Testing the activitystat hypothesis: a randomised controlled trial protocol

Background: The activitystat hypothesis proposes that when physical activity or energy expenditure is increased or decreased in one domain, there will be a compensatory change in another domain to maintain an overall, stable level of physical activity or energy expenditure. To date, there has been no experimental study primarily designed to test the activitystat hypothesis in adults. The aim of this trial is to determine the effect of two different imposed exercise loads on total daily energy expenditure and physical activity levels. Methods. This study will be a randomised, multi-arm, parallel controlled trial. Insufficiently active adults (as determined by the Active Australia survey) aged 18-60 years old will be recruited for this study (n=146). Participants must also satisfy the Sports Medicine Australia Pre-Exercise Screening System and must weigh less than 150 kg. Participants will be randomly assigned to one of three groups using a computer-generated allocation sequence. Participants in the Moderate exercise group will receive an additional 150 minutes of moderate to vigorous physical activity per week for six weeks, and those in the Extensive exercise group will receive an additional 300 minutes of moderate to vigorous physical activity per week for six weeks. Exercise targets will be accumulated through both group and individual exercise sessions monitored by heart rate telemetry. Control participants will not be given any instructions regarding lifestyle. The primary outcome measures are activity energy expenditure (doubly labeled water) and physical activity (accelerometry). Secondary measures will include resting metabolic rate via indirect calorimetry, use of time, maximal oxygen consumption and several anthropometric and physiological measures. Outcome measures will be conducted at baseline (zero weeks), mid- and end-intervention (three and six weeks) with three (12 weeks) and six month (24 week) follow-up. All assessors will be blinded to group allocation. Discussion. This protocol has been specifically designed to test the activitystat hypothesis while taking into account the key conceptual and methodological considerations of testing a biologically regulated homeostatic feedback loop. Results of this study will be an important addition to the growing literature and debate concerning the possible existence of an activitystat. Trial registration. Australian New Zealand Clinical Trials Registry ACTRN12610000248066

Device‐measured light‐intensity physical activity and mortality: A meta‐analysis

INTRODUCTION: The association of light‐intensity physical activity (LPA) with mortality is poorly understood. This meta‐analysis of cohort studies aimed to examine the dose‐response relationships between daily device‐measured LPA and mortality in adults aged 18 or older and to explore whether the associations were independent of moderate‐to‐vigorous physical activity (MVPA). METHODS: Searches for prospective cohort studies providing effect estimates of daily LPA (exposure) on all‐cause mortality (outcome) were systematically undertaken in electronic databases up to April 30, 2019. Subgroup analyses and meta‐regression analyses with random‐effects models were performed to quantify the dose‐response relationships between daily LPA and mortality. Sensitivity analyses were also conducted to assess the stability of the results. RESULTS: A total of 11 studies were included in the meta‐analysis. Analyses contained 49 239 individuals (mean age 60.7, SD = 13.6) who were followed up for a mean 6.2 years (2.3‐14.2 years), during which 3669 (7.5%) died. In comparison with the reference group (<3 h/d), the pooled HRs (and 95% CIs) of mortality were 0.71 (0.62‐0.82), 0.68 (0.59‐0.79), 0.56 (0.44‐0.71) for groups 3 to <5 h/d, 5 to <7 h/d, and more than 7 hours a day LPA, respectively. Meta‐regression models indicated that there was a log‐cubic dose‐response relationship between daily LPA and mortality in adults and older people, independent of MVPA. CONCLUSIONS: Time spent in daily LPA was associated with reduced risks of mortality in adults and older people. These data support the inclusion of LPA in the future physical activity guidelines