A new study published in Nature Metabolism (pdf) has revealed that the basal metabolic rate (BMR) in people in the United States and Europe has decreased over the past three decades, potentially contributing to the growing obesity epidemic in both regions.
Basal metabolic rate, or basal energy expenditure, refers to the energy required per unit of time for the body to maintain vital functions such as breathing, blood circulation, and maintaining body temperature. Put simply, BMR is the number of calories the body burns while at rest. BMR is one component in the body’s total energy expenditure. The other is activity expenditure, the number of calories burned during physical activity, such as running or walking.
According to the study, which analyzed data from the present day back to the late 1980s from nearly 4,800 adults in Europe and the United States, the adjusted total daily energy expenditure has decreased significantly since the 1990s. The data indicated a decline of approximately 7.7 percent in men and 5.6 percent in women. In terms of adjusted basal energy expenditure, men experienced a drop of 14.7 percent over time, while women’s decline was 2 percent and not deemed significant. However, the authors noted that a larger dataset of BMR measurements of nearly 10,000 adults across 163 studies going back 100 years confirms the decline in both men and women.
“The surprising conclusion is we spend less energy when resting now than individuals did 30-40 years ago!” John Speakman, a professor at the Chinese Academy of Sciences in Shenzhen, China, and a lead author of the study, wrote on Twitter. “The magnitude of the effect is sufficient to explain the obesity epidemic.”
Speakman addressed the lack of significance of the basal energy expenditure drop in women, attributing it to the inclusion of data from just one study. “If data from that single study was removed the trend was also highly significant in females.”
It has been widely accepted that changes in body weight are linked to an imbalance between the energy (calories) consumed through food and the energy expended (calories burned) to sustain life and engage in physical activity.
The BMR plays a crucial role, accounting for around 60 to 75 percent of an individual’s total daily energy expenditure, particularly for those with sedentary jobs. This rate directly impacts the rate at which a person burns calories and ultimately influences whether a person maintains, gains, or loses weight.
It is generally believed that the obesity epidemic has been primarily caused by decreased physical activity levels and increased food intake. However, the study revealed that physical activity levels have actually increased in both men and women, but the total energy expenditure has significantly decreased, alongside a corresponding decline in basal energy expenditure. One possible explanation is that increased physical activity during leisure time—such as jogging or swimming—is offsetting the progressive rise in sedentary behavior.
Why Has Basal Metabolic Rate Been Declining for Decades?
A factor that may be important in explaining the decline is food. “Diets have changed enormously over the past 100 years,” Speakman said.
The authors of the study pointed out that over the 20th century, people’s diets underwent numerous changes, including the amounts and types of carbohydrates, fiber, and fats consumed. For example, in 1910, animal fats accounted for over 90 percent of the fat intake, but currently, they account for less than 15 percent.
Researchers conducted experiments on mice to explore the possible impact of these dietary changes, allowing for well-controlled and monitored diets. The results suggested that intake of saturated fats could be an essential factor in lower basal energy expenditure. Saturated fat, found in cheese, butter, and meat, is generally considered unhealthy, while unsaturated fats, found primarily in plant-based foods such as nuts, avocadoes, and olive oil, are considered healthy. These findings could indicate that our dietary shift from predominantly animal-based to plant-based fats over the past century may have contributed to the decline in energy expenditure, affecting our metabolic rate and thus making obesity more likely.
However, the researchers acknowledged that further studies in humans are necessary.
Additionally, other aspects of the diet, such as fiber intake, may also impact metabolic rate and have declined in recent years. In fact, a randomized, controlled trial from 2017, observing 81 adults, found that fiber intake affects resting metabolic rate. However, the exact mechanisms of how fibers may boost metabolism are still being studied.
Other Possible Factors for the Decrease in Metabolic Rate
“The first place to look is our food supply, but we also need to look at environmental toxins, such as plastics, pesticides, other chemicals, etc.,” Dr. Christopher Palmer, an assistant professor of psychiatry at Harvard Medical School specializing in the connection between metabolism and mental health, who was not part of the study, wrote on Twitter. “Something in our environment is poisoning our mitochondria.”
Mitochondria, the tiny organs within a cell, play a central role in energy metabolism by converting energy from food into a form the body can use.
Dr. Anders Rehfeld, a Danish researcher in human sperm physiology, shares the same concern and notes the decline in sperm counts over the past 40 years. He wrote on Twitter that “such rapid, widespread changes clearly suggest environmental causes.” Exposure to environmental chemicals and unhealthy lifestyle behaviors are the two main reasons for the global sperm count drop, Dr. Shanna Swan, one of the world’s leading environmental and reproductive epidemiologists and an author of the study on sperm, told The Epoch Times in March.
How to Speed Up Metabolism
Why metabolism slows down is a “complex phenomenon,” Dr. Konstantinos Spaniolas, director of the Bariatric and Metabolic Weight Loss Center at Stony Brook University, told The Epoch Times. However, discovering why it happens will lead to new ways of combating obesity. “I think the important part is what can we do about it.”
Spaniolas added that increasing physical activity is one way to improve metabolic profile and that medications and surgery can be necessary for treating obesity, too.
Some naturopathic doctors claim that healing the metabolism should precede weight loss. A 2005 scientific article suggested that balanced, whole-food diets—designed to reduce or prevent insulin resistance, improve insulin sensitivity, and build muscle—can increase metabolism.
Improving metabolism may also involve adequate sleep, stress management, and nutrient supplementation.
The study’s authors acknowledged that one of the main limitations is its cross-sectional design, which does not allow for establishing a causal link between changes in metabolic rate and the changes in obesity rates. Furthermore, despite adjusting the basal energy expenditure for age and body composition, other factors may need to be considered.
The study’s participants may not have been representative of the underlying population, and a long-term decrease in BMR may have come from methodological factors.