Limits in Energy Expenditure

Abstract

The research on the ceiling value of four times resting energy expenditure , as observed in birds, got support from observations on endurance performance in man. House Martins, Sand Martins and Swallows reach a performance ceiling in the most active phase of the annual cycle: the time when they are feeding chicks at the nest. Man nowadays reaches a ceiling during performances like endurance sport events. Here, limits in energy expenditure are illustrated with typical examples: endurance athletes while participating in the Tour de France; Olympic crosscountry skiers during a training stage; and sailors during a leg of the 'Whitbread race'. Sailing does not seem to be a high intensity exercise, however , there is a continuous need to counterbalance the movements of the boat leading to an unexpectedly high workload. Non-athletes can be trained to reach an energy ceiling. Here, the examples include: subjects preparing to run a half-marathon; overweight boys training on a cycle ergometer; and men performing weight training in a fitness centre. In all studies, the energy ceiling is assessed by measurement of total energy expenditure with doubly labelled water. In the non-athletes, the effect of training on the activity factor was assessed with measurements before and after the training. The energy ceiling for non-athletes occurs at an activity factor of 2.0 to 2.5. A further increase in activity is difficult to sustain for more than a week, but if one does so, one loses weight. This loss of weight will eventually result in a loss of performance. Exceptions are professional endurance athletes. They are a selection of the population, born to be athletes, training for many years to reach their high level of performance. The training includes exercise and the maintenance of energy balance at a high level of energy turnover. The latter implicates the supplementation of the diet with energy drinks. Highly trained athletes have learned to eat the maximum amount of food during hard physical work. Endurance athletes like professional cycle racers and Olympic crosscountry skiers can reach an activity factor around 4.0 and maintain this value for one or more weeks. Non-athletes reach an energy expenditure ceiling at an activity factor of 2.0-2.5. This value is already reached after a 5-10-week endurance training program. Maintaining this high level of energy expenditure requires higher and higher training intensities because of the increased movement efficiency. A higher body weight decreases the likelihood of compliance with such a training program, especially when the training involves body displacement as in running.