Ask anyone who exercises regularly and the chances are they will claim that physical activity plays a major role in keeping them lean. The same people when asked about the obese will often suggest they are lazy because they do not exercise enough. Accusing the obese of laziness and greed is common is Western society because most people do not understand the reason for weight gain and therefore do not understand the reasons for weight loss. This makes them extrapolate their experiences unjustifiably onto others. This unempathetic attitude has resulted in a belief that if we can only get the obese exercising, they will lose weight. This is the currently accepted mainstream medical position, and most people accept without question that exercise causes weight loss. In fact, most are so convinced of this ‘fact’ that to question it results in derision. Indeed, the nutritional literature supports the observation that the overweight do not exercise as much as the lean. In fact studies consistently show an association between exercise and body weight.
For example, a study investigating the association between participation in youth sports and the subsequent development of metabolic syndrome showed that three years of intense physical activity during childhood was associated with a reduced risk of developing metabolic syndrome when older1. Another study investigating the association between leisure time and the degree of intramuscular fat in twins showed that inactive twins had 50 % increased visceral fat, a 170 % increased liver fat and a 54 % increased intramuscular fat, when compared to active twins2. The fitness levels of the twins, measure by peak oxygen uptake, was inversely associated with visceral and intramuscular fat. The authors from this study concluded that ‘regular physical activity seems to be an important factor in preventing the accumulation of high risk fat over-time’. Many such other studies have been performed and the vast majority show that physical activity is associated with improvements in body composition.
However, associations are correlations between variables, they do not allow ascription of cause and effects. While the overwhelming weight of evidence shows that physical activity is associated with improved body composition, should we assume that the cause of body fat losses is the physical activity? It could for example be that the reduction in body weight and the participation in physical activity are caused by a third unidentified variable. Alternatively, it could be that those with lower body weights are more likely to perform physical activity. To clarify that exercise is the cause of body weight loss, we need to assess the clinical trials data. In this sort of study, a group of subjects are randomly assigned to either an exercise or non-exercise group, and their body weight measured over time. All other variables are controlled and because the only difference between the groups is the activity levels, it allows us to make more definitive conclusions about the effects of exercise. So when we look at such studies, do they show that exercise causes weight loss?
Unfortunately for those who advocate exercise to cause weight loss, results from clinical trials investigating the benefits of exercise on weight loss are far more inconsistent than results cohort studies. Many such studies are actually poorly designed because many use treatments that include both forced energy restrictive diets and changes to activity levels concomitantly. This makes it impossible to ascribe the cause of any weight loss to exercise alone. Other studies are of such short duration that the results are not relevant to free living individuals in real world situations. The measurement of body weight rather than fat mass in some studies also leads to a loss of clarity in drawing conclusions. However, notwithstanding this, some well designed studies have reported on the weight loss effects of dietary changes in comparison to exercise. Results from such studies generally contradict the exercise induced weight loss hypothesis because the subjects that make changes to their diet tend to lose more weight that those who exercise.
For example, one study investigated the effects of dietary changes on parameters of metabolic syndrome in the overweight3. In this study, the authors randomly assigned the subjects to undergo improvements to their diets in addition to a modest energy restriction, to undertake an exercise programme, or both. After 12 months, the diet only group had lost 12 kg in weight, whereas the exercise group had only lost 4 kg. Even more interesting, the diet and exercise group lost 10 kg after 6 months but actually gained weight in the following 6 months. Therefore not only were dietary changes superior when compared to the exercise group, the addition of exercise to dietary changes actually attenuated the beneficial effects of the dietary improvements. This supports other studies showing that exercise can be detrimental to resting metabolic rate and can actually hamper long term weight loss if not properly implemented. Other studies (here and here and here) support the viewpoint that dietary improvements are superior to exercise at causing weight loss.
Results from studies such as these suggest that exercise is not effective at causing weight loss in those with obesity. So why then is there an association between physical activity and body weight? This may relate to the fact that athletes and those who are interested in physical activity happen to eat higher quality diets. Diets containing healthier foods might therefore be the confounding variable that causes the weight loss and distorts the beneficial effects of exercise. Alternatively, those with lower body weights are more likely to be free from metabolic dysfunction and energy metabolism irregularities that are characteristic of metabolic syndrome. This means they likely have the ability to generate energy more efficiently and may therefore be more likely to exercise. Lastly, it might relate to the fact that for exercise to be effective it must be of a specific intensity, a level that is too high for most to attain. Lower intensity, longer duration exercise with lower force outputs might simply not trigger adequate adaptive responses to cause weight loss.
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