Fitness moderates glycemic responses to sitting and light activity breaks

Purpose: Regular engagement in sedentary behaviours can lead to major public health consequences. This study aimed to experimentally determine whether cardio-respiratory fitness modifies postprandial glycemia during prolonged sitting and investigated the potentially blunting influence this may have upon the benefits of interrupting postprandial sitting time with light activity breaks. Methods: Thirty–four adult volunteers (18female; 16male; mean±SD age: 40±9 years, BMI: 24.5±3 kg/m2) undertook two 7·5 hour experimental conditions in a randomized order: 1) Prolonged sitting; 2) Sitting interspersed with 5 minute light walking bouts every 30minutes. Blood samples were obtained while fasting and throughout the postprandial period following ingestion of two identical meals. Incremental Area Under the Curve (iAUC) was calculated for glucose and insulin throughout each experimental condition. Maximal exercise testing quantified VO2 peak as a measure of cardiorespiratory fitness (CRF) prior to experimental conditions. A repeated measures ANOVA investigated whether VO2 peak modified iAUC data between conditions. This trial is registered with ClinicalTrials.gov (Reg no.NCT0493309). Results: Interrupting prolonged sitting time with light walking breaks reduced blood glucose iAUC from 3.89 ± 0.7 to 2·51 ± 0.7 mmol·L-1·h (p = 0.015) and insulin iAUC from 241 ± 46 to 156 ± 24 mU·L-1·h (p = 0.013) after adjustment for VO2 peak and sex. A significant interaction between treatment response and VO2 peak was observed for glucose (p = 0.035), but not insulin (p = 0.062), whereby the treatment effect reduced with higher levels of fitness. Average blood glucose iAUC responses for a man at the 25th centile of CRF (42.5 mL∙kg-1∙min-1) within our cohort went from 5.80 to 2.98 mmol·L-1·h during the prolonged sitting and light walking breaks conditions respectively, whereas average responses for a man at the 75th centile of CRF (60.5 mL∙kg-1∙min-1) went from 1.99 to 1.78 mmol·L-1·h. Similar trends were observed for women. Conclusions: Individuals with low levels of CRF gained the most metabolic benefit from breaking prolonged sitting with regular bouts of light walking. Future interventions aimed at alleviating the deleterious impacts of sedentary behavior may be optimized by tailoring to cardio-respiratory fitness levels within the general population.