Effects of Microbouts of Activity on Metabolic Health

Over each 2 week period, participants will be instructed to wear an ActiPAL (PALTechnologies: Glasgow, Scotland) activity monitor on their right leg. Participants will be instructed to wear the monitors at all times except when sleeping or participating in water-based activities. The time-stamped "event" data file from the activPAL software will be used to determine time spent sitting/lying, standing and stepping per day.

Insulin sensitivity will be determined via an insulin modified frequently sampled intra-venous glucose tolerance test (IVGTT). After baseline samples, intravenous glucose (0.3 g/kg) will be infused over 1 minute, followed by insulin at 0.03 U/kg, 20 minutes after glucose administration. Blood samples will be frequently sampled over 5 hours, and insulin sensitivity will be calculated using the Bergman minimal model.

Incremental area under the curve of plasma glucose concentration measured in fasting and every hour for 14 hours.

Total energy expenditure (TEE) will be measured before and at the end of the intervention using doubly labeled water (DLW) over a 10-day period, as previously described (31). Subjects will ingest premixed 2g/kg total body water (TBW), estimated as 73% of fat free mass) dose of DLW composed of 0.2 and 0.15 g/kg estimated TBW of H218O and 2H2O, respectively. Urine samples will be collected in duplicate on days 0, 6, 8 and 10. 2H and 18O enrichment will be determined in urine samples by isotope ratio mass spectrometry (Delta V Advantage IRMS)

Over the same 10-day period, participants will be instructed to wear an Actigraph GT3X+ accelerometer (Actigraph, LLC, Fort Walton Beach, FL) on their hip. Participants will be instructed to wear the monitors at all times except when sleeping or participating in water-based activities.

Gas exchange are measured for 24hr by whole room calorimetry and nitrogen excretion is measured in the urines collected for 24hr.

24hr energy expenditure is then calculated using 24hr VCO2, 24hr VO2 and 24hr urinary nitrogen excretion with the Weir equation.

24hr carbohydrate, fat and protein oxidation are calculated using 24hr VCO2, 24hr VO2 and 24hr urinary nitrogen excretion with the Frayn equation.

After the investigators have shown the subjects the procedure, subjects will collect their own hourly breath sampling for CO2 by blowing through a straw into two 15ml Vacutainers. Breath CO2 will be sampled directly from the Vacutainer with a syringe, and 13CO2 /12CO2 measured with IRMS.

Hourly urine sampling will be collected by the subjects for 24 hours and stored in 5mL cryovials at -20C until further analysis. To measure 2 hour fat oxidation, 2 hour/1 hour ratios from urine samples will be analyzed, as above described for the doubly labeled water method. The oxidation rate of glucose/water will be calculated from the cumulative recovery of 2 hours in total body water (TBW) as detailed previously.

Incremental area under the curve of plasma insulin concentration measured in fasting and every hour for 14 hours.

Incremental area under the curve of plasma NEFA as measured in fasting state and every hour for 14 hours.

Incremental area under the curve of plasma TG as measured in fasting state and every hour for 14 hours.

Daily interstitial glucose mean and oscillations as measured by continuous glucose monitoring for 14 consecutive days.

Freshly isolated skeletal muscle biopsies will be cut into small samples, the fibre bundles separated mechanically and partially teased apart and permeabilised. Small samples (2-5mg) will be added to the 2ml chamber of an Oroboros high-resolution respirometer with a mitochondrial respiration buffer. With the addition of several substrates, different states will be reached to analyse mitochondrial function. Pyruvate, malate and glutamate will be added as substrates to examine respiration through Complex I, succinate will be used to examine respiration through Complex II and octanoylcarnitine will be used to examine respiration through the electron-transferring flavoprotein, ETF.

Fasting blood samples will be obtained for the measurements of Interleukin 1 alpha (IL1a), Interleukin 6 (IL6), Interleukin 10 (IL10), Tumour Necrosis Factor alpha (TNFa) and cardiometabolic (triglycerides, cholesterol, C-Reactive Protein (CRP).

Fasting plasma blood draw for the measurement of total cholesterol, HDL and LDL.

Self-perceived hunger, satiety and prospective food consumption as measured by visual analog scale before and after standard breakfast, lunch, and dinner.