There is enough information about exercise physiology to support the well-documented public health guidelines promoting at least 150 minutes of moderate-to vigorous physical activity (MVPA)(1). However there is an emergence of inactivity physiology studies. If we divide a day into periods of MVPA, light intensity PA (LIPA), sedentary behavior and sleep we observe a large proportion of the time taken up by time spent sitting, such as TV viewing, car driving and computer use. In fact, evidence exists that excessive time spent in sedentary behavior (SB) is a health risk that is not mitigated by performing half an hour of MVPA per day (2, 3). Life expectancy at birth may increase by \~2y if SB is reduced to \<3h/day (4) whereas prospective associations exist between SB with mortality and morbidity (2, 3). Short term experimental studies indicate that physical inactivity affects energy balance and is considered conducive to weight gain (5, 6). A decrease in PA has a considerable ability to decrease total energy expenditure (TEE) without any compensatory changes in energy intake, which generate a positive energy balance. The extent to which reducing and breaking up SB over sustained periods of time considerably increases TEE has never been investigated under free-living conditions.

Our hypothesis are that, a daily 3h reduction of SB during 1-week, either by increasing the number of breaks and by shifting SB to low intensity physical activity (LIPA), mainly through standing and walking activities, would substantially increase physical activity energy expenditure (PAEE) in male and female overweight/obese inactive computer desk workers compared to 1-week of usual prolonged SB. Prior to intervention, number of steps/day and PA were assessed through the use of a pedometer and accelerometer to respectively identify the habitual daily steps and to assure participants are inactive (\<30 min/day of MVPA and \~ 5000 steps/day). Eligible participants were enrolled in a crossover experiment with two conditions performed in a random order: intervention (3h-reduction in SB) and control (habitual SB), both under free-living conditions. Each condition last for 1 week and participants were instructed to keep the same eating patterns while wearing an accelerometer, pedometer, a combined accelerometer and heart rate device, and an inclinometer (activpal). Doubly labeled water (DLW) was administered in both conditions to assess TEE, indirect calorimetry was employed to measure resting energy expenditure (REE), and PAEE subtracted from the sum of REE and thermogenic effect of food (assumed as 10% of TEE). Body composition was assessed at baseline and in the last day of the intervention week with DXA and participants wore the devices 24 hours a day during the two weeks and did the food records in three days in each week. In practice, at the workplace, our intervention to reduce SB include a software that hourly alert the participants to break up SB for approximately 7 minutes through adopting walking behaviors (\~30-60 minutes/day) while during transportation, home/domestic, and leisure time contexts, an individual goal for number of steps/day was set based on an expected step cadence for ambulatory activities (\~90-120 minutes/day). Also a number of strategies to break up SB were transmitted to the participants in the several contexts for accomplishing their goals.

At the workplace, daily breaks were automatically generated and registered through the software. Daily adherence in breaking up SB was supervised using phone calls during the day as well as compliance with the individual steps/day goal, self-registered in a diary at the end of the day. During the control week, supervision was performed to assure that participants remained inactive with a similar SB and number of steps/day, as observed at baseline.

During the trial, a 3-day food intake record was collected and analyzed at each condition. We anticipate that by using objective measures of transitions from sitting to standing and stepping, we will provide important methodological information, as sedentary time comprises a large proportion of waking hours and small changes may go undetected using self-report SB. A unique aspect of the present study is the utilization of state of-the-art technologies to investigate differences in daily EE and activity patterns in overweight/obese individuals.

The results of this project may have remarkable public health relevance. Most of the population weight gain in the past could have been avoided if a negative energy balance of 100 Kcal/day was achieved. We expect that our findings reveal a meaningful difference in energy expenditure by breaking up SB. We anticipated a public health message emphasizing "standing and walking more" as a simple approach to prevent weight gain and the rise of obesity in developed countries. This project may also contribute to disclose innovative energy balance -based methodologies for designing long-term intervention studies examining the effect of breaking up sedentary time on health-related parameters.