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What’s really driving the childhood obesity epidemic?

While popular media focus on a simplistic too-much-food-not-enough-activity bottom line, scientists are grappling to understand the myriad factors that have contributed to the tripling of obesity rates among Canadian children in just three decades. Clearly, our environment has undergone dramatic change in recent years, but why aren’t all children equally susceptible to the weight-promoting lifestyle of the 21st century?  

If a social engineer were to intentionally design an environment with the goal of increasing the average weight of a society’s children, it would be difficult to surpass what we have today in North America. The list of obesigenic features begins with (but is certainly not limited to) an abundance of  relatively cheap, high-calorie, nutrient-poor processed food that is intensely marketed to children, urban sprawl that promotes driving and discourages walking and biking, an emphasis on sedentary entertainment (TV, computers), reduced resources for physical education and recreational activities and hectic lifestyles that preclude sit-down family meals.

“We need to understand how these environmental factors — and other influences, such as cultural differences, parenting style and socioeconomic status —contribute to pediatric obesity,” says Katherine Morrison, Associate Professor, Department of Pediatrics, McMaster University. She is part of a research team exploring how the built environment and nutrition and tobacco policies affect children’s risk of obesity and other health outcomes. 

Her work is complicated by the multiple interconnections between risk factors. For example,studies have shown that there is a positive association between childhood obesity and television viewing. This association may be influenced by snacking while watching TV (children consume an extra 167 calories per hour of TV watched), increased exposure to food marketing, less activity, less social interaction and/or sleep disruption.

In addition to these large-scale environmental influences, childhood obesity is also being driven by hundreds of tiny shifts in behaviour brought about by the prevalence of labour-saving devices that today’s children use. From the television remote control to automatic car windows to battery-operated toothbrushes, each device incrementally reduces calorie expenditure. 

“Each time a child changes the TV channel or roll s down the car window, he uses five or maybe 10 fewer calories than a child a generation ago used. Multiply that by 50 times a day, and it’s a significant change,” says Jean-Pierre Chanoine, Clinical Professor, Department of Pediatrics, University of British Columbia.

LESS ACTIVE OR MORE SUSCEPTIBLE?

Many pundits favour “lazy kid syndrome” as a leading driver of childhood obesity. At first glance, a recently released Statistics Canada report supported the idea that our obesity epidemic is caused by an inactivity epidemic.Based on data from the 2007–2009 Canadian Health Measures Survey, the report showed that only 5% of Canadian children are meeting the recommended levels for vigorous physical activity per week.

A closer look, however, revealed that overweight and “normal” weight youth are equally inactive. Specifically, data showed that overweight and obese girls aged six to 19 years have the same minutes of moderate-to-vigorous physical activity as girls who are neither overweight or obese (between 44 and 48 minutes per day). While a discernible relationship between activity and BMI is seen in boys (overweight and obese boys are active 14 and 22 minutes per day, respectively, while normal weight boys are active 65 minutes per day), the actual calories spent in activities are negligible when you factor in the increased effort required to move higher body weights.(Similar findings were reported for adults.)

The report begs the question: if few children are getting enough activity, if they are all exposed to more or less the same environmental risk factors, why aren’t all kids overweight?

Work by German researcher Andreas Beyerlein may offer a clue. His team analyzed data on more than 13,000 children and youth to assess the impact of maternal BMI, maternal smoking in pregnancy, low parental socioeconomic status, exclusive formula feeding, and high TV viewing time on BMI (PLoS One, 2011). The data showed that the estimated effects of all risk factors (except formula-feeding) on BMI were greatest for children with the highest BMI. For example, increased time spent watching TV was associated with a much greater impact on BMI in the overweight and obese children than in the normal weight children. Beyerlein suggests, “genetic variants with a possibly increased susceptibility of carriers to certain risk factors might offer an explanation for differences in the effect magnitude of risk factors by BMI percentiles.” In other words, children who are genetically predisposed to obesity are far more likely gain weight when spending hours in front of the TV than children who are genetically less obesity prone.

“Some people are genetically predisposed to be more sensitive to changes in the environment,” says Dr. Chanoine. “While our genes don’t change, they can be programmed in a different way, especially prenatally or in early life.” (The study of heritable changes in gene functions is known as epigenetics.)

For example, studies have shown that low-birth-weight babies have an increased risk of obesity later in life. The hypothesis is a baby deprived of sufficient nutrients in utero becomes “programmed” to anticipate a compromised environment and is then unprepared to deal with the environment of plenty at birth. Similarly, babies born from mothers with gestational diabetes tend to be heavier at birth and are at increased risk for obesity and type 2 diabetes in adulthood. “The additional fat and glucose received in utero changes the way the individual metabolizes these later in life,” says Dr. Chanoine. “The whole machinery works differently because of the prenatal environment.”

The worrisome implication is that the impact may extend beyond the current generation. As obesity rates go up, more mothers will be overweight and have gestational diabetes, giving birth to more babies who are at higher risk to become overweight mothers themselves when they grow up. “It
becomes an accelerating cascade. That is why it is so important that we understand what is driving obesity so that we can find effective interventions to break the cycle,” says Dr. Morrison.

The most difficult element when talking about causation is to avoid blame. 

“It’s easy to say it’s the child’s fault or his mother’s fault that he is overweight. But to develop effective solutions, we have to step out of our comfort zone and realize that the roots of obesity are incredibly complex, involving everything from societal decisions down to the biology of the individual,” she adds.

Dr. Arya M. Sharma, Scientific Director of the Canadian Obesity Network, agrees. 

“We must ensure that any messages that link ‘healthy’ weights to unhealthy lifestyles do not perpetuate and reinforce the stereotype that obese kids (or their parents) are simply overindulgent and lazy.”

COMPLEX SOLUTIONS REQUIRED

The recently announced National Dialogue on Healthy Weights may be a step in the right direction. Announced in March by Federal Health Minister Leona Aglukkaq, the initiative seeks to identify ways to promote and maintain healthy weights for children and youth. Areas of discussion will include making our social and physical environments more supportive of physical activity and healthy eating,identifying and addressing obesity risks in children early, increasing access to nutritious foods and decreasing the marketing of foods and beverages high in fat, sugar and/or sodium to children.
The initiative recognizes that the causes of childhood obesity are multi-factorial and the problem requires a multi-pronged approach.

“Simple solutions — just telling kids to be more active and eat less — just don’t work,” says Dr. Chanoine. “Our research into what causes obesity and how to effectively treat it has a long way to go.”

Dr. Sharma agrees, saying that addressing childhood obesity “requires a complete upheaval and change in how we run our lives — unfortunately, small changes will have small effects.”