95% CI 0.01 to 0.4; p=0.011) greater than Regular Activity Breaks. The mean total estimated energy expenditure for Regular Activity Breaks was 0.8 MJ (95% CI 0.5 to 0.9; p<0.001) greater than Prolonged Sitting.
Regular Activity Breaks lowered plasma insulin iAUC by 96.30 IU·L-1·h (95% CI 56.22 to 136.38, p<0.001) when compared to Prolonged Sitting, and by 60.23 IU·L-1·h (95% CI 19.98 to 100.47, p=0.003) when compared to Physical Activity. Regular Activity Breaks lowered plasma glucose iAUC by 2.10 mmol·L-1 ·h (95% CI 1.11 to 3.10 p<0.001) when compared to Prolonged Sitting and by 1.93 mmol·L-1·h when compared to Physical Activity (95% CI 0.93 to 2.92 p<0.001). There was no difference in triglyceride iAUC between Prolonged Sitting and either Physical Activity or Regular Activity Breaks. However, Physical Activity lowered mean plasma triglyceride iAUC by 0.69 mmol·L-1·h (95% CI 0.20 to 1.19 p=0.006) when compared to Regular Activity Breaks.
Interestingly, despite the proposed link between sedentary behaviour and lipoprotein lipase neither Regular Activity Breaks nor Physical Activity were effective at lowering postprandial lipidaemia. In spite of the higher estimated energy expenditure with Physical Activity, breaking prolonged sitting with regular, short brisk walks was more effective than a single bout of continuous physical activity at significantly decreasing postprandial glycaemia and insulinaemia. These findings provide evidence of the profound metabolic benefits of regularly breaking prolonged sitting with short bouts of physical activity. We propose that these findings inform modifications to New Zealand public health guidelines that include the recommendation that all New Zealanders should regularly break periods of prolonged sitting regardless of their other exercise habits, age, or risk of cardio-metabolic disease.