An international study, published in the journal of Science, has found that, after accounting for body size, energy expenditure peaks in infancy and then steadily declines until the age of about 20 years. Contrary to the popular belief that metabolism slows in middle age, the research suggests that energy expenditure does not change until close to the age of 60, when it starts to fall again.
Total daily energy expenditure (“total expenditure”) reflects daily energy needs and is a critical variable in human health and physiology, but its trajectory over the life course is poorly studied. The researchers analyzed a large, diverse database of total expenditure measured by the doubly labeled water method for males and females aged 8 days to 95 years and living in 29 different countries. This method requires study participants to drink water that contains unusual isotopes of hydrogen and oxygen atoms.
Energy expenditure varies considerably among individualls, even after accounting for body size, sex and age. The researchers were surprised to discover that 1 year olds burn calories 50% faster than adults.
The coauthor of the study Dr. John Speakman, of the University of Aberdeen in the United Kingdom said that, “Perhaps the most unexpected feature was the constancy of metabolic rate in both males and females between the ages of 20 andd 60. This suggests that if you are experiencing middle-aged spread, it’s more likely to be because you are eating more rather than expending less.”
In the past, research into energy expenditure has mostly focused on resting or basal metabolism, which is the number of calories burned just to keep the body ticking over. Basal metabolism includes the energy that the body devotes to vital functions, such as breathing, digesting food and pumping blood in the body. This only accounts for 50-70% of all the calories that we burn. It does not include commonplace but energetic activities, such as walking, climbing stairs, jogging, or doing other activities.
The authors believe that the surprisingly high metabolic rate found in the tissues of infants might relate to their rapid growth and development and that the reduced energy expenditure in older people may reflect a decline in metabolism in their organs. They also believe that the metabolic changes identified will lead to further investigations of disease progression, drug activity and healing. They also note that their research identified considerable differences in energy expenditure among individuals, even after accounting the body composition, sex and age.
Herman Pontzer, et al. Daily energy expenditure through the human life course. Science 13 Aug 2021. Vol. 373, Issue 6556, pp. 808-812.