There is no question that as you get older, you start to notice changes in your metabolism, but in what point of life do these changes actually occur? For a long time, metabolism (total energy expenditure) has been assumed to correlate with organ-specific needs with respect to growth and development. In other words, metabolic activity would be higher in young people (infants, children, and adolescents), declines throughout adulthood, and be lowest in older age. However, research is beginning to show us that this assumption isn’t entirely correct, and unfortunately the extra weight gain that many of us see throughout middle adulthood (especially around the holidays) can no longer be blamed on so-called “slower metabolism” than that of your adolescent years.
Research conducted by Pontzer and colleagues explored metabolism and aging by studying over 6,400 people across 29 different countries and evaluated total daily energy expenditure. Study participants ranged from eight days to 95 years old. To control for body size differences (fat-free mass) between subjects of different age/sex, researchers used a variety of mathematical models to generate size-adjusted energy expenditure values for individuals based on their age groups. Interestingly, their data showed that there are four major stages where energy metabolism is different: the neonatal period (1-12 months), childhood and adolescence (1-20 years), adulthood (20-60 years) and older adulthood (over 60 years old).
Their results showed that throughout the first month of life the neonatal group had energy expenditure values that were very similar to that of adults, however these values rapidly increase over the next 11 months and are highest in life around 8-9 months. Interestingly, energy expenditure values steadily decline throughout childhood and adolescence long before growth and development have finished, and eventually plateau around 20 years of age. Contrary to our previous assumptions, average energy expenditures stabilize just as we enter adulthood (20-60 years old), and only begin to decline in older adulthood around the age of 70-75 years old.
The timelines in which metabolic expenditures are highest is much earlier than previously thought, and researchers have attributed these to the rapid growth phase that is required during infancy to support survival. The reduced metabolic demand likely reflects organ-level metabolic decline, with most changes occurring in the childhood and adolescent years rather than adulthood. These findings provide healthcare workers and researchers with a tool to better deliver healthcare, such as an improved understanding of fluctuations in therapeutic efficacies, wound healing, and the disease kinetics associated with age. As research continues in this field, our ability to treat disease will undoubtedly improve, along with our capability to provide the highest level of healthcare.