Obesity is caused by an energy intake that exceeds energy expenditure. It is vital to understand how much increased food intake or reduced energy expenditure contribute to the growing obesity epidemic. A new study published in Nature Metabolism has examined the changes in total energy expenditure and its components over the past 30 years.
Total energy expenditure is composed of energy used during activities and energy used for basic body functions, such as breathing and circulation, known as basal energy expenditure. So far, it has been assumed that the obesity epidemic has been caused by increasingly sedentary, or inactive, lifestyles and reduced physical activity over the past decades, combined with increased food intake. Using the IAEA's Doubly Labelled Water (DLW) database as a basis, the peer-reviewed study co-authored by IAEA scientists, found that total energy expenditure has declined in both men and women over the past 30 years. Surprisingly for researchers, this was due to a reduction in basal energy expenditure and not in activity expenditure as previously assumed. Actually, activity energy expenditure had significantly increased.
The IAEA DLW database, established in 2018, provides a repository of data collected from studies, including information on the participants - their age, body composition, activity levels - as well as the isotopic data generated by the DLW analysis. The DLW method uses water containing two stable isotopes, deuterium (hydrogen-2) and oxygen-18, to determine the amount of energy a person has expended, or in other words, how many calories they have burned. Each participant consumes a dose of a doubly labelled water, before resuming their normal activities. Urine samples are usually collected for a period of 10 to 14 days to determine how quickly the two isotopes leave the body. By calculating the rate at which these isotopes are eliminated, the amount of carbon dioxide produced can be estimated, a figure which is related to energy expenditure.
For the recent paper, DLW measurements collected since the 1980s on energy expenditure of more than 4500 adults from Europe and United States were analysed, combined with measurements of basal energy expenditure, which allowed investigators to evaluate energy used for basic body functions and energy used during activities. They found that total energy expenditure had declined since the 1990s by about 7.7 per cent in males and 5.6 per cent in females.
John Speakman, lead author of the study and Professor at the Shenzhen Institute of Advanced Technology in China and the University of Aberdeen in the United Kingdom, said: "It turns out that actually the activity expenditure has gone up slightly over time. But what's really gone down is your basal energy expenditure. What that means is that your resting metabolic rate now, as a person living in 2023, is lower than a person of your age and body composition from the late 1990s. And that's pretty unexpected. And we're not really sure why that is."
"There are several potential contributing factors to why basal energy expenditure has decreased, including diet changes that may affect energy expenditure. More research is needed so that we may understand how to reverse this decline, potentially by changing what we eat," Speakman said. "This may be the basis for a useful strategy in obesity management. However, at present, obesity remains best avoided by not overeating."
The DLW database contains over 8000 measurements from 37 countries collected using the DLW method since 1981. Because the data is largely from research carried out in Western countries, the IAEA has begun broadening the dataset through a coordinated research project initiated this year, to incorporate information from Asia, Africa and Latin America.
"The DLW database has enabled a better understanding of the obesity epidemic and provided evidence for the first time on how energy expenditure has declined over the past 30 years," said Cornelia Loechl, Head of the IAEA's Nutritional and Health-related Environmental Studies Section and co-author of the paper. "Often individual studies are small and not generalizable due to their small sample sizes. However, when combined into one database, big questions on the causes of obesity can be addressed." The database and its ongoing expansion provide scientists a resource to answer these questions.
