Junkfood Science: The myth of sloth slayed... again

June 06, 2008

The myth of sloth slayed... again

The popular belief that we’ve become a nation, and an industrialized world, of increasingly sedentary sloths has become so widespread, that research to the contrary travels slow... if it even makes it out of the medical journals at all. A new study using the latest scientific measuring tool of physical activity energy expenditure, the doubly labeled water method, compiled the largest collection of international data going back to the 1980s. Guess what it found?

You can probably guess simply by the fact that this extensive study barely made a blip on the news. Most readers probably heard nothing about it.

Researchers from Maastricht University in The Netherlands and the University of Aberdeen in Scotland found that actual measures of energy expenditure of adults in the United States and Europe show no evidence of a decline over time. “It seems that we have been misled by the anecdotal information about levels of our physical activity,” said co-author, professor John Speakman. Daily physical activity hasn’t changed, he said. “In the time we spend watching television today, people probably listened to radio in the 1950s and read books in the 1920s.”

Researchers have used many different methods to measure the physical activity and fitness levels of children and adults through the decades. While each one has shown we’re no less active or fit today, this is the first study to examine research data on what some consider especially controlled, the doubly labeled water method (DLW).

This complicated study was just published in the online issue of the International Journal of Obesity. For this study, the researchers collected information from four sources.

The first was from the town of Maastricht, which between 1983 and 2005 has measured energy expenditure in 366 free-living adults, using DLW and direct measures of their basal energy expenditure by respirometry. This single site data was consistently gathered and well-controlled, allowing a reliable look at changes over the decades.

The second source of information in this paper was original DLW data from 13 North America studies of 393 people (85% women, ethnically diverse, and across a large geographic area) published in peer-reviewed journals between 1982 and 2005. To ensure that the DLW data was reliable and representative of adults in the general population, they included information on people who were healthy, not pregnant, and not part of diet or physical activity interventions.

Third, to compare modern data with underdeveloped countries more likely to reflect energy expenditure before modernization of our travel and lifestyles, they gathered DLW data from rural third world cultures.

Finally, in order to evaluate any reductions in energy expenditure compared to what would be naturally expected for bodies of the same size, they compiled DLW data for mammals living in the wild. [They recognized that the data on wild animals used a different statistical model (single-pool) than in the human calculations, which would slightly overestimate the predicted energy demands.]

This may not sound like impressive numbers of subjects, but it will when you realize what’s involved in this method and that this study compiled virtually all such data available to date. The DLW method is the first noninvasive method for measuring the energy expended by people over a period of 2 to 3 weeks in their normal, real-life activities.

It was invented by professor Nathan Lifson and colleagues at the University of Minnesota in the 1940s after discovering that the oxygen in respired carbon dioxide is in isotopic equilibrium with the oxygen in body water. It was an expensive high-tech method that for years was mostly used small birds and animals until isotopes became more cost-effective and gas isotope ratio mass spectrometers became commercially available. DLW wasn’t used in human until the early 1980s and it’s been increasingly refined over the years.

It’s an intricate method. The authors described the Maastricht protocol in their study [*see below], as well as the different ways that the data on basal energy expenditure and daily energy expenditure can be statistically used to determine physical activity level for people of different body masses. They used all three calculation methods in this study. More detailed information on DLW is found in the IDECG (International Dietary Energy Consultancy Group) working group consensus report from Vienna. While there are complex issues surrounding this method beyond the scope of this post, the bottom line is that the method has been validated in more than a dozen studies by four independent research groups and shown to be valid, with an accuracy around 1-3% and precision of 2-8%, for a range of activity levels of people in daily life. Researchers at the University of Chicago, for instance, found it reliable for measuring daily energy expenditure among women within a 7.8% variation.

The findings

Among the Maastricht people, the researchers found that the energy they expended in physical activity each day slightly increased over the decades. The calculated calories people burned in physical activity showed no significant changes between 1988 to 2006. The amount of energy individuals expend in physical activity varies by age, gender and body size, of course, but there was no difference over the decades. A correlation that puzzled the authors was that during these same decades, ‘obesity’ rates in the Netherlands doubled from 5 to 10%.

Among the DLW data from North America, activity increased over the decades and the daily energy people expended in activity rose. Again, this was while ‘obesity’ rates increased from 8 to 22%.

Despite popular beliefs that our modern, Western lifestyles has turned us into slugs, the overall DLW data from third world, nonindustrialized peoples was similar to those of people from Maastricht and North America. The energy expended by modern people in Westernized societies is completely in line with people in third world countries, the authors concluded.

Finally, using the animal data to predict what daily energy demands would be for modern humans of similar size, they found that people were expending more energy than predicted. None of the values was lower than predicted based on body mass and living temperature.

In each group of humans and animals and using each mathematical method, the total physical activity levels for size and gender were amazingly similar.

