The answer to this question was contained in one of the most important studies published in 2021. Herman Pontzer from the Department of Evolutionary Anthropology at Duke University in the United States and colleagues conducted the first ever meta-analysis of precise energy expenditure in humans across the whole lifespan . In total, they studied over 6000 people in 29 countries, starting with babies only 8 days old and finishing with 95-year-olds. All had previously participated in experiments employing two isotopes of water (doubly labelled water), the gold standard method for measuring the amount of energy (calories or kilojoules) spent in daily activities.
They found that total energy expenditure was strongly correlated with the amount of lean body mass (i.e., the weight of your organs, bones, muscles, blood and skin, and everything else which is not fat, but has mass or weight), and yet they detected 4 distinct life stages where it differed. For each kilogram of lean body mass, babies had the highest energy expenditure, which accelerated rapidly after birth to 50% higher than typical adult values at 1 year of age. Energy expenditure then declined slowly during childhood until the age of 20 where it remained stable until 60 years of age. After that it slowed very gradually. As expected, the new study found males had higher total energy expenditure, but sex had no detectable effect on the rate of decline.
So…can you blame a slowing metabolism for incremental weight gain creep during adulthood. Short answer: yes and no. Previous studies on weight gain across adulthood have shown that adults in the age range 22-32 years gained weight more steeply than in any other decade of adulthood . The authors reasoned that twenty-somethings are suddenly more sedentary with new jobs, and often new families, that curtail involvement in sports and exercise. However, the new study showed that total energy expenditure across the age range 20 to 60 years was stable. In fact, childhood was the time where they detected the biggest decline – about 3% per year from the age of 1-2 years to 20-25 years, after which it plateaued. This applied to both total daily as well as resting (basal) energy expenditure.
At around 60 years of age, the new study found that total and basal (expenditure at rest) energy expenditure began to decline more steeply than expected, along with lean tissue and fat mass. In people in their 90s, total energy expenditure was about 25% below that of middle-aged adults. This means that older people are theoretically at risk of weight gain. And yet, the authors found that neither total fat mass nor percentage fat mass increased in this period. This implies that energy intake in the form of food is primarily determined by expenditure of energy, an important observation. If we expend more energy, we automatically eat more. Thus, a deliberate increase in physical activity is not necessarily a recipe for weight loss, but for eating more. However, certain kinds of exercise like lifting weights can increase lean body mass in the form of muscle.
Puberty and pregnancy
Even more unexpected, there appeared to be no increase per unit of lean mass during puberty or pregnancy, times when new tissues are being rapidly synthesised. By the end of pregnancy, the foetus and associated tissues weigh on average about 13-15 kg, in other words, a large proportion of the mother’s starting weight. Nonetheless, when adjusted for body size, total energy expenditure and resting energy expenditure were similar to those of non-pregnant females. Even during the postpartum period when many women produce up to a litre of milk a day, total energy expenditure based on lean body mass and fat mass is similar to that expected of non-lactating females.
Am I average?
Perhaps the most important take-away from the new study by Pontzer and colleagues is that energy expenditure varied markedly from one person to another, even after controlling for lean mass, fat mass, sex and age. Some people were expending 2 and even 3 times as much energy, even at rest. Explaining why this is so is not yet possible – we must await further studies from this group and others. In the meantime, it may well be true to say ‘I have a slow metabolism’. We can only envy those who seem able to eat twice as much as others.
The assumption that individual organs and tissues have the same metabolic rate throughout life is wrong. Babies have the highest rate of tissue and energy expenditure adjusting for body size and yet they are gaining weight rapidly. Elderly individuals show reduced expenditure and yet they tend of lose weight as they age.
- Pontzer, H. and colleagues. Daily energy expenditure through the human life course. Science 2021.
- Allman-Farinelli, M.A. and colleagues. Age, period and birth cohort effects on prevalence of overweight and obesity in Australian adults from 1990 to 2000. European Journal of Clinical Nutrition 2008.
Professor Jennie Brand-Miller holds a Personal Chair in Human Nutrition in the Charles Perkins Centre and the School of Life and Environmental Sciences, at the University of Sydney. She is recognised around the world for her work on carbohydrates and the glycemic index (or GI) of foods, with over 300 scientific publications. Her books about the glycemic index have been bestsellers and made the GI a household word.