Cardiorespiratory fitness and risk of type 2 diabetes mellitus: A 23-year cohort study and a meta-analysis of prospective studies
Introduction
Type 2 diabetes mellitus (T2DM) is a complex metabolic disorder approximately doubling the risk of a wide range of vascular diseases [1], [2], [3]. Amongst the risk factors associated with T2DM, overweight/obesity, physical inactivity, and low cardiorespiratory fitness (CRF) have a major role [4], [5].
CRF reflects the ability of the lungs and cardiovascular system to transport oxygen and the ability of the tissues and organs to extract and use oxygen during sustained exercise [6]. Aerobic exercise training improves CRF in most adults [7], although the extent of adaptation may be partially influenced by genetics [8]. Published studies suggest an inverse association between CRF and incident T2DM [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27]. These studies have mainly included American and Asian people and have assessed the relationship between CRF and risk of T2DM adjusting for several, well-known cardiometabolic risk factors. Moreover, some of these studies have reported associations in the same population for different follow-up times. However, they use different definitions of CRF and the associations between CRF and T2DM are estimated in inconsistent ways, thus complicating attempts to quantify the magnitude of any relationship.
In a previous meta-analysis Kodama and colleagues estimated the extent to which CRF was inversely associated with the risk of cardiovascular disease and all-cause mortality [28]. Although a similar relationship between CRF and risk of T2DM is expected combining available evidence, the degree of such relationship is not well defined, thus limiting the possibility to comparatively assess the relevance of CRF on the risk of cardiovascular disease and T2DM. The aims of this study were twofold. First, we examined the relationship between CRF and incident T2DM in a population-based sample of men from Eastern Finland. Second, we conducted a systematic review and meta-analysis to estimate the magnitude of the association between CRF and T2DM and quantify the potential impact of CRF improvement on T2DM prevention.
Section snippets
The Kuopio Ischaemic Heart Disease study
The Kuopio Ischaemic Heart Disease (KIHD) risk factor study was designed to investigate risk factors for atherosclerotic cardiovascular outcomes in a population-based sample of men from Eastern Finland. The subjects were a randomly selected sample of 3433 men 42–60 years of age resident in the town of Kuopio or its surrounding rural communities. The study is described in detail elsewhere [29]. Briefly, baseline examinations were conducted between March 1984 and December 1989. Of those invited,
Cardiorespiratory fitness and incident type 2 diabetes mellitus
Baseline characteristics of the study population and correlation coefficients are reported in Table 1 and Supplementary Table S1, respectively. Baseline mean age and CRF were 53 years (SD, 5) and 8.7 METs (SD, 2.1), respectively; CRF levels were moderately correlated with other risk factors.
During a median follow-up time to incident T2DM or the end of follow-up of 23 years (interquartile range: 18–25 years), a total of 153 (6.1%) new cases of T2DM were recorded. In an analysis adjusted for age,
Discussion
In the present study, we did not find a statistically significant association between CRF and incident T2DM in a randomly selected population cohort of middle-age Finnish men. Notwithstanding, in a pooled analysis of published reports from eight prospective cohort studies, each 1-MET increment in CRF was associated with a 5% (95% CI: 2, 7) relative reduction in T2DM risk. One MET equates to a relatively small difference in absolute fitness levels. For example, it has consistently been shown to
Author contribution
FZ study idea and design, data analysis and collection, manuscript draft.
GO, DRW, TY, KK, MJD study critical revision, manuscript draft.
SK data collection, manuscript draft.
JAL study design, critical revision, manuscript draft.
All authors provided final approval of the version to publish. FZ is the study guarantor.
Ethics
The study was approved by the Research Ethics Committee of the University of Eastern Finland.
Conflict of interest
KK has acted as a consultant and speaker for Novartis, Novo Nordisk, Sanofi-Aventis, Lilly and Merck Sharp & Dohme. He has received grants in support of investigator and investigator initiated trials from Novartis, Novo Nordisk, Sanofi-Aventis, Lilly, Pfizer, Boehringer Ingelheim and Merck Sharp & Dohme. KK has received funds for research, honoraria for speaking at meetings and has served on advisory boards for Lilly, Sanofi-Aventis, Merck Sharp & Dohme and Novo Nordisk.
MJD has acted as
Fundings
FZ is a Clinical Research Fellow funded by Sanofi-Aventis. The research was supported by the National Institute for Health Research (NIHR) Diet, Lifestyle & Physical Activity Biomedical Research Unit based at University Hospitals of Leicester and Loughborough University; the NIHR CLARHC East Midlands; and the Finnish Foundation for Cardiovascular Research, Helsinki, Finland. The views expressed are those of the authors and not necessarily those of the NHS, the NIHR or the Department of Health.
Acknowledgements
We thank the staff of the Kuopio Research Institute of Exercise Medicine and the Research Institute of Public Health, University of Eastern Finland, for data collection. We acknowledge the support of the following institutes in this work: the National Institute for Health Research, Collaboration for Leadership in Applied Health Research and Care – East Midlands (NIHR CLARHC East Midlands); the National Institute for Health Research (NIHR) Diet, Lifestyle & Physical Activity Biomedical Research
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