Energy Expenditure and Substrate Oxidation Associated with Prolonged Sitting, Regular Activity Breaks and Physical Activity

Abstract

Increased sedentary time is associated with an increased risk of cardiovascular disease and metabolic disorders. Regular activity breaks from sedentary behaviour impart acute metabolic benefits in terms of postprandial glycaemia and insulinaemia, however to date the energy expenditure (EE) and substrate oxidation associated with this pattern of activity has not been investigated or described. The study aims to compare EE and substrate oxidation profiles between breaking up prolonged sitting with regular activity breaks, prolonged sitting followed by a similar amount of continuous physical activity, and a combination of regular activity breaks and continuous physical activity, along with examining any effect on ad libitum energy intake. Thirty-six sedentary individuals (male = 11, age 26±4 y, BMI 24.1±4.6, V̇O2max 36±8 mL.kg-1.min-1, mean±SD) performed four, two day trials, in a randomised-controlled crossover design: (1) prolonged sitting (SE), (2) sitting with regular activity breaks every 30 min for 2 min (RAB), (3) prolonged sitting with 30-min continuous exercise at the end of day 1 (PA), (4) RAB and PA combined (RAB + PA). All exercise was undertaken on a treadmill at a speed and incline to elicit 60% V̇O2max. Participants consumed weight based standardised meals during the interventions and an ad libitum meal was fed at the conclusion of day 2. Indirect calorimetry was used to estimate EE, carbohydrate and fat utilisation during the distinct sitting, exercise, and post-exercise periods. Mixed-model regressions using generalised least squares were conducted for all measures of energy expenditure, carbohydrate and fat oxidation. Total EE on day one (7 h) in SE (2401±92kJ, mean ± SE) was less than PA (3110±92kJ), RAB (3267±92kJ), and RAB + PA (4150±92kJ); all p< 0.001. There was no difference in total EE on day one between PA and RAB (p= 0.516). Post-exercise EE in PA (64±4kJ), on day one was less than RAB (697±31kJ), and RAB + PA (615±26kJ); both p< 0.001. Total EE rate on day two in RAB (9.01±0.23kJ·min-1) was greater than PA (5.91±0.23kJ·min-1) and SE (5.84±0.23kJ·min-1); both p< 0.001. Respiratory exchange ratio across all components of day one in RAB (0.90±0.01) was less than SE (0.93±0.01), PA (0.96±0.01), and RAB + PA (0.92±0.01); all p≤ 0.01. Similarly, total carbohydrate oxidation on day one in RAB (90±6g) was less than PA (144±6g) and RAB + PA (129±6g); both p < 0.001. Total fat oxidation in SE (7.4±1.2g) on day one was less than RAB + PA (12.9±1.5g); p = 0.027, similarly fat oxidation during exercise in PA (2.3±0.6) was less than RAB (4.8±0.3g); p< 0.001. There were no differences (all p≥ 0.07) in ad libitum energy intake between SE (2581±140kJ), PA (2452±136 kJ), RAB (2644±171kJ) and RAB + PA (2559±164kJ) respectively, however PA tended to be less than RAB (p= 0.07). The current study builds on previous biochemical findings, providing additional evidence concerning the underlying mechanisms contributing to the metabolic benefits of breaking sedentary time. These inform the development of more complete public health recommendations with respect to the impact of frequency and duration of activity punctuating prolonged sitting.