Human metabolism revisited, from birth to old age

How does our energy expenditure vary with age? The response, published in the journal Science on August 12, was measured in 6,421 people aged 8 days to 95 years – 64% of them women – from twenty-nine countries.

It reveals several surprises. Did you think that puberty, thirties or forties and menopause marked so many breaks in our basic energy expenditure? Well no. "It may seem strange, but the calendar of the different metabolic periods of our life does not coincide with these major stages of existence", summarizes Herman Pontzer, of Duke University, Durham (United States). He is the first author of this international study coordinated by John Speakman, of the University of Aberdeen (United Kingdom).

The researchers used a reference technique – reliable, precise and non-invasive – used since the 1980s to measure human energy expenditure in daily life situations. This is the “double-labeled water method”. The principle: the person ingests a dose of water enriched with two stable (non-radioactive) isotopes, deuterium ( ^2H ) and oxygen ¹⁸ (18O). On urine samples taken regularly for one to three weeks, the elimination kinetics of these two isotopes are measured. This makes it possible to calculate the quantity of carbon dioxide (CO ₂ ) produced, from which the energy expenditure is deducted. This method, however, requires a tracer and time-consuming and expensive mass spectrometry analyses. Hence the interest of this pooling of data collected by half a dozen laboratories over the past forty years.

Very spendthrift babies

Their analysis draws a four-step model, once the data is averaged and adjusted according to the lean body mass of each.

Early childhood, first. For the same lean mass, toddlers have the maximum energy expenditure – which skyrockets during the first year of life. Between the age of 9 and 13 months, they are 50% higher than those of adults, always at equal lean mass. Is it because children triple their birth weight in their first year of life? Not only. “Processes occur inside a baby's cells that make them more active – the nature of which we don't know,” observes Herman Pontzer.

This very energy-intensive metabolism would partly explain why nutritional deficiencies, during this key period, lead to severe growth disorders that can be life-threatening.

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