It is claimed that aerobic exercise is beneficial at causing weight loss. This claim is based largely on the ‘eat-too-much, do-too-little’ theory of obesity that explains weight gain in terms of an energy surplus caused by too much food and too little exercise. By performing aerobic exercise it is claimed that a negative energy balance can be created and that this in turn will cause weight loss. However, aerobic exercise is very inefficient at causing weight loss, and evidence from the scientific literature does not support the contention that performing aerobic exercise causes any more weight loss compared to sedentary behaviour. In fact, eating a high quality diet without any forced caloric restriction or aerobic exercise has been shown to cause superior weight loss compared to those who performed both energy restriction and exercise. In contrast to aerobic exercise, resistance training appears to produce improvements in body composition which may result from increases in muscle mass and concomitant reductions in percentage body fat.
Part of the problem with aerobic exercise is that is does not cause sustained increases in metabolic rate following exercise, and thus only causes energy utilisation while the exercise is being performed. The thermogenic effects of moderate aerobic exercise have been investigated in healthy subjects and generally the inability of this type of exercise to cause long term elevations in metabolic rate is consistently reported. For example, in one study1, fasted subjects performed moderate exercise on a cycle ergometer for twenty minutes. Following two successive 20 minute exercise periods interspersed with forty minutes rest, a similar protocol was performed but this time the subjects consumed a 800 kcal mixed meal prior to exercise. Oxygen consumption increased 13.6 % following both fasted and fed exercise sessions, but increased 22 % following the mixed meal on a non-exercise day. By sixty minutes post-exercise the thermic effect of the exercise had diminished to baseline levels.
These results support the contention that aerobic exercise does not cause prolonged elevations in metabolic rate, even following repeated session. Further, the rise in metabolic rate seen during aerobic exercise is no greater that seen with consumption of a mixed meal on a non-exercise day. This is in contrast to resistance training that is known to cause prolonged elevations in metabolic rate for upto 12 hours. Resistance training is able to cause these changes because the muscle tissue damage sustained during contraction requires extensive tissue remodeling in the subsequent days. This is a highly energy intensive process that explains the muscle building and fat burning effects of intense resistance training. Because this tissue damage does not occur during moderate aerobic exercise this tissue remodeling phase is absent and as such the thermogenesis caused by these processes is absent. The thermic effect of activity for aerobic exercise is not conducive to weight loss and may actually be detrimental based on its damaging effects to resting metabolic rate.
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