Imagine this scene – a personal trainer barking at his flabby pen-pushing charges to push themselves through the pain barrier and climb those steps because “the human body wasn’t designed to sit at a computer all day”. It’s easy to imagine because of the common perception that the root cause of the current obesity epidemic is a radical shift in human behaviour – from the hunter-gatherer ways of our ancient ancestors to our current sedentary lifestyles with diets high in energy-dense and highly-processed foods.
But, in a paper just published in PLoS One researchers from the United States, Tanzania and England have shown that there’s actually no difference in the energy expenditure of the modern American in comparison to the closest modern comparator we have to the hunter-gatherer lifestyle, the Hadza foragers of Northern Tanzania.
According to the authors, although the hunter-gatherers do indeed walk far more than your average American, they actually expend the same amount of total energy. Based on these findings, the authors challenge the view that obesity in Western populations results from decreased energy expenditure, placing the blame squarely on our high-energy diets.
This is an intriguing finding but what are we to make of it when all the advice we receive (including an article on this very site not long ago) is to get moving to halt our expanding waistlines? Is all this advice based on insufficient evidence?
The best place to start answering this question is through a closer examination of the validity of this latest hypothesis.
The first assumption implicit in the research is that “globesity” is the result of a change in lifestyle from ancient hunter-gatherer to modern human behaviours of sloth and over-indulgence. But anyone from the generations that preceded X,Y and Z will tell you that when they were growing up, there was no such thing as an obesity epidemic.
In Australia, as in much of the world, it was really only in the 1980s that obesity started to become common. In fact, the roots of the obesity epidemic stretch back no further than the early twentieth century.
So although it’s interesting that energy expenditure is similar in the modern American and the hunter-gatherer from Tanzania, to get to the root cause of the obesity epidemic the question that really needs asking may be how is the lifestyle of Americans in 2012 is different to that of an American in 1920? And the modern love affair with computers, cars, labour-saving devices, video games and television will tell you the answer to that one.
But an even more important issue with this research is its sole focus on activity. By comparing the energy expenditure of a hunter-gatherer from Tanzania with a modern-day American, the researchers have lost touch with half of the obesity equation.
If I were living on the diet of a forager from Tanzania, I doubt I would feel much like going for a jog at lunchtime, going to the gym or riding my bike home from work. Energy intake (eating) matters too!
It seems amazing to me that on their limited diet, the hunter-gatherers still managed equal energy expenditure and far greater levels of physical activity than well-fed Americans. They exist on limited calories but maintain high levels of physical activity while the latter takes in copious calories and has little physical activity, despite similar levels of total energy expenditure.
It isn’t really surprising that the body fat of the Americans is twice as high. In reality, we still have a fairly limited conception of how human physiology works in response to different levels of activity. To compare the activity of two very different groups is a further challenge. And to not account for the gross discrepancy in energy intake between them makes that comparison even more fraught.
We would never assume that the enormous appetite of an Olympic athlete lead to high levels of body fat or is linked to low activity levels. Intake is a critical part of the energy balance equation and we would be unwise to write off the obesity prevention advice to stay active based on a study that doesn’t account for diet.
The soaring increase in average BMI (body mass index) over the last 30 years is a result of a multitude of factors, some of which may not yet even be apparent. The evidence we do have suggests that it’s not just physical activity or diet that matter in obesity – it’s both acting together.
Modern sedentary lives, while apparently requiring that people expend as much energy as hunter-gatherers, are not sufficiently active to combat the large portions or energy-rich, processed food that’s offered at every turn.
We have increasing evidence that sedentary behaviour is making us sick and even killing us, and to deny that would be to stick one’s head in the Tanzanian sand.