My argument is that if, as seems to be the case, calorie consumption has fallen over time, increases in body weight must be due to fewer calories being expended and, therefore, that a reduction in physical activity is the most likely culprit. I appreciate that there are people who believe that a calorie is not a calorie and that changes in the diet could therefore be the issue. I tend to side with the traditional consensus view that a calorie is, in fact, a calorie, but even I wasn't so inclined, it so happens that the consumption the ingredients that some claim are uniquely fattening (notably saturated fat and sucrose) have also declined, so that line of argument seems like a dead end.
Physical inactivity is not the sole explanation for why people are expending fewer calories than they used to. It has been argued that central heating means that people burn off less energy through keeping warm. The decline of smoking has probably also had some effect; smokers weigh several kilograms less than nonsmokers on average.
There may be other factors that have affected metabolism over time, but physical inactivity remains the main contender for why obesity has risen. All sorts of evidence can be given on this count, only a little of which was documented in The Fat Lie. For example, I read an interesting post by Tim Olds at The Conversation this week:
In 1919, a young woman named E.M. Bedale started postgraduate research at University College London, an uncommon undertaking for a woman at that time. Her studies focused on energy balance in children, which led her to spend several years at a serendipitously eponymous school called Bedales in rural Hampshire.
During her two years at Bedales, Miss Bedale measured the energy expenditure and intake of the school’s students, using methods that are still considered to be gold standards today.
Her data provide a startling contrast to our time. Children from almost 100 years ago were 50% more active than kids today. They accumulated over four hours more of physical activity and sat for three hours less than today’s kids - every day.
Too historical for you? Not 'evidence-based' enough? Then how about this?
In the 1960s, half the jobs in private industry in the United States required at least moderate-intensity physical activity, compared to less than 20% today.
Work in factories and farms has given way to office work, and that has amounted to over 400 kilojoules less each day that adults expend at work. This difference alone results in a weight increase of about 13 kilograms over 50 years, which pretty closely matches actual changes in weight.
And in the home...
In many ways, the whole ethos of ease now saturates our society, and efficiency is the hallmark of modernity.
Think about it this way - nobody is in the market for a labour-creating device. Sit-on mowers, leaf blowers, self-opening doors and automatic car windows, robot vacuum cleaners, sensor lighting, dishwashers and microwaves all yield daily microsavings in energy expenditure that add up to hundreds of kilojoules.
In 1900, the average American housewife spent an estimated 40 hours every week in food preparation. Today, that time is barely four hours — and it appears to have reached an absolute minimum.
When it comes to physical activity in one's leisure time, some interesting research was published this year in the American Journal of Medicine. I mentioned it briefly in The Fat Lie but some of the statistics are shocking and require another look. The graphic below shows the proportion of Americans who engage in no leisure activity whatsoever. Click to engorge.
Between 1988-94 and 2009-10, the proportion of men who did no leisure-time physical activity rose from 11.4% to 43.5%. Amongst women the rate rose from 19.1% to 51.7%. These are enormous changes in a relatively short period of time.
There's more to come but this blog post is long enough so I'll come back to it in the near future.