Good news for couch potatoes, said a story in the Daily Mail today – lack of exercise is not to blame for the obesity crisis. Its report says that contrary to popular opinion, 'Westerners' surrounded by mod cons don't burn any fewer calories than hunter-gatherer African tribes.
The news is based on research that assessed how many calories members of an African tribe burned during the course of a day. They then compared the average 'burn rate' to previous studies into Western habits.
After adjusting for body size and weight they found that the total energy expenditure was roughly the same between people from the Hadza tribe and people in developed countries.
The Mail's headline overlooks two important points:
- while the Hadza may burn off the same amount of energy during a day (metabolic rate) they are still much more physically active than most people in the West – for example Hadza men walked an average of 11.2 km a day
- the Hadza eat far less high-calorie and unhealthy food than people in the West; researchers noted that their diet contained high levels of tubers and berries
The results of the study seem to suggest that it is the trend towards eating 'high fat, low value' food, rather than lack of exercise, that could be partially responsible for the rise in obesity rates. But obesity is a complex condition and there are many factors involved.
The researchers themselves stressed that they were not studying the benefical effects of exercise and this study should not be taken as proof that exercise did not bring health benefits.
There is plenty of evidence that physical activity has many positive effects on health, such as reducing the risk of heart disease and some types of cancer. Increasing physical activity, along with a healthy diet, is an important part of any weight loss programme.
Where did the story come from?
The study was carried out by anthropologists from Hunter College, New York, and several other academic institutions in the US. Funding was received from Washington University, the National Science Foundation and the University of Arizona.
The study was published in the science journal PLoS ONE, a peer-reviewed freely available online journal.
Media coverage of the study overstated its results, implying that obesity is not caused by lack of exercise. Although the authors state that their study challenges current models of obesity that suggest Western lifestyles lead to decreased energy expenditure, their study did not address the issue of what does cause obesity or what can be done to reduce it. Obesity is a complex disorder in which energy consumption (in the form of calories) is more than energy expenditure. Very little about the factors involved in obesity can be deduced from this study.
It is worth pointing out that this research, which looked at energy expenditure rather than energy intake, had nothing to do with the popular 'hunter-gatherer diet' (or 'caveman' or 'paleo' diet).
What kind of research was this?
This was a study that looked at the total daily energy expenditure (as measured by kCal per day), body composition and physical activity levels of the Hadza tribe. These are a population of traditional hunter-gatherers living in northern Tanzania, Africa. Hadza tribespeople spend their time trekking long distances on foot to forage for wild plants and game, using traditional tools such as bows and small axes. The researchers noted that this type of lifestyle was shared by our human ancestors thousands of years ago.
Researchers compared Hadza adults’ energy expenditure, body composition and physical activity levels with data from Western populations.
The authors say that with one in 10 people projected to be obese by 2015, the causes of obesity remain a focus of debate. Western lifestyles differ markedly from our hunter-gatherer ancestors, they and the differences in diet and activity levels are often implicated on the obesity pandemic. However, little physiological data from hunter-gatherer populations is available to test our models of obesity. Their study aimed to test the hypothesis that hunter-gatherers use more energy each day than their Western counterparts.
What did the research involve?
The researchers measured body composition (for example, weight and fat-free mass) in the participants. They then measured total daily energy expenditure (in kCal/day) over an 11-day period in 30 Hadza adults (13 men and 17 women, aged 18-65). They did this using a technique called the doubly labelled water method, which is used to indirectly measure basic metabolic rate. People drink a given volume of water that has been chemically altered for tracking purposes. This allows researchers to calculate the carbon dioxide production and oxygen consumption and, as a result, the energy used in resting and active states can be inferred.
The researchers also carried out various other assessments. They measured the Hadza's daily walking distances using wearable GPS devices, and they measured energy expenditure during both resting and walking. They adjusted their results for mass, height, sex and age.
The researchers compared the data collected from the Hadza with similar data, taken from previous studies, from other populations in the US, Europe and from non-Western market and farming economies. They also compared average daily energy expenditure of the Hadza with an analysis of average expenditure among another sample of 4,972 subjects.
What were the basic results?
A range of results was reported by the researchers. Importantly Hadza were highly active and lean, with body fat percentages on the low end of the normal healthy range for Western populations. Other results show that:
- physical activity levels were generally greater among Hadza tribespeople than Westerners
- body fat percentages for Hadza adults were lower than individuals from Western populations
- total energy expenditure among Hadza adults (both men and women) were similar to those in Western populations, as measured by individual studies
- the results were unchanged when the Hadza were compared to all market economy individuals (rather than just Westernised populations)
- total energy expenditure was also similar when population studies were used
How did the researchers interpret the results?
The researchers point out that despite high physical activity levels, the Hadza's total daily energy expenditure was similar to that of Westerners. This, they say, challenges the view that Western lifestyles result in abnormally low energy expenditure and that this is a primary cause of obesity in developed countries. They say that differences in obesity rates between different populations may be the result of differences in how much and what we eat, rather than how much activity we do. Human daily energy expenditure may be an 'evolved physiological trait' largely independent of cultural differences, they argue.
This study is of anthropological interest, but it should not be interpreted as suggesting we should all give up on physical activity, which is a crucial part of a healthy lifestyle.
Diet is another vital element. Both diet and physical activity play a part in the fight against overweight and obesity. Put simply, most people need to eat less and move more.
Independent of its role in weight management, physical activity is also important to keep the heart healthy and to promote mental wellbeing.
It should be noted that the study had some limitations:
- its assessment of energy expenditure and other factors was based on only 30 Hadza adults
- its assessment was also short term, carried out over an 11-day period
- the data it used for comparison of the Hadza with Western and other populations was taken from several different sources, including some small studies of individuals (one such study included just 68 adults)
As the authors point out, this study only measured energy expenditure between different populations. This study does not examine the effects of changing levels of physical activity on obesity and did not report the long-term dietary patterns or calorie intakes of the people studied. It therefore cannot answer the question of which is more important, a calorie-dense diet or lack of physical activity as a cause of obesity.
It does not address the important public health question of how best to combat rising overweight and obesity levels.
It is well recognised that weight loss may be difficult to achieve and even more difficult to maintain. Research on the most effective way to address this issue is urgently needed.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
BBC News, 26 July 2012
The Daily Telegraph, 26 July 2012
Daily Mail, 26 July 2012
Links to the science
PLoS One. Published online July 25 2012