Letter in response to BJSM Open Access Review: Chastin SFM, Craemer M de, Cocker K de, et al. How does light-intensity physical activity associate with adult cardiometabolic health and mortality? Systematic review with meta-analysis of experimental and observational studies. Br J Sports Med 2018.
We read the recent systematic review with meta-analysis on the association of light intensity physical activity (LIPA) with adult cardiometabolic health and mortality by Chastin and colleagues with great interest . We appreciate the authors’ contribution to this emerging field however, we have some comments and concerns regarding their paper.
The major challenge in providing a systematic review in this field, as the authors acknowledge, is the identification of relevant studies. The information provided by the authors on the search strategy and inclusion criteria raises the question whether they indeed succeeded in “fully captur[ing] the literature comprehensively”.
First of all, the issue of how LIPA is defined is crucial for all subsequent steps of the analysis. We disagree with the authors’ statement that there is a “lack of a standardised definition and operationalisation of LIPA”. Even if not absolutely standardized, there is a commonly accepted definition of LIPA in terms of metabolic cost (≥1.5 <3 MET), and there are references for the operationalisations in terms of cardiorespiratory load as well as perceived exertion, such as the ACSM classification (57-63% HRmax, 37-45% VO2max, 30-39% HRR/VO2R, 9-11 RPE) . We wonder why the authors do not make any reference to this or any other classification.
Further, the authors provide very little information on the validity of self-report PA instruments and how self-reports “translate” into MET-intensities. Also, among intervention studies there is a large and an unexplained heterogeneity of how intensity is defined. Intensity is given e.g. as „50% oxygen consumption“ (maximal or peak oxygen uptake?), “lactate threshold”, “7 to 8 km walks per day”, “yoga class”. Clearly, the volume (7 to 8 km per day) or the type of activity (yoga) do not define intensity. Based on the ACSM classification, 50% VO2max falls into the moderate intensity range. If for any reason (which is not stated) the authors decide to include exercise studies at 50% VO2max, we wonder why some of the well-known dose-response studies, e.g. STRIDE study (exercise intensity of 40-55% VO2peak in one of the intervention groups) , or the DREW study (exercise intensity of 50% VO2peak in one of the intervention groups)  are not included.
Also, the cardiometabolic effects of yoga have been examined in a number of recent reviews , , , , so again we are not sure why these studies are not mentioned, if one yoga study is included.
In our recent systematic review we summarized the then available evidence on LIPA and health based on the NHANES dataset . We wonder why some of the studies we identified were not included in the current review , , .
Another important issue to consider is the choice of the study population. The authors state that their focus is on primary prevention, and exclude therefore “cardiac rehabilitation programmes and secondary cardiovascular disease prevention trials and longitudinal studies” and “studies which investigated only populations with diagnosed cardiovascular diseases or patients with cancer”. It seems incongruent that at the same time studies in participants who “have some form of metabolic impairment (eg, obesity or type 2 diabetes)”, conditions which evidently increase the risk of cardiovascular disease and mortality, were included. Whereas in the case of “acute mechanistic studies” separate analyses were conducted for healthy and non-healthy samples, no such distinction was made in case of the training studies and prospective studies. A simple and strict dichotomy (healthy vs. non-healthy) would have been more rigorous.
The main outcomes of the review are “cardiometabolic health markers” and mortality. It is surprising to find one single training study with the outcome fitness to be included, since effects of low-intensity training on cardiorespiratory fitness have been subject of quite a number of studies [13-16].
A discussion of study findings in the light of the systematic reviews on LIPA and cardiometabolic health cited  and not cited  would have been interesting. And finally, we miss a discussion on the adjustment or lack thereof for major confounding variables, first and foremost of moderate-vigorous PA activity.
Eszter Füzéki, Ph.D.
Tobias Engeroff Ph.D., M.D.
Winfried Banzer, Prof. Ph.D., M.D.
Department of Sports Medicine, Goethe University Frankfurt
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