Welcome to Part 1 in a series on potential contributors to the pediatric obesity epidemic.This series is based on a recent paper in the journal ISRN Pediatrics, which is available for free here. Big thanks to the University of Ottawa Author Fund for covering the Open Access publication costs.
Throughout the week we will examine the following potential contributors to the pediatric obesity epidemic:
Part 1: Physical Activity and Sedentary Behaviour
Today we have a brief introduction to the factors affecting energy balance (and therefore adiposity), before examining the evidence linking physical activity and sedentary behaviour with the recent increases in childhood obesity rates. To skip to Part 2, which focuses on changes on food intake, click here.
As noted by Jéquier and Tappy, the first law of thermodynamics—which states that energy can neither be created nor destroyed—applies to humans . With respect to body weight, this means that changes in stored energy (e.g., adiposity) are equal to energy intake (EI) minus energy expenditure (EE) . Energy expenditure can be broken down into three separate components :
(i) basal metabolic rate (BMR),
(ii) diet-induced thermogenesis, and
(iii) energy used for exercise and physical activity (PA).
Energy intake, on the other hand, is simply the sum of the energy consumed by an individual, minus approximately 5–10% that is excreted in urine and feces . When EI exceeds EE, the result is an increase in energy stores, and therefore weight gain. Thus, any putative cause of the childhood obesity epidemic must influence either EI, EE, or both. With that in mind, let us now evaluate the role of both conventional and unconventional factors in the etiology of childhood obesity.
Reduced Physical Activity
Given its role in the energy balance equation it is quite obvious that, all else being equal, a reduction in the number of calories burned through PA will directly lower EE and result in positive energy balance. Regular bouts of PA are also known to result in substantial elevations in BMR in both lean and obese individuals [12, 13], suggesting that reductions in PA may further reduce EE by deleterious changes to BMR. Similarly, it has also been suggested that regular PA results in more accurate coupling of EI and EE [13–17]. Taken together, these findings suggest that reductions in PA may negatively impact both sides of the energy balance equation by directly reducing EE and by inhibiting the proper regulation of EI. Not surprisingly, available observational evidence also suggests that PA plays a role in the prevention of excess weight gain in children and youth.
Numerous cross-sectional studies report that overweight and obese children are less active than their lean peers, while the majority of longitudinal studies report small, inverse associations between high levels of PA and the accumulation of excess body weight [18–20]. For example, Berkey and colleagues report that every hour of self-reported daily PA in girls aged 9–14 is associated with a −0.0284 kg/m2smaller increase in BMI over a one-year period (the relationship was of borderline significance in boys) . Similarly, a recent systematic review by Connelly and colleagues reports that compulsory PA is the single most defining factor of controlled trials that successfully prevent the development of childhood overweight or obesity . It should be noted that current findings are based mainly on self-reported levels of PA, which are known to be substantially less accurate than objective measures such as pedometry and accelerometry [23, 24]. However, despite these methodological limitations, the balance of evidence suggests that low levels of PA are likely to predispose to future weight gain.
While the above evidence suggests that low levels of PA are likely to result in increased risk of future weight gain, at present it is unclear whether current levels of PA in Canadian youth are lower than those of previous generations, which would be necessary in order for PA to play a causal role in the current obesity epidemic . Self-reported leisure-time PA among Canadian adolescents actually increased during the 1980’s and remained stable throughout the 1990’s , suggesting that current PA levels may be higher than they were before the obesity epidemic. However, this data conflicts with other lines of evidence, which suggest that total PA levels among Canadian children may be lower than they were in the past. For example, it has been reported that the proportion of trips to school that involves active transportation decreased by roughly 20% between 1986 and 2006 among Canadian children in the Toronto region . Similarly, children who live in Canadian Old Order Amish and Mennonite communities, where lifestyles are similar to those in contemporary Canadian society 60–100 years ago , accumulate roughly 50% more steps per day than their contemporary Canadian peers , as well as 30–50% more moderate-to-vigorous PA . In the absence of more complete and objective data on the PA of past generations of Canadian youth and given that less than 10% of Canadian youth are currently meeting PA guidelines [30–32], it appears relatively safe to conclude that total PA-related EE of Canadian youth is at or near historic lows.
