Reply to Dutheil et al.

to the editor: On March 11, 2020, the outbreak of COVID-19 was characterized as a pandemic [World Health Organization (WHO)]. In the following weeks more than 3 billion people were subjected to home-based confinement.

It is with great interest we read the Letter “Will the COVID-19 pandemic decrease the FatMax?” by Dutheil and colleagues (1). Although we do not think that the COVID-19 pandemic will influence the reported lack of a relationship between menstrual cycle phase and maximal fat oxidation during exercise (MFO), we very much agree with the authors that in countries with population confinement policies necessary to control the contagion, a sedentary lifestyle has become a public health problem following the outbreak of COVID-19. There has so far been no published data on the actual reduction of physical activity during the COVID-19 pandemic, but preliminary Danish data indicate a 20–27% decrease in daily physical activity, with the elderly population (>60 yr) having the largest relative reduction (5). The Danish population has not been subjected to home-based confinement; hence the reduction in physical activity in countries with strict home-based confinement is most likely larger.

The adverse metabolic consequences of this pandemic of sedentarism is difficult to foresee; however, a large body of literature exists regarding the adverse metabolic effects of a sedentary lifestyle in both young and elderly people (4). After only 3 days of step count reduction from ~10,000 to 1,000 steps/day, a marked reduction in glycemic control and indices of insulin sensitivity were observed, and after 2 wk insulin sensitivity was decreased by 17–44% in healthy young people, and even more severe effects were seen in elderly prediabetic people (4). Seven days of complete bed rest in young healthy people resulted in severe metabolic consequences with major decreases in whole body glucose uptake, insulin action, cardiorespiratory fitness, and plasma free fatty acid concentrations (3), all of which evidently will result in a decrease in MFO.

Recently, we found that MFO was decreased in physically trained middle-aged compared with young equally trained subjects, and that in middle-aged subjects there were no difference between trained and sedentary middle-aged subjects, indicating that to increase the positive metabolic effects of regular exercise training in middle-aged subjects more strenuous exercise training is needed (2).

Based on the existing literature, we, in accordance with the authors (1), estimate that the reduction in daily physical activity will lead to a decreased MFO which may cause an increase in a number of other adverse metabolic effects during and after this period of COVID-19-induced confinement. Thus, to attenuate the risk of a metabolic pandemic and to counteract the induced negative health consequences, population-wide attention to the benefit of physical activity must be prioritized when the COVID-19 contagion is fully under control.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

J.F., N.P., J.P.Q., F.J.A.-G., S.L., F.D., and J.W.H. edited and revised manuscript; J.F., J.P.Q., F.J.A.-G., S.L., F.D., and J.W.H. approved final version of manuscript.