Regular physical activity only associated with low sedentary time increases survival in post myocardial infarction patient

Regular physical activity reduces the risk of all-cause and cardiovascular mortality in healthy individuals in a dose–response fashion, and in cardiac patients,1 and plays an important role in successful recovery following myocardial infarction (MI).2 Moreover, sedentary lifestyle is one of the major risk factors for cardiovascular disease, independent of participation in physical activity. Sedentary behaviour refers to any waking behaviour characterised by an energy expenditure of 1.5 metabolic equivalents (METs) or less while in a sitting, reclining or lying posture. Strong evidence demonstrates a significant relationship between a greater time spent in sedentary behaviour and higher all-cause mortality, and that this relationship varies by the amount of moderate to vigorous physical activity.3 The detrimental effect of sedentary behaviour on all-cause mortality is higher among people who perform low amounts of moderate to vigorous physical activity.4 For example, Ekelund and co-authors, in a harmonised meta-analysis using individual-level data from more than one million adults, reported that increasingly higher amounts of moderate to vigorous physical activity attenuated the relationship between sedentary behaviour and all-cause mortality, and the relationship between self-reported sitting and mortality was not significant among those who reported participating in at least moderate intensity physical activity for 60–75 minutes per day.5 Few studies addressed the effect of sedentary behaviour and physical activity after MI. Ejlersen and co-authors showed that physical activity prior to MI reduced case fatality, but survivors were not protected against subsequent heart failure or mortality.6 Maessen et al. found benefit on left ventricular ejection fraction and circumferential strain after MI in athletes with more than 20 years training and a weekly volume of 30 MET hours/week compared with sedentary controls.7 In this issue of the European Journal of Preventive Cardiology Wu and co-workers present a well-conducted prospective community-based cohort study including 101,510 Chinese adults in service or retired employees of the Kiluan Coal Group, living in Tangsha.8 The authors investigated whether sedentary time measured before disease onset was associated with all-cause mortality among MI survivors. Furthermore, they used mediation analysis to assess how the relation between sedentary time spent before MI and all causes of mortality after MI was mediated by physical activity and metabolic phenotypes such as diabetes mellitus, dyslipidaemia and abdominal obesity. During a follow-up period of 7.46 years they identified 989 MI survivor cases, which were followed until death or December 2014, and among them 180 deaths occurred. At baseline, in 2006 all participants completed health examinations including a standardised questionnaire, a physical examination and a laboratory test. Health examinations were administered biennially. Sedentary behaviour over 4 hours per day was associated with a higher risk of mortality after MI. The combination of sedentary behaviour and inactive individuals (practising 20 minutes of physical activity less than once per week) had an increased risk of all-cause mortality compared with persons with sedentary time of less than 4 hours a day and practising moderate or vigorous physical activity. Surprisingly, sedentary individuals with more than 4 hours per day sitting engaging in moderate to vigorous activity remained at higher mortality risk than the reference group. This is a large prospective study, in which sedentary time assessed at baseline (2006) predicted all-cause mortality among individuals who later developed MI. By reporting sedentary time assessed at baseline, the authors did not take into account changes that might occur during the time lapse before the onset of MI, such as coalminers or blue and white collar workers by retiring might have modified their sedentary habits, and after MI due to the MI extension or the severity of the disease, the individual might have increased their sedentary behaviour. Changes in physical activity before and after MI occurrence, as well as other lifestyle modifications that might also play a role in the prognosis, such as smoking cessation, cardiovascular rehabilitation, nutrition modification or medication, were lacking. A study limitation is that sedentary and physical activity habits were not measured objectively but by questionnaires not externally validated. The physical activity questionnaire used included different types of activity at high and low intensity, such as aerobic activity (running and walking), but also strength training such as weight lifting or callisthenic exercise and balance exercise such as tai chi/qi-gong. The intensity was not objectively defined as energy cost in METs.9 Individuals were defined as inactive, moderate and vigorous, by the frequency of performing 20 or more minutes of activity during the week, rather than by the total energy expenditure expressed in MET hours/week as the summed product in intensity, frequency and duration.10 The misclassification in assessment of sedentary time and physical activity could attenuate the strength of the observed risk estimates. The population studied included only a small proportion of women (81,110 men and 20,400 women), therefore the sex-related effect of sedentary behaviour and physical activity on mortality after MI was not taken into account. Minges and co-authors found that women recovering from acute MI were less likely to meet the physical activity recommendations than men.11 Postmenopausal women of the Women’s Health Initiative – Observational Study, who increased or maintained high physical activity levels after a first MI, showed a lower risk of all-cause mortality compared with women who maintained low physical activity. Increased sitting time post-MI was associated with an increased risk of all-cause mortality among individuals with a low sitting time prior to MI (<8 hours/day).12 Due to this limitation the results of the Kiluan Coal Group population study, suggesting that high sedentary time is associated with all-cause mortality among MI survivors and that physical activity may not counterbalance the increased risk of all-cause mortality associated with prolonged sitting, need further confirmation. Survivors after MI might be motivated to change their behaviour participating in secondary prevention programmes to prevent future mortality. The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. The author(s) received no financial support for the research, authorship, and/or publication of this article.