Doctor: Manasa shanta Yerramalla

Title: Longitudinal associations of objectively-assessed sedentary behaviour with cardiovascular didease and all-cause mortality among older adults.

Supervisor: Séverine Sabia

Doctoral school: ED 393 Epidemiology and Biomedical Information Sciences, Université Paris Cité

Date of thesis defense: 09/2022

Thesis abstract

Background : Physical activity is an important modifiable protective risk factor for cardiovascular diseases (CVD), and all-cause mortality. Emerging evidence has shown that sedentary behaviour (SB) such as sitting might be deleterious to health, leading to a recent shift in focus pf physical activity guidelines from solely requiring adults of all ages to undertake at least 2 and a half hours of moderate-to-vigorous physical activity (MVPA) per week to also suggesting them to reduce time spent in SB. The MVPA recommendations are not met by the majority of the population, especially by older adults in part due to declining physiological ability to perform higher intensity activity. Also, older adults spend almost 80% of their time being sedentary. Light intensity physical activity (LIPA) such as strolling may be easier than MVPA and could potentially confer some benefits in those not fit enough to engage in physical activity at higher intensity. Additionally, it has been suggested that not just the total duration of SB, but also the manner of its accumulation throughout the day might be important for health outcomes, but there is insufficient evidence to attest this point.

Methods : The main objectives of the thesis were :1) to examine the how different compositions of objectivity-assessed MVPA, LIPA, and SB in a waking day are associated with incident CVD, using an innovative approach, compositional Cox regression ; 2) to determine the association of total sedentary time and pattern of its accumulation with incident CVD and all-cause mortality, using Cox proportional hazards regression. Data from the Whitehall II accelerometer sub-study from wave 2012-2013 was used.

Results : An increase in MVPA duration at the expense of time in either SB or LIPA was found associated with lower incidence of CVD. Among highly sedentary individuals, it was better to increase MVPA by reducing SB alone. There was an asymmetrical response to the increment or decrement of MVPA duration ; reducing time in MVPA was far more detrimental to CVD risk than the gains obtained from increasing MVPA, especially if it exceeded the recommended guideline.

Our study did not find any associations of total sedentary time and sedentary accumulation patterns with incident CVD and all-cause mortality in the total sample once MVPA was considered. There was evidence of higher mortality risk with increased total and less interruption of prolonged SB independently from MVPA in younger older adults.

Conclusion : Main conclusionary notes is that MVPA is cardioprotective against CVD risk among all older adults and that a more interrupted pattern of sedentary time independent of MVPA prevents premature death in younger older adults.

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