One of the main topics of study in physiology focuses on investigating whether there is a limit to exercising. How much energy can we expend above our basal metabolism in a sustained manner? This is a critical question because humans cannot develop high workloads indefinitely, as it would involve excessive caloric expenditure that could produce a catabolic deterioration of the body's systems. This occurs because caloric intake seems limited to 2,5 times the basal metabolism, so that a negative energy balance could not be compensated for a long time. Therefore, as the duration of sports tests is longer, the total energy expenditure tends to decrease to preserve the body's homeostasis (1).
One of the most illustrative physiological models is that of pregnancy, since, during 9 months, the mother must increase her metabolic rate to give life to her child. This places the total energy expenditure/basal metabolism ratio at 2, which, added to caloric intake, causes the mother to gain around 0,04 kg per day (2). That is, during the months of pregnancy, the mother has on average twice the energy expenditure compared to what her basal metabolism accounts for. If we consider sports disciplines, Tour de France cyclists over 22 days maintain a metabolic rate 5 times their basal metabolism, one of the highest recorded over such a time period (3). If we reduce the time interval to 1 day, the test that recorded the highest sustained energy expenditure was measured in Kona, in the famous IronMan race, where a rate of 9,4 over the basal metabolism was reached.
On the contrary, if we increase the duration, we can find one of the toughest tests of human endurance, where runners cross the United States for 140 days running 6 marathons per week. A study has recently been published showing that the metabolic rate in these runners decreases in an L-shaped pattern during the 20 months, going from 3,79 the first week to 3,21 the last. That is, in the first week the runners spent an average of 6200 to 5300 kcal over the 20 weeks that the test lasted (1). This confirms that the human being cannot sustain a high energy expenditure indefinitely, as mechanisms are triggered that aim to preserve the homeostasis of the tissues, preventing their catabolism.
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References
- Thurber C, Dugas LR, Ocobock C, Carlson B, Speakman JR, Pontzer H. Extreme events reveal an alimentary limit on sustained maximal human energy expenditure. Sci Adv. 2019 Jun 1;5(6)
- Butte NF, King JC. Energy requirements during pregnancy and lactation. Public Health Nutr. 2007/01/02. 2005;8(7a):1010–27.
- Westerterp KR, Saris WH, van Es M, ten Hoor F. Use of the doubly labeled water technique in humans during heavy sustained exercise. J Appl Physiol. 1986 Dec 1;61(6):2162–7.
AUTHOR:
Adrián Castillo García
Graduate in Physical Activity and Sport Sciences, official Master in Integrative Physiology, University Specialist in Personal Training, NSCA – CPT.
Web: www.fissac.com
