It is a common belief about metabolism that people gain weight year after year after the age of 20 because their metabolism slows down, especially in middle age. Women have a slower metabolism than men, which makes it harder for them to control their weight. Menopause worsens the situation by further slowing down women’s metabolism.
As it turns out, none of this is true. According to a new and comprehensive analysis of the body’s use of energy, published in the journal Science, the slowing of metabolism throughout life is not directly related to age. A study of 6,400 people in 29 countries, ranging in age from eight days to 95 years, shows that a person’s metabolism does not change during the average lifespan. It peaks at one year of age, remains stable from 20 to 60 years of age, and then declines steadily.
These discoveries make us rethink our understanding of the body. The research is likely to change our understanding of human physiology and may have implications for medicine, for example, in determining the appropriate dosage of medication for children and the elderly.
Metabolism is a chemical process necessary to maintain the viability of an organism. And the larger the size of that organism, whether it’s a giant biceps or an excess of belly fat, the more energy it requires. Therefore, the researchers adjusted their measurements for height in order to compare the metabolism of people with different body types and heights.
The study, published in the journal Science, identified four stages of metabolism throughout life:
As expected, this formula works on average for the entire population, but there are individual differences. Some people’s metabolic rate is 25% lower than the average for their age, while others’ metabolic rate is a quarter higher than expected. However, these boundary values do not change the overall picture shown in the graphs that show the trajectory of the metabolic rate throughout life.
“This is a picture we have never seen before and it is full of surprises,” says one of the researchers, Professor John Spikman of the University of Aberdeen. “What surprised me most is that there are no changes in adulthood – if you ‘fall apart’ in old age, you can no longer blame it on a decline in metabolism.” It was also strange that the researchers did not find it. There was no metabolic surge during sexual maturation or pregnancy, nor was there a slowdown during menopause.
A high metabolism in the early years of life also emphasizes how important this moment is for development and why the consequences of malnutrition in childhood can last a lifetime. “When people talk about metabolism, they mean diet and exercise – but it’s really about the functioning of your body, your cells,” said Professor Herman Pontzer of Duke University, according to the BBC. “At one year old, they are incredibly busy working, and when we see a decline with age, we see that the cells stop working.”
One of the discoveries that surprised Ponce the most was the metabolism of infants. He expected, for example, that a newborn baby would have an incredibly high metabolism – after all, biologists know that small animals burn calories faster than larger ones.
But it turns out that during the first month of life, babies have the same metabolic rate as their mothers. But shortly after the baby is born, something triggers the baby’s metabolism to increase dramatically.
Scientists also expected to find a slowing of the metabolism in adults around the age of 40 or, in women, with the onset of menopause. However, as Dr. Ponzer, the author of the study, explained, the researchers simply did not observe this.
People’s metabolism was measured using so-called double-labeled water, which consists of heavier isotopes of hydrogen and oxygen atoms. Scientists measured the amount of calories burned by tracking the amount of carbon dioxide exhaled by the participants during everyday activities. But such water is very expensive, so it took a concerted effort by researchers from 29 countries to collect data from 6400 participants in the experiment.
The researchers said a full understanding of the age-related changes in metabolism could be valuable in medicine. Professor Ponser believes that this will help to identify the relationship between the spread of cancer and changes in metabolism, as well as the need to adjust the dosage of drugs at different stages. Scientists also speculate that drugs that affect metabolism may slow the development of disease in the elderly.
Rosalin Anderson and Timothy Rhodes of the University of Wisconsin stated that this study has already led to new important discoveries in the field of human metabolism, and it cannot be considered a coincidence that age-related diseases appear and develop precisely at the moment when the level of metabolism decreases. Around the age of 60, no matter how young people may appear, they undergo radical changes. “There’s a myth about eternal youth,” Anderson says. “But biology disagrees. Around the age of 60, everything starts to change. There comes a moment when everything is not what it used to be.”
Professor Tom Sanders of the Royal College of London said: “It is interesting to note that very small differences in total energy expenditure were found between young and middle age – a time when most adults in developed countries typically gain weight. These data support the view that the obesity epidemic is driven by excessive food consumption rather than a decrease in human energy expenditure.”
According to Dr. Samuel Klein, director of the Human Nutrition Center at Washington University School of Medicine in St. Louis, the energy requirements of the heart, liver, kidneys and brain account for 65% of total resting metabolism, even though they make up only 5% of body mass. He added that a slower metabolism after the age of 60 may mean that people start to function worse as they age. This may be one of the reasons why chronic diseases are more common in older people. He added, “I don’t think any new clinical conclusions can be drawn for individual people. According to him, when it comes to weight gain, the problem remains the same: people consume more calories than they burn. In general, says Klein (who was not involved in the study), the research has limited value for public health and understanding of diet and nutrition because it provides a ‘bird’s eye view of energy exchange.’