The Daily Mail announced today that, despite being commonly blamed, lack of exercise has not fuelled the obesity crisis and we are as active as we were 20 years ago. The article reports that researchers say the “real cause is over-eating”. It said we are as active as people in Third World countries and, weight-for-weight, use the same amount of energy as wild animals.
The researchers collected a number of experimental studies, which all used a sophisticated technique that directly measures an individual’s energy demands throughout the day. This was a reliable study, and the results appear to show that people are no less active than they were 20 years ago. However, measuring energy expenditure using this method does not directly measure the timing or type of physical activity. Both of these factors are thought to have an effect on general health outcomes independent of energy expenditure, and may be relevant to extremely obese or sedentary people.
Although this study found no evidence of a decline in energy expenditure, it's possible that there have been changes in the type or timing of physical activity that explains the rising obesity levels in some very sedentary groups of people. On its own, this study doesn’t overturn conventional wisdom that changes in energy intake and physical activity both have a part to play in the emerging obesity epidemic.
Where did the story come from?
Professor Klass R Westerterp from Maastricht University in the Netherlands and Professor John R Speakman from the University of Aberdeen in Scotland carried out the research. Sources of funding are not reported in the journal paper. The study was published in the peer-reviewed medical journal International Journal of Obesity .
What kind of scientific study was this?
It is generally accepted that obesity is caused by consuming more energy than is expended through physical activity. However, it is unclear to what extent the imbalance is caused by excessive energy consumption through eating too much food or high-fat food, or by increasingly inactive lifestyles.
In this study, the researchers tested the theory that reduced levels of physical activity have “driven” the obesity epidemic. To do this, they looked at three different areas. First they examined the levels of daily energy expenditure (DEE) over a 20-year period in western society (Europe and North America). They then compared the energy expenditure of people in modern western societies with that of people in Third World countries. Finally, they compared the energy expenditure and physical activity levels of modern humans with animals living in the wild.
For the first part of the study, the main source of data was a time series study carried out in Maastricht in the Netherlands. Over a 20-year period, the researchers measured the energy expenditure of healthy volunteers over 18 years of age. To be eligible, the subjects couldn’t be involved in athletic activities or be pregnant.
Between 1983 and 2005, 167 women and 199 men were measured using the double-labelled water (DLW) technique. This technique involved the volunteers drinking water that was labelled with two isotopes of oxygen and hydrogen. The researchers then used mass spectrometry, a method which identifies and measures the chemical make-up of a substance, to measure the exact rate at which water (H2O) disappeared from their bodies, and the rate at which carbon dioxide (CO2) was generated. They then calculated the energy expenditure on the basis that CO2 is produced when oxygen is used to metabolise the energy stores in the body. From this, the researchers calculated the subject’s basal energy expenditure rate (BEE), for when the individual was at rest, and the daily expenditure rate (DEE), the amount used in a day.
As the Maastricht data was collected from a single site, and the obesity problem in the Netherlands is “relatively modest” compared to that in the US, the researchers also searched and systematically reviewed the literature for studies in North America that used the DLW technique to look at energy expenditure. From this, they obtained estimates of DEE for 393 subjects from 13 studies.
For the part of the study that compared the energy expenditure of people in modern western societies with that of people in Third World countries, the researchers analysed several population studies that used the DLW method. They also conducted a separate search and analysis of studies that predicted the daily energy expenditure of animals living in the wild.
The researchers used three statistical methods to assess how much of the subjects’ total energy expenditure was due to physical activity. In the first, they assessed the relationship between BEE and DEE to show the proportion that was due to physical activity. In the second, they calculated the ratio of daily expenditure to basal (or resting) expenditure. Finally, in some cases - such as the data from the North American studies - the basal data was not available, meaning the researchers had to estimate average levels of basal expenditure based on the age and sex of the participants. From this, they could then estimate the expected ratio.
What were the results of the study?
The first of the statistical methods showed that in Europe, physical activity expenditure (the amount of energy used up through physical activity) increased slightly but significantly since the 1980s. However, the other two methods found no trend in physical activity expenditure over time.
In the North American part of the study, the researchers used the third method, which gave an index of physical activity expenditure based on adjustments for average weight, age and sex. They found that physical activity expenditure had also significantly increased over time in North America.
The daily expenditure of energy in the groups studied in Europe and North America was not significantly different from individuals measured in the Third World.
What interpretations did the researchers draw from these results?
The researchers say “as physical activity expenditure has not declined over the same period that obesity rates have increased dramatically, and daily energy expenditure of modern man is in line with energy expenditure in wild mammals, it is unlikely that decreased expenditure has fuelled the obesity epidemic.”
What does the NHS Knowledge Service make of this study?
This was a reliable study. It had some limitations, some of which the authors acknowledge:
- The most complete data for this study came from a single city, Maastrict, in the Netherlands, and therefore provides the strongest evidence for the trends in energy expenditure in this selected population. However, as the usual amount of exercise undertaken wasn’t reported, it’s not possible to tell if the 366 participants were generally more or less active than the rest of the population. This is important, as without more details on how this selection was made, it’s not possible to be certain that the changes noted in the study reflect changes in the level of activity in the rest of the Dutch population.
- The varied techniques used to calculate energy expenditure due to physical activity mean that the results from Europe and the US cannot be directly compared.
- The 13 US studies were small and some were conducted in either men or women only. Their findings may not therefore be representative of the energy expenditure of the general US population.
- Although data was available on the body mass index (BMI) of the volunteers in the Netherlands, there was no data on BMI of the North American or Third World volunteers. It’s therefore not possible to tell whether there were differences between the different regions, or changes in average BMI over time.
This study has attempted to resolve some controversy about whether obesity results from excessive energy intake, lowered physical activity or both. However, the volunteers studied may not be typical of the general populations in the countries concerned.
Time series such as these can suggest further areas for study, but on their own they cannot be used to identify causes because many other unmeasured factors also change over time.
This study on its own does not resolve the issue or overturn the conventional wisdom that changes in energy intake and physical activity both have a part to play in the emerging obesity epidemic.
Sir Muir Gray adds...
I don’t believe it; in England the decrease in active travel, walking, cycling and public transport has reduced energy expenditure over the last 30 years.