Acute metabolic responses to breaking up sedentary time in older adults

Most older adults spend most of their waking hours sedentary, a behavior associated with adverse metabolic outcomes, including reduced metabolic flexibility. While interrupting sedentary time has shown benefits, its impact on metabolic flexibility remains unexplored. This study investigates whether different doses of timed interruptions of sedentary behaviors with light activity elicit acute adaptations in metabolic flexibility in older adults. Twelve sedentary older adults, free of any chronic conditions, (9W/3 M; 72.3 ± 4.8 years; BMI: 26.8 ± 3.6 kg/m²) completed three randomized 8-hour sedentary conditions: noBreaks, Break30 (3-minute breaks every 30 min), and Break60 (6-minute breaks every 60 min). Breaks involved treadmill walking at light intensity (i.e. 30% of VO_2max reserve). Metabolic flexibility was determined by combining hourly measurements of respiratory exchange ratio (RER) and plasma insulin levels (T0-T8). Meals (consumed after T0 and T4) were standardized for the day before and during the experiment day. Mixed linear models included fixed effects (condition, time and condition x time) and random effects (time, condition, participant). Break30 exhibited a greater variance in RER throughout the day compared to noBreak whereas no difference was observed for insulin. Significant condition-by-time interactions were observed for RER at T4 (noBreak=0.83±0.04, Break30=0.86±0.04, Break60=0.83±0.04, p = 0.035) and T7 (noBreak=0.87±0.04, Break30=0.90±0.05, Break60=0.89±0.05; p = 0.002). A significant interaction was also observed for non‑esterified fatty acid concentrations at T5 (noBreak=0.251 ± 0.042 mmol/L, Break30=0.393 ± 0.042 mmol/L, Break60=0.373 ± 0.042 mmol/L; p = 0.041). Compared with prolonged sedentary behavior, interrupting sitting every 30 or 60 min appears to induce acute alteration in substrate oxidation. Further studies are needed to confirm these results and to evaluate potential long‑term adaptations.