Statistics Canada recently released the findings of the 2007-2009 Canadian Health Measures Study. Fewer than one Canadian in 20 meets recommended levels of physical activity. Yet, some Canadians, even some obesity specialists, are in denial that inactivity accounts for Canada's obesity epidemic. Interpretation of the Canadian findings are of direct relevance to U.S. residents who have the same problems of overweight and inactivity.
I previously made the case that the modern obesity epidemic is more attributable to under activity than to overeating. The big picture is fairly easily stated and it is supported by the Canadian study, despite some intriguing wrinkles.
When it comes to putting on weight, farmers know that to fatten up an animal, one needs both plenty of food and restricted activity. This idea can be generalized to human populations. Everywhere on the globe that inactive populations consume energy-dense foods, one gets an epidemic of obesity.
Much of the focus among health professionals has been on dietary restriction which is difficult, dangerous, and often ineffectual. Perhaps they feel that getting a sedentary population to move is more difficult than putting people on a diet.
That is unfortunate because no physically active population throughout history ever had an obesity problem. Whether one observes the lean and well-toned bodies of hunter-gatherers, or the slender frames of subsistence farmers, a high activity level prevents obesity.
A common reaction to this evidence is that these poor people are starving because it is so difficult to acquire food. Yet the truth is otherwise. Pound for pound, people in subsistence societies where there is zero obesity consume at least twice as much food as people in sedentary modern societies where obesity is rife. The argument that physical activity prevents obesity is supported by the broad picture of the Canadian data also.
Canadian activity levels
For adults over 20 years, obese men and women are only half as active as those of normal weight in terms of daily time devoted to moderate or vigorous physical activity (women 13 vs. 35 minutes, men 19 vs. 35 minutes, 1). These data were collected using state of the art accelerometers worn by volunteers.
Yet, a curious wrinkle emerged in the data for children aged 6-19 years (2). Obese boys were less active than those at a healthy weight (44 minutes versus 65 minutes). Obese girls were just as physically active as their healthy-weight counterparts, however (48 minutes in each case.
If physical activity can protect young people against obesity, why were Canadian girls not protected from gaining weight? The obvious answer is that they were not sufficiently active to prevent weight gain. Another possible reason is that obese girls spent more time sitting down (544 versus 522 minutes per day "sedentary"). Moreover, the data involve such a wide age range that they are difficult to interpret with activity declining from 58 minutes per day at ages 6-10 years to 39 minutes at ages 15-19 years.
It is certainly possible that in a sedentary population, some young women are biologically more vulnerable to weight gain than others. Yet the low level of activity found for modern populations is an anomaly.
This point emerges from research on the Pima tribe of Arizona who are vulnerable to obesity and metabolic disorders. Yet that vulnerability is recent. When the Pima worked as farmers a century ago, they were universally lean.
The same is true of Pima agriculturalists that survive in a mountainous region of Mexico. Despite a hearty diet, their heavy work load as subsistence farmers keeps them lean.
For sedentary populations, high-fat food is dangerous because it gets stored as adipose tissue and reduces physical fitness. Yet, the inactivity is a more fundamental cause of obesity. It is what distinguishes us modern humans from our lean subsistence ancestors. It challenges Canadians and Americans alike.
1. Coley, R. C., et al. (2011). Physical activity of Canadian adults: Accelerometer results from the 2007 to 2009 Canadian Health Measures Survey. Statistics Canada Health Reports, No. 82-003-X.
2. Coley, R. C., et al. (2011). Physical activity o f Canadian children and youth: Accelerometer results from the 2007 to 2009 Canadian Health Measures Survey. Statistics Canada Health Reports, No. 82-003-X.