Conclusion and body of evidence to date

The authors concluded that, using a variety of different techniques to analyze the date on daily and basal energy expenditures of people since the 1980s, this study found no indication that physical activity has declined in North America or Europe.

In fact, the trends “were actually in the opposite direction, suggesting levels of activity energy expenditure may have actually increased over this time interval,” the authors concluded.

These findings are at odds with anecdotal reports, as they went on to talk about. When people observe ‘obese’ people seeming to be less physically active than their lean counterparts, this observation is popularly misunderstood. Although ‘obese’ people may appear to do less physical activity, they are actually expending more energy, said the authors, and can be more fit than their thinner counterparts.

These findings have been confirmed in numerous studies, using a variety of methodologies. For example, although not mentioned in this paper, this lead author has conducted some especially relevant research. Several years ago, Dr. Klaas R. Westerterp had specifically studied the physical activity among ‘obese’ people using DLW measurements. The results were published in the International Journal of Obesity. He found “activity associated energy expenditure increases with body mass index” even while physical activity level doesn’t change.

He also found that “the majority of ‘obese’ subjects are moderately active.” Most fat people are not sedentary, as is popularly believed. However, increasing activity among the ‘obese’ is “not rewarded by weight loss,” he found. And higher intensity exercise and training programs are not tolerated without raising risks for them.

A more recent study by pediatric researchers at Maastricht University compared daily energy expenditure among teens of differing sizes and body fat in The Netherlands, Britain and America. They found that the basal metabolic rate and the average daily metabolic rate are higher among the fat teens as compared to lean ones. Their basal metabolic rates remain higher even when adjusting for age, the percentage of body fat and gender, and fat teens proved actually to be equally active as lean teens.

Deja vu

As has already been covered, the carefully worded admonishments of government agencies and stakeholders in exercise can mislead us to believe in sloths. Definitions are everything. But the research and conclusions of experts that counter popular anecdotes rarely make the news.

Michael Gard, a physical and health educator at Charles Stuart University’s Bathurst campus dispelled the belief that kids today are sedentary and explained our lack of understanding of children’s physiology that attempts to put adult measures and exercise solutions on kids. Despite popular notions of kids being shuttled around in cars and spending their days plopped in front of a screen, children’s play experiences and the amount of physical activity they derive from it and walking and biking, also appear to have changed very little since the 1940s. Research led by Colin Pooley, Ph.D. at Lancaster University in England, followed over 895,000 individual trips and collected 160 hours of taped interviews and data among four age groups of children and found that walking still accounted for about two-thirds of their means of transport and that their actual movements and play activities had changed very little in the past 60 years, despite greater affluence, media and car ownership.

Nor is the physical fitness of kids in the toilet, as popular anecdotes tell us. A review of U.S. studies examining actual peak oxygen consumption measurements indicated that there has been little change in absolute and relative peak V-O2 levels in children from the 1930s through the 1990s.

A review of the evidence for children’s physical activity in the U.S. and internationally, conducted by Adelaide, Australian researchers, concluded that overall, the data does not support the view that today’s generation of young people is less active. Overall population survey data shows little, if any, change for decades. But trend data also suggests dramatic increases since the 1960s in extracurricular sports and physical activities, according to a recent review by researchers at the Division of Epidemiology, School of Public Health, University of Minnesota, Minneapolis. The most recent Child Trends Data from the U.S. Department of Education, National Center for Education Statistics, reported that although socioeconomic disparities were evident, between 2001 and 2005, the percentage of students participating in after-school sports has increased to the highest levels ever.

Examining the telly tubby myths and popular assumptions of an obesogenic environment and calls for more physical education for kids, found no relationship between sedentary or physical activities and ‘obesity’.

“Yes, children today are surrounded by an array of new and exciting forms of technology, but they have not, as a group, become morally degenerate overnight,” wrote professor Gard.

The government’s own evidence doesn’t support fears that we’re a nation of couch potatoes or that sedentary behaviors are a new public health crisis. The latest Behavioral Risk Factor Surveillance System (BRFSS) survey data defused the myth of sloth, showing that activity levels increased between years 2001 and 2005 to 49.7% and 46.7% among men and women, respectively. Previous BFRSS surveys had reported that 31.5% of Americans were supposedly inactive in 1989.

The National Health Interview Surveys from the CDC’s National Center for Health Statistics, which asks slightly different questions, has also shown that the numbers of us getting regular physical activity rose from 1998 to 2005.

Continued findings that dispel popular beliefs of obesity leave people, including the authors of this latest study, perplexed and tempted to speculate about the cause of the obesity epidemic using another set of anecdotes, rather than consider the more obvious: if they are even asking the right question.

© 2008 Sandy Szwarc

* The Maastricht protocol

The measurement of daily energy expenditure in adult subjects usually covers an interval of about 2 weeks [click on image to enlarge]:

Bookmark and Share