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The endurance athlete's circulation: Ultra-risky or a long road to safety?

      The positive effects of regular participation in physical activity are well documented. Meeting the minimum physical activity guidelines reduces the risk of many diseases, and physically active individuals generally live longer, healthier lives than inactive people [
      • Runacres A.
      • Mackintosh K.A.
      • McNarry M.A.
      Health Consequences of an Elite Sporting Career: Long-Term Detriment or Long-Term Gain? A Meta-Analysis of 165,000 Former Athletes.
      ]. Accordingly, current guidelines [
      • Bull F.C.
      • Al-Ansari S.S.
      • Biddle S.
      • et al.
      World Health Organization 2020 guidelines on physical activity and sedentary behaviour.
      ,
      • American College of Sports Medicine
      • Riebe D.
      • Ehrman J.K.
      • et al.
      ,
      • Piercy K.L.
      • Troiano R.P.
      • Ballard R.M.
      • et al.
      The physical activity guidelines for Americans.
      ] recommend 2.5–5h/week of moderate-intensity exercise or 1.25–2.5h/week of vigorous-intensity exercise (a metabolic equivalent of 8–17 MET-h/week). However, the health status of individuals who markedly exceed the minimum recommendations remains a matter of debate. One major concern pertains to the possible mal-adaptation of the cardiovascular system that would predispose athletes to a greater risk of all-cause and cardiovascular morbidity and mortality, raising the question whether excessive volumes of exercise could be “too much of a good thing” [
      • La Gerche A.
      • Heidbuchel H.
      Can intensive exercise harm the heart? You can get too much of a good thing.
      ]. In agreement with this concern is the observation of a reverse J-curve association between exercise and cardiovascular health (Fig. 1) [
      • O'Keefe E.L.
      • Torres-Acosta N.
      • O'Keefe J.H.
      • et al.
      Training for longevity: the reverse J-curve for exercise.
      ]. Specifically, the reverse J-curve represents the fact that mild-to-moderate volume and intensity exercise (in accordance with physical activity guidelines) seems to improve cardiovascular health compared with inactivity, while cardiovascular risk in individuals who perform a large volume of exercise may increase again. Such a concern is far from trivial because the participation in ultra-endurance events has steadily increased over the last decades. In 2017, more than 300,000 men and women participated in ultra-marathons alone, and several thousand more took part in other long-distance competitions [
      • Scheer V.
      Participation trends of ultra endurance events.
      ]. Reports of increased coronary artery calcium scores and “hearts of stone” [
      • Baggish A.L.
      • Levine B.D.
      Coronary artery calcification among endurance athletes: "hearts of stone.
      ] in some endurance athletes have alarmed medical professionals and indicated the need for further data on the health risks associated with excessive amounts of endurance exercise. On the other hand, readers of this journal will be interested to note that several studies have reported an increased coronary artery calcium score (CACS) in middle-aged athletes, but with more stable atherosclerotic plaques and a similar or reduced morbidity and mortality compared with less active, age-matched individuals [
      • DeFina L.F.
      • Radford N.B.
      • Barlow C.E.
      • et al.
      Association of all-cause and cardiovascular mortality with high levels of physical activity and concurrent coronary artery calcification.
      ,
      • Aengevaeren V.L.
      • Mosterd A.
      • Braber T.L.
      • et al.
      Relationship between lifelong exercise volume and coronary Atherosclerosis in athletes.
      ,
      • Merghani A.
      • Maestrini V.
      • Rosmini S.
      • et al.
      Prevalence of subclinical coronary artery disease in masters endurance athletes with a low atherosclerotic risk profile.
      ,
      • Aengevaeren V.L.
      • Mosterd A.
      • Sharma S.
      • et al.
      Exercise and coronary Atherosclerosis: observations, explanations, relevance, and clinical management.
      ]. In accordance, a recent meta-analysis surmises that the previously proposed reverse J-curve association between volume of exercise and cardiovascular health can now be “refuted” [
      • Runacres A.
      • Mackintosh K.A.
      • McNarry M.A.
      Health Consequences of an Elite Sporting Career: Long-Term Detriment or Long-Term Gain? A Meta-Analysis of 165,000 Former Athletes.
      ]. Yet, the story seems far from over because some humans are pushing the boundaries of exercise further, and the effects of extreme volumes of exercise on cardiovascular adaptation and health remain poorly understood.
      Fig. 1
      Fig. 1Possible associations between exercise volume and cardiovascular (CV) risk.
      Several associations have been proposed in the literature, with the only consistent finding related to the clear benefits of exercise at mild-moderate volumes (8-17 MET-hours/week). It remains unclear whether high or even very high volumes of exercise progressively result in a further benefit (long-term), a similar benefit (L-curve), or indeed an increased cardiovascular risk (reversed J-curve and U-turn). Graph based upon data shown by O'Keefe et al. [
      • O'Keefe E.L.
      • Torres-Acosta N.
      • O'Keefe J.H.
      • et al.
      Training for longevity: the reverse J-curve for exercise.
      ].

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