It is assumed by many that physical activity causes weight loss. We are told that the overweight are lazy because they do not perform enough exercise and recommendations encourage increased physical activity, either during leisure or daily tasks, in order to address the increased prevalence of obesity. However, that exercise causes effective long term weight loss is a fallacy that is not supported by scientific data. In fact, studies investigating the effects of aerobic style low intensity exercise on weight loss in the obese have generally reported short term weight loss, but the long term success of such regimens is questionable. Further, the weight loss seen in such studies can be accounted for almost entirely by lean mass, which causes detrimental changes to the resting metabolic rate, and as a result the risk of future weight gain is increased. Associations between physical activity and body weight do show a positive correlation. However, the possibility that this association has been misinterpreted is credible and evidenced.
A recent study investigating Chinese workers reported on the associations between physical activity and anthropometric parameters in just under half a million subjects1. The work performed by the subjects in the course of their day was converted to a metabolic equivalent figure for comparison purposes (Metabolic equivalent task hours per day; Met-h/d). The mean physical activity of the workers was 22 MET-h/d and the mean leisure time was 3 hours. Unsurprisingly, factory workers had levels of physical activity that were ~50 % higher than professionals and office workers, with farmers having intermediate levels of physical activity. The retired, homemakers and unemployed had descending levels physical activity, all below those of office and professional workers. Rural workers had ~20 % higher physical activity levels and ~5% less leisure time than urban equivalents. Education was correlated inversely with physical activity, with college educated individuals having 30 % less activity than those with no formal education.
In both men and women, BMI, waist circumference and percentage body fat were inversely associated with physical activity, and positively with sedentary leisure time. At first glance these results appear to support the contention that physical activity is a cause of improvements to body composition. However, the cause and effect of this relationship cannot be determined without further data, and the possibility that confounding variables may have influence the association should be considered. Interestingly, the BMI was 28 or greater in 6 % of those with high physical activity levels and in 12 % of those with low physical activity levels. In women the corresponding figures were 9 % and 15 %, respectively. This suggest that physical activity is not the driver of body composition, but that improvements of body composition are caused by some other factor or factors (because clearly some sedentary subjects are thin, and some physically active people are fat). A consideration of the factors that may really drive body weight increases is therefore justified.
While evidence suggests that low intensity aerobic exercise (that would be similar to the physical activity performed during free living non-exercising conditions) is not effective at causing improvements in body composition, dietary changes are. It is becoming increasingly evident that poor diet, and in particular high intakes of fructose and trans fats, are the cause of the metabolic changes that lead to weight gain. In terms of leisure time exercise, the association between body weight and physical activity can be explained in terms of the fact that once the obesity disease has developed in an individual, they have less available energy for physical activity and are therefore likely to avoid it. Body weight may be the cause of the reduced physical activity, rather than the physical activity being the cause of reduced body weight. Diet could therefore be the driver of the association that we see between physical activity and body composition. However, it is harder to explain the association between body composition and work performed during daily tasks.
Certainly factory workers are a self selecting subgroup of the population. Generally to be successful at acquiring a job in a factory where physical activity is to be performed, a certain level of physical fitness is required. With this likely comes improved body composition, however we cannot make assumptions to the cause of this improved body composition. In addition, those who are successful at acquiring a job in a factory, but who are not physically fit, would be more likely to lose their job due to not being able to maintain high force outputs. While office workers and professional may have higher body weight compared to factory workers, they may also be more highly paid. With affluence comes access to less nutritious more processed foods that characterise the Western diet and this could therefore explain the association. Farmers likely have higher intakes of whole foods and lower intakes of processed foods and sugar, and so the associations between body weight and physical activity may not be as first appears.
RdB
