Everyone knows the conventional wisdom about metabolism: people gain weight year after year from the age of 20 because their metabolism slows down, especially in middle age. Women have a slower metabolism than men. This makes it harder for them to control their weight. Menopause only makes things worse and slows women’s metabolism even more.
All wrong, according to a paper published in Science on Thursday. Using data from nearly 6,500 people between the ages of 8 days and 95 years, the researchers discovered that there are four different stages of life when it comes to metabolism. They also found that there were no real differences between men’s and women’s metabolic rates after controlling for other factors.
The results of the research are likely to change the science of human physiology and could also have implications for some medical practices, such as determining appropriate drug doses for children and the elderly.
“It will be in textbooks,” predicted Leanne Redman, an energy balance physiologist at Pennington Biomedical Research Institute in Baton Rouge, La., Who also called it “a critical paper.”
Rozalyn Anderson, a professor of medicine at the University of Wisconsin-Madison who studies aging, wrote a perspective that accompanies the article. In an interview, she said she was “overwhelmed” with the results. “We’re going to have to revise some of our ideas,” she added.
However, the effects of the results on public health, nutrition and nutrition are currently limited as the study offers “a 30,000 foot view of energy metabolism,” said Dr. Samuel Klein, who was not involved in the study and is the director of the Center for Human Nutrition at Washington University School of Medicine in St. Louis. He added, “I don’t think you can make new clinical statements” for a person. When it comes to weight gain, the topic is the same as always: people eat more calories than they burn.
Metabolic research is expensive, and so most published studies had very few participants. But the new study’s lead researcher, Herman Pontzer, an evolutionary anthropologist at Duke University, said the project’s participating researchers agreed to share their data. There are more than 80 co-authors on the study. By combining the efforts of half a dozen laboratories collected over 40 years, they had enough information to ask general questions about changes in metabolism over the course of life.
All of the research centers involved in the project studied metabolic rates using a method that is considered the gold standard – double-labeled water. It measures calorie consumption by measuring the amount of carbon dioxide a person exhales during daily activities.
Investigators also had the participants’ height and weight, as well as body fat percentage, which enabled them to study basic metabolic rates. Of course, a shorter person burns fewer calories than a taller person, but to correct size and fat percentage, the group asked: Was their metabolism different?
“It was really clear that we didn’t have a good overview of how height affects metabolism or how aging affects metabolism,” said Dr. Pontzer. “These are basic fundamentals that you would think would have been answered 100 years ago.”
Central to their results was that the metabolism is different in all people in four different phases of life.
There are infants, up to the age of 1, when calorie burning peaks and accelerates until it is 50 percent above the adult rate.
Then, from the first to the 20th year of life, the metabolism gradually slows down by about 3 percent per year.
It remains stable between the ages of 20 and 60.
And from the age of 60, it drops by around 0.7 percent per year.
After checking people’s height and muscle mass, the researchers found no differences between men and women either.
As would be expected, metabolic rate patterns vary for the population, but individuals do vary. Some have a metabolic rate 25 percent below their average age and others have a metabolic rate 25 percent higher than expected. But these outliers do not change the general pattern that is reflected in graphs that show the course of metabolic rates over the years.
The four stages of metabolic life outlined in the new paper show that “there is no constant energy expenditure per pound,” noted Dr. Redman. The price is age-dependent. This contradicts the longstanding assumptions that she and others have held in nutritional science.
The pathways of metabolism throughout life and the individuals who are outliers will raise a number of research questions. For example, what are the characteristics of people whose metabolism is higher or lower than expected and is it related to obesity?
One of the findings that Dr. Most surprising to Pontzer was the metabolism of infants. For example, he expected a newborn to have sky-high metabolism. After all, the general rule in biology is that smaller animals burn calories faster than larger ones.
Instead, says Dr. Pontzer, babies have the same metabolic rate as their mothers in the first month of life. But shortly after the baby was born, he said, “Something happens and your metabolism picks up.”
The group also expected metabolism to slow down in adults over the age of 40 or in women with the onset of menopause.
But, said Dr. Pontzer, “we just didn’t see that.”
The slowdown in metabolism that begins around the age of 60 leads to a 20 percent drop in metabolic rate by the age of 95.
Dr. Klein said that while people gained more than a pound and a half on average in adulthood, they can no longer attribute it to a slowed metabolism.
The energy needs of the heart, liver, kidneys, and brain make up 65 percent of resting metabolic rate, even though they make up only 5 percent of body weight, said Dr. Small. A slower metabolism after age 60 could mean that important organs function less well as you get older. This could be one reason why chronic diseases are most common in the elderly.
Even college students could see the effects of the metabolic shift around the age of 20, said Dr. Small. “When you finish college, you burn fewer calories than you did when you started.”
And by the age of around 60, young people, no matter what they look like, change fundamentally.
“There is a myth about keeping youth,” said Dr. Anderson. “Biology doesn’t say that. At the age of 60 things start to change. “
“There comes a time when things are no longer the way they used to be.”