Everyone knows conventional wisdom about metabolism: people put on weight year after year from their 20s onward because their metabolisms slow down, especially in middle age. Women have slower metabolisms than men. That’s why they have a harder time controlling their weight. Menopause only makes things worse, slowing women’s metabolisms even more.
All wrong, according to a paper published on Thursday in Science. Using data from nearly 6,500 people, ranging in age from eight days to 95 years, researchers discovered that there are four distinct periods of life, as far as metabolism goes. They also found that there are no real differences between the metabolic rates of men and women after controlling for other factors.
The findings from the research are likely to reshape the science of human physiology and could also have implications for some medical practices, like determining appropriate drug doses for children and older people.
These are basic fundamental things you'd think would have been answered 100 years ago
"It will be in textbooks," predicts Leanne Redman, an energy balance physiologist at Pennington Biomedical Research Centre in Baton Rouge, Louisiana, who also calls it "a pivotal paper".
Rozalyn Anderson, a professor of medicine at the University of Wisconsin-Madison, who studies ageing, wrote a perspective accompanying the paper. In an interview, she said she was “blown away” by its findings. “We will have to revise some of our ideas,” she added.
But the findings' implications for public health, diet and nutrition are limited for the moment because the study gives "a 30,000ft view of energy metabolism", says Dr Samuel Klein, who was not involved in the study and is director of the Centre for Human Nutrition at the Washington University School of Medicine in St Louis. He adds: "I don't think you can make any new clinical statements" for an individual. When it comes to weight gain, he says, the issue is the same as it has always been: people are eating more calories than they are burning.
Metabolic research is expensive, and so most published studies have had very few participants. The new study's principal investigator, Herman Pontzer, an evolutionary anthropologist at Duke University, said that the project's participating researchers agreed to share their data. There are more than 80 co-authors on the study. By combining efforts from six labs collected over 40 years, they had sufficient information to ask general questions about changes in metabolism over a lifetime.
All of the research centres involved in the project were studying metabolic rates with a method considered the gold standard – doubly labelled water. It involves measuring calories burned by tracking the amount of carbon dioxide a person exhales during daily activities.
The investigators also had participants’ heights and weights and per cent body fat, which allowed them to look at fundamental metabolic rates. A smaller person will burn fewer calories than a bigger person, of course, but correcting for size and per cent fat, the group asked: were their metabolisms different?
“It was really clear that we didn’t have a good handle on how body size affects metabolism or how ageing affects metabolism,” Pontzer says. “These are basic fundamental things you’d think would have been answered 100 years ago.”
Central to their findings was that for all people metabolism differs across four distinct stages of life:
– There’s infancy, up until the age of one, when calorie burning is at its peak, accelerating until it is 50 per cent above the adult rate;
– Then, from age one to about age 20, metabolism gradually slows by about 3 per cent a year;
– From age 20 to 60, it holds steady,
– And, after age 60, it declines by about 0.7 per cent a year.
Once the researchers controlled for body size and the amount of muscle people have, they also found no differences between men and women.
At age 60, no matter how young people look, they are changing in a fundamental way
As might be expected, while the metabolic rate patterns hold for the population, individuals vary. Some have metabolic rates 25 per cent below the average for their age and others have rates 25 per cent higher than expected. But these outliers do not change the general pattern, reflected in graphs showing trajectory of metabolic rates over the years.
The four periods of metabolic life depicted in the new paper show “there isn’t a constant rate of energy expenditure per pound”, Redman notes. The rate depends on age. That runs counter to the long-standing assumptions she and others in nutrition science held.
The research group also expected the metabolism of adults to start slowing when they were in their 40s or, for women, with the onset of menopause.
But, Pontzer says: “We just didn’t see that.”
The metabolic slowing that starts at about age 60 results in a 20 per cent decline in the metabolic rate by age 95.
Dr Klein said that although people gain on average more than 1½ pounds (that is, more than 0.6kg) each year during adulthood, they can no longer attribute it to slowing metabolisms.
And at age 60, no matter how young people look, they are changing in a fundamental way.
“There is a myth of retaining youth,” Prof Anderson says. “That’s not what the biology says. In and around age 60, things start to change.
“There is a time point when things are no longer as they used to be.” – New York Times
