Obesity is caused by a prolonged positive energy balance. Whether reduced energy expenditure stemming from reduced activity levels contributes is debated.
Here we show that in both sexes, total energy expenditure (TEE) adjusted for body composition and age declined since the late 1980s, while adjusted activity energy expenditure increased over time. We use the International Atomic Energy Agency Doubly Labelled Water database on energy expenditure of adults in the United States and Europe (n = 4,799) to explore patterns in total (TEE: n = 4,799), basal (BEE: n = 1,432) and physical activity energy expenditure (n = 1,432) over time.
In males, adjusted BEE decreased statistically-significantly, but in females this did not reach statistical-significance. A larger dataset of basal metabolic rate (equivalent to BEE) measurements of 9,912 adults across 163 studies spanning 100 years replicates the decline in BEE in both sexes.
We conclude that increasing obesity in the United States/Europe has probably not been fuelled by reduced physical activity leading to lowered TEE. We identify here a decline in adjusted BEE as a previously unrecognized factor.
…Rather than adjusted AEE declining, it has statistically-significantly increased over time in both sexes. Yet TEE (adjusted for age and body composition) has declined statistically-significantly in both males and females over the past 3 decades. As adjusted AEE has increased at the same time that TEE has declined, there has been a corresponding reduction in adjusted BEE (which only reached statistical-significance in males). The observation that adjusted AEE (and PAL in males) has statistically-significantly increased over time is counterintuitive given the patterns established in work-time PA and the suggested progressive increase in sedentary behavior4,6–8. One possibility is that lowered work-time PA may have been more than offset by increased engagement in leisure time PA. For example, sales of home gym equipment in the United States increased from $2.92$2.42017 billion to $4.51$3.72017 billion 1994–23201719. Time spent in leisure time PA in the United States also increased 1965–30199529ya20, suggesting leisure time PA has replaced the decline in work-time PALs20. Leisure time PA has also changed in other Westernized populations21. Although time spent on computers has increased, much of the increase in this time has largely come at the expense of time spent watching television. Since these activities have roughly equivalent energy costs22, this change would not be apparent as a decline in overall adjusted AEE.
The reduction in adjusted BEE is less easily understood but is consistent with the recent observation that body temperatures have also declined over time23, over the same interval as the reduction of BMR in the wider dataset we analysed (Figure 3b). The magnitude of secular change in BMR is consistent with studies measuring BMR and body temperature in several contexts, including calorie restriction, ovulation and fever, which show a 10–25% increase in BMR per 1℃ increase in core temperature24,25. It was recently suggested that changes in both activity and basal metabolism may have contributed to the decline in body temperature26, but our data suggest this is probably dominated by a BMR effect. The reduction in body temperature has been speculated to be a consequence of a reduction in baseline immune function, because we have greatly reduced our exposure to many pathogens. However, the links between immune function and metabolism are not straightforward. For example, artificial selection on metabolic rate leads to suppressed innate but not adaptive immune function27, and studies of birds point to no consistent relation between immune function and metabolism, either within or between subjects28. Experimental removal of parasites in Cape ground squirrels (Xerus inauris) led to elevated rather than reduced resting metabolic rate29. Nevertheless, some studies in forager-horticulturalist societies in South America have noted elevated BMR is linked to increased levels of circulating immunoglobulin (Ig)G30 and cytokines31, supporting the view that a long-term decline in BEE may be mediated by reduced immune function. Whether this has any relevance to changes in the United States and Europe in the past 30 years is unclear. It is also possible that the long-term reduction in BMR represents methodological artefacts. In the early years, measurements of BMR were often made using mouth-pieces to collect respiratory gases, and recently such devices have been shown to elevate BMR by around 6%32. A second possibility is that early measurements paid less attention to controlling ambient temperature to ensure individuals were at thermoneutral temperatures33.