Piling on the pounds in middle age is an affliction familiar to many, but the common excuse of a slowing metabolism is no longer a valid excuse, according to a new study.
Instead, the dreaded middle-age spread is likely down to lifestyle factors such as exercise and diet.
An international team of researchers, led by Duke University in North Carolina, analysed the speed of a person's metabolism throughout their life.
Data show that when a baby is born, they use the same amount of energy as adults when accounting for the difference in body size and mass.
However, their metabolism almost instantly goes into hyperdrive, becoming 50 per cent faster by the time they are one year old.
"Something is happening inside a baby's cells to make them more active, and we don't know what those processes are yet," said Herman Pontzer, co-author of the study and associate professor of evolutionary anthropology at Duke University.
This intense pace continues until a child is two, but it then slows by about three per cent a year. The gradual fall continues until adulthood and stabilises around 20 years old.
This finding surprised scientists as it flies in the face of common wisdom, which states that a person's metabolism speeds up during adolescence.
Myth of the metabolic go-slow debunked
However, it was not the only unexpected result of the landmark study as it also revealed metabolism remains steady between 20 and 60 years old.
Contrary to what many people think - and tell themselves when they look in the mirror - there is no metabolism go-slow as a person progresses from their 30s to 50s. Metabolism speed was found to decline only when a person reaches 60.
More than 6,600 people from 29 countries took part in the study, with individuals ranging in age from one week to 95 years old.
Participants drank special water which consisted of "heavy" isotopes of hydrogen and oxygen that can be tracked to see how swiftly these are flushed out a person's body.
This allowed scientists to precisely gauge a person's metabolic speed, and takes into account energy consumed by all bodily processes, from breathing and digesting to working out and walking the dog.
This technique is tried and tested, and decades of data from various institutions was pooled for the study, published in the journal Science. As a result, this study is the most thorough analysis ever done of metabolic speed throughout life.
"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," said Dr Pontzer.
Dr Timothy Rhoads, from the department of medicine at the University of Wisconsin-Madison, was not involved in the study and wrote an article on the new findings with Dr Rozalyn Anderson, from the same university.
"The decline from age 60 is thought to reflect a change in tissue-specific metabolism, the energy expended on maintenance," they write.
"It cannot be a coincidence that the increase in incidence of noncommunicable diseases and disorders begins in this same timeframe."
Noncommunicable diseases are conditions that cannot be passed from one person to another, such as strokes, heart disease, diabetes and dementia. They become more common with age and account for around 70 per cent of all deaths, according to the World Health Organisation.