Thus, given the multiple biological mechanisms linking reduced PA with increased adiposity, consistent but relatively small longitudinal associations between PA and weight gain, and evidence suggesting that Canadian children are likely less active than in previous generations, there is currently moderate evidence that insufficient PA plays a causal role in the current epidemic of childhood obesity.
Increased Sedentary Behaviour
Sedentary behaviour is defined as “a distinct class of behaviours (e.g., sitting, watching TV, driving) characterized by little physical movement and low energy expenditure (≤1.5 METs)” . At present, it is unclear whether the prevalence of sedentary behaviours in Canada and other industrialized nations has increased in recent decades. For example, a review by Marshall and colleagues reports that the screen time (time spent watching television, playing videogames, or using computers) of children in modern nations has not increased since the 1950’s . In contrast, Nelson and colleagues report that weekly computer usage increased by roughly 4 hours/week between 1999 and 2004 in American youth . Similarly, a recent report suggests that Canadian children average more than 6 hours of screen time on weekdays, and that even preschoolers watch an average of almost 2 hours of television per day —amounts that seem highly unlikely 40 years ago. Similar trends are seen in sedentary modes of transportation such as driving, which have also increased dramatically in recent decades . Finally, accelerometry data from the nationally representative Canadian Health Measures Survey suggests that Canadian youth spend an average of 8.6 hours per day (more than 60% of their waking hours) engaging in sedentary behaviour . Taken together, these reports suggest that Canadian children are likely more sedentary than previous generations.
In addition to reports of increasing levels of sedentary behaviour in Canadian youth, there is also an accumulating body of evidence which suggests that high levels of sedentary behaviour may predispose to weight gain, especially in young children, while reductions in sedentary behaviour may promote weight loss or weight maintenance [18, 37]. For example, Burke et al.  report that every hour of television watching at age 6 was associated with a 50% increased risk of overweight or obesity at age 8 in a sample of Australian children, independent of other risk factors for weight gain. Similarly, a randomized controlled trial which reduced screen time resulted in significant reductions in both weight gain and the accumulation of abdominal fat in elementary school children . However, few studies have found relationships between sedentary behaviour and weight gain in older children, suggesting that sedentary behaviour may only be a risk factor for obesity in young children [18, 37].
The potential mechanisms which are thought to link sedentary behaviour and adiposity involve deleterious changes to both EE and EI. Most obviously, sedentary activities are defined by having low EE  and may also displace PA, although there is currently little evidence that such displacement takes place [18, 40–42]. With respect to EI, excess sedentary behaviour may also result in “uncoupling” between EE and EI . Further, it has been suggested that television viewing may exert a particularly negative impact on pediatric EI. For example, Wiecha and colleagues report that every one-hour increase in daily television viewing among school children is associated with an extra consumption of 167 calories . In addition, it has also been suggested that exposure to television food advertisements increases children’s food intake at subsequent meals [44, 45]. Thus, through its impact on both EE and EI, it is very plausible that sedentary behaviour has a deleterious impact on energy balance, and therefore body weight.
Given the strong and consistent relationship between sedentary behaviours and weight gain in early childhood, temporal trends which suggest that sedentary behaviours have increased in recent decades, and numerous plausible biological mechanisms, there is currently strong evidence that increases in sedentary behaviour play an important role in the epidemic of childhood obesity.
Coming Up Next
Come back tomorrow (now online here) for a look at the evidence related to total energy intake, fat intake, sugar sweetened beverage intake, and calcium intake.
See you then!
Saunders, T. (2011). Potential Contributors to the Canadian Pediatric Obesity Epidemic ISRN Pediatrics, 2011, 1-10 DOI: 10.5402/2011/917684
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Contributors to the Pediatric Obesity Epidemic Part 1: Energy Balance, Physical Activity & Sedentary Behaviour by Obesity Panacea, unless otherwise expressly stated, is licensed under a Creative Commons Attribution 3.0 Unported License.