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

What is metabolism?

Metabolism is the process by which your body converts what you eat
and drink into energy. During this complex process, calories in food and beverages are combined with oxygen to release the energy your body needs to function.

Even at rest, your body needs energy for all its ‘hidden’ functions to keep you alive and your organs functioning normally, such as breathing, circulating blood, adjusting hormone levels, growing and repairing cells, and digesting food.

The minimum amount of energy your body requires to perform these chemical processes is called the basal metabolic rate (BMR).

Your individual basal metabolic rate, or metabolism, is determined by your body size and composition, sex and muscle composition. In addition to your BMR, thermogenesis (food processing) and physical activity determine how many calories your body expends, or burns, each day.

People who struggle to lose weight often blame a slow metabolism. But there’s actually little evidence to support this claim.

Conversely, research shows that overweight people have faster metabolisms than thinner people. Larger bodies require more energy to carry out basic bodily functions. More often than not, the reason you’re putting on weight is not because of a slow metabolism; it’s because you’re eating and drinking more calories than you’re burning.

The 4 stages of metabolism

  1. From one week old up until age 1, your metabolism is at its highest level, accelerating until it is 50% above the adult rate.
  2. From age 1 to 20, metabolism slows down gradually at a rate of around 3% per year.
  3. Between ages 20 to 60, your metabolism remains relatively stable.
  4. From age 60 onwards, it slows down by about 0.7% a year.
Aimee Rodrigues
Senior content writer and strategist with more than 15 years’ experience in health, beauty and lifestyle.

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