ACCORDING TO AN unprecedented analysis of the body’s energy use, the middle-aged spread cannot be blamed on a waning metabolism. This comprehensive study of 6,400 individuals, ranging from eight days old to 95 years, across 29 countries reveals that the metabolism remains “rock solid” throughout mid-life. It peaks at one, remains stable from ages 20 to 60, and gradually declines.
Unraveling the Mysteries of Metabolism
The metabolism is the sum of all the chemical processes that keep the body functioning—the body’s size influences whether it comprises lean muscle or excess fat. Larger bodies require more energy to maintain. Researchers, therefore, adjusted their measurements to account for body size, comparing metabolisms “pound for pound.”
The study, published in the journal Science, identified four distinct phases of metabolic life:
- Birth to age one: Metabolism shifts from matching the mother’s to reaching a lifetime high, 50% above that of adults.
- Age one to 20: A gentle slowdown occurs, with no significant changes during puberty.
- Ages 20 to 60: Metabolism remains constant with no change.
- Age 60 and beyond: A permanent decline begins, with metabolism decreasing yearly, culminating in a 26% reduction by age 90 compared to mid-life levels.Â
“It is a picture we’ve never really seen before, and there are a lot of surprises in it,” said Prof. John Speakman from the University of Aberdeen. “The most surprising thing for me is there is no change throughout adulthood—if you are experiencing mid-life spread, you can no longer blame it on a declining metabolic rate.”
Childhood Metabolism and Lifelong Impact
One of the key revelations was the absence of metabolic surges during puberty or pregnancy and no slowdown around menopause. The high metabolism in the first years of life underscores the critical nature of early development and the severe impact of childhood malnutrition.
“When people talk about metabolism, they think diet and exercise—but it is deeper than that; we are watching your body and cells at work,” explained Prof. Herman Pontzer from Duke University. “They are swamped at one year old, and when we see declines with age, we see your cells stopping working.”
Groundbreaking Methodology
The study utilised “doubly labelled water,” which contains heavier forms of hydrogen and oxygen, allowing researchers to track its movement through the body. This method is costly, necessitating international collaboration to gather data on 6,400 people.
Implications for Medicine
Understanding the shifts in metabolism has significant implications for medicine. Prof. Pontzer suggested it could inform cancer research, indicating how cancers might spread differently as metabolism changes. Adjustments in drug dosages for different life stages might also become feasible. “There is even discussion about whether drugs that modify the metabolism could slow diseases of old age,” he added.
Drs. Rozalyn Anderson and Timothy Rhoads from the University of Wisconsin described the study as “unprecedented” and noted it had led to “important new insights into human metabolism.” They observed, “It cannot be a coincidence that diseases of old age kick in as the metabolism falls.”
The Obesity Epidemic
Prof. Tom Sanders from King’s College London remarked, “Interestingly, they found minimal differences in total energy expenditure between early adult life and middle age—when most adults in developed countries put on weight. These findings would support the view that the obesity epidemic is fuelled by excess food energy intake and not a decline in energy expenditure.”
Dr Soren Brage from the University of Cambridge highlighted the difficulty in measuring total energy expenditure accurately. He stated, “We urgently need to turn our attention not only to the global energy crisis defined by the burning of fossil fuels but also the energy crisis caused by not burning enough calories in our bodies.”
Conclusion
The study provides a paradigm shift in understanding human metabolism, revealing that the commonly held belief that metabolism declines in mid-life is a myth. Instead, it remains stable until around age 60. This insight challenges long-standing assumptions and emphasises the importance of addressing lifestyle factors such as diet and physical activity in combating obesity and related health issues. The findings also open new avenues for medical research, potentially leading to better management of metabolic-related diseases.
