Recent research has shaken widely held weight loss theories to the core. As it turns out, you won’t be able to blame a slow metabolism for your weight gain anymore.
Remember those heady days of youth when it seemed we could eat anything we want and not gain weight? And now even so much as a glance at a Tim Tam will send the scales in the wrong direction? Our metabolism has taken the brunt of the blame for decades, but a landmark 2021 study* published in the journal Science says metabolism in adulthood does not slow as commonly believed.
‘There are lots of physiological changes that come with growing up and getting older,’ says study co-author Herman Pontzer, associate professor of evolutionary anthropology at Duke University. ‘Think puberty, menopause, other phases of life. What’s weird is that the timing of our “metabolic life stages” doesn’t seem to match those typical milestones.’
Total daily energy 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. Pontzer and an international team of scientists analysed the average calories burned (daily energy expenditure) by more than 6,600 people ranging from one week old to age 95 across 29 countries as they went about their daily lives.
Smashing Metabolism Myths
Analysing energy expenditures across the entire lifespan revealed some surprises. Some people think of their teens and 20s as the age when their calorie-burning potential hits its peak. But the researchers found that infants had the highest metabolic rates of all; a one-year-old burns calories 50% faster for their body size than an adult.
After this initial surge in infancy, the data show that metabolism slows by about 3% each year until we reach our 20s, when it levels off into a new normal.
Despite the teen years being a time of growth spurts, the researchers didn’t see any uptick in daily calorie needs in adolescence after they took body size into account. ‘We really thought puberty would be different and it’s not,’ Pontzer says.
Midlife was another surprise. We’ve been told that it’s all downhill after 30 when it comes to our weight. But while several factors could explain the thickening waistlines that often emerge during our prime working years, the findings suggest that a changing metabolism isn’t one of them.
In fact, the researchers discovered that energy expenditures during these middle decades – our 20s, 30s, 40s and 50s — were the most stable. Even during pregnancy, a woman’s calorie needs were no more or less than expected given her added bulk as the baby grows.
Why 60 is the new 30
The data collected in the study suggest that our metabolisms don’t really start to decline again until after age 60. The slowdown is gradual, only 0.7% a year. But a person in their 90s needs 26% fewer calories each day than someone in midlife.
The findings suggest that other factors lie behind the so-called ‘middle-age spread. Lost muscle mass as we get older may be partly to blame, the researchers say, since muscle burns more calories than fat. But it’s not the whole picture. ‘It’s because [our] cells are slowing down,’ Pontzer says.
The patterns held even when differing activity levels were taken into account. For a long time, what drives shifts in energy expenditure has been difficult to parse because ageing goes hand in hand with so many other changes, Pontzer says. But the research lends support to the idea that it’s more than age-related changes in lifestyle or body composition.
‘All of this points to the conclusion that tissue metabolism, the work that the cells are doing, is changing over the course of the lifespan in ways we haven’t fully appreciated before,’ he says. ‘You really need a big data set like this to get at those questions.’
‘These changes shed light on human development and ageing and should help shape nutrition and health strategies across the life span,’ the authors conclude. CBM
*Daily Energy Expenditure Through the Human Life Course,’ Herman Pontzer, Yosuke Yamada, Hiroyuki Sagayama, et al. Science, Aug. 12, 2021. DOI: 10.1126/science.abe5017