Exercise and obesity - are they related?
It seems obvious that since obesity reflects an imbalance between energy in and energy out, that both ‘input’ - diet, and ‘output’ - physical activity, have an impact on the development and reduction of obesity. We will look at the development issue in this section, and at the role for physical activity in the management of obesity in the next section.
Evidence in favour of the relation between physical activity and obesity.
Physical inactivity and obesity: A vicious circle summarises: "Physical inactivity in adolescence strongly and independently predicts total and especially abdominal obesity in young adulthood, favoring the development of a self-perpetuating vicious circle of obesity and physical inactivity. Physical (in)activity should be a major target of obesity prevention in the young."
Obesity, Abdominal Obesity, Physical Activity, and Caloric Intake in US Adults: 1988 to 2010 concludes: "Average BMI and waist circumference both increased among US adults from 1988 to 2010. Abdominal obesity, an independent predictor of mortality, has become more prevalent among normal-weight women and overweight women and men. At the population level, we found associations between the reported level of leisure-time physical activity and obesity, but even among adults reporting an ideal level of leisure-time physical activity, obesity prevalence has increased."
Relationship of obesity to physical activity, domestic activities, and sedentary behaviours: cross-sectional findings from a national cohort of over 70,000 Thai adults finds: "Increasing obesity with increasing screen-time was seen in all population groups examined; there was an overall 18% (15-21%) increase in obesity with every two hours of additional daily screen-time. There were 33% (26-39%) and 33% (21-43%) reductions in the adjusted risk of being obese in men and women, respectively, reporting housework/gardening daily versus seldom or never. Exercise-related PA, screen-time and housework/gardening each had independent associations with obesity."
Evidence against the relation between physical activity and obesity.
The role of physical activity and exercise in obesity and weight management: Time for critical appraisal suggests: "The increase in obesity epidemic is occurring against the background of continuous decline in the energy expenditure required for daily living. However, the idea that obesity is increasing because of consistent decline in daily energy expenditure is not supported either by objective measures of energy expenditure or physiological theory of weight gain. Clearly, obesity results from excessive energy intake that has sustained over a long period of time."
A similar message here: Physical activity does not influence obesity risk: time to clarify the public health message
- Long-term secular trends in activity are not consistent with the dynamics of the obesity epidemic
- Energy expenditure does not vary between societies with low vs high obesity prevalence
- Energy intake increases to balance expenditure
- Conflicting evidence from clinical trials on effect of increased energy expenditure on weight
- Observational studies show no association between energy expenditure and subsequent weight change
- Extremely small proportions of our society engage in levels of energy expenditure at a sufficiently high level to impact on long-term energy balance
- A sophisticated, coherent body of data now supports the transition to a new food environment as the fundamental cause: the ‘time cost’ of food has been lowered, and the inducements to eat high calorie items have increased, leading the susceptible half of the world’s populations to gain 0.5–1 kg per year from age 18 years onward.
And here: Why is the developed world obese? "The available data on energy expenditure, albeit limited, suggest that physical activity has declined but that the magnitude of the change is probably too small to explain most of the rise in adult obesity. With the exception of Australia and Finland, our analyses suggest that increased caloric intake is the driving force behind the growing obesity epidemic. However, we do not diminish the importance of energy expenditure to weight management and overall health. ..our data consistently demonstrate that caloric supply, driven by changing technological and sociodemographic factors, is highly associated with the increase in obesity among the OECD countries. Our findings also highlight potential unintended consequences of positive societal trends such as increased food availability and increased participation of women in the workforce."
Data equivocal - more research needed
A review of longitudinal studies Long-term health benefits of physical activity – a systematic review of longitudinal studies found few studies - “Two of the three longitudinal studies with at least 5-year follow-up focusing on the development of obesity over time showed a negative relationship between physical activity and obesity. Surprisingly, results of one study indicated that high leisure-time physical activity increased the risk of becoming obese in the following ten years for men. The reason for this remains unexplained. Overall, the results of the studies included in this review are inconclusive regarding the required minimum level of physical activity for preventing obesity. There is no evidence for the type, intensity and frequency of activities that lead to positive health results.”
The same root cause for both?
This book is one suggestion of how one factor has been responsible for both obesity and physical inactivity - oil:
"This exciting new book argues that the pulse of fossil fuel energy released from the ground after the discovery of oil not only started the process of catastrophic climate change, but also propelled the average human weight distribution upwards. The author presents a frightening vision of humans besieged by a food industry that uses sophisticated marketing techniques to sell mountains of energy-dense food to those who are 'functionally paralysed', with fewer opportunities to move our bodies than ever before. We see why the accumulation of body fat is a political, not a personal, problem."
This has not been an exhaustive review, but illustrates that the link between physical activity and obesity is not as simple as you might think. Make up your own mind! Let's also not forget that in this section we are talking about causation - the next section discusses intervention, where there is a different story about how increasing physical activity through exercise might be a way of reducing obesity.