Metabolism occurs very rapidly in newborns, slows down from the age of 20, and gradually decreases after 60.
It is often said that to maintain a healthy weight, the calories burned must equal the calories consumed. If you consume more calories (or energy) than you use, you will gain weight. If the calories consumed are less than the calories burned, you will lose weight. However, 55% to 70% of the calories we intake through food are used to fuel the invisible chemical reactions that occur in the body to sustain life, according to Herman Pontzer, an associate professor of evolutionary anthropology at Duke University. He states that calculating total energy expenditure reveals how many calories a person needs to survive and also indicates how the body functions.
Running is one of the activities that helps burn calories. (Photo: Coachmag).
Despite studying metabolism for at least a century, scientists have yet to accurately measure it under real-world conditions with a sufficiently large sample across different ages to examine how this process changes throughout a person’s lifespan. Clearly, larger individuals have more cells, thus burning a higher total number of calories each day. However, assessing whether variations in age, gender, illness, and lifestyle affect energy expenditure is much more challenging.
The lack of data leads to assumptions based on personal experiences. For instance, significant hormonal changes during puberty and menopause can accelerate or slow down metabolism, leading to higher or lower calorie consumption each day. Another assumption is that men have faster metabolisms than women, as they seem to lose weight more easily; or that energy expenditure slows down in middle age, leading to weight gain.
However, an article published by Pontzer and over 80 other authors last month in the journal Science reveals that much of our understanding of metabolism is incorrect. Using data collected from over 6,400 volunteers aged from 8 days to 95 years and adjusting for body size, fat, and muscle, the research team found that metabolism occurs in four distinct stages throughout life. The metabolism of newborns is similar to that of adults. At one month old, a baby’s metabolic rate begins to rise rapidly. By 9 to 15 months, infants have a metabolic rate that is 50% faster than that of adults, equivalent to an adult burning about 4,000 calories per day. According to estimates from the U.S. Department of Health and Human Services, on average, women require 1,600 to 2,400 calories per day, while men need 2,000 to 3,000 calories per day.
Between the ages of 1 and 2, energy expenditure begins to decline and continues to decrease until the age of 20. From that age, the expenditure remains stable for the next 40 years, even throughout pregnancy and menopause. At age 55, you burn calories as efficiently as you did at 25. By age 60, energy expenditure begins to decrease again and continues to decline until the end of life. Researchers also observed that men do not have faster metabolisms than women. Instead, they tend to burn more calories each day, corresponding to their body size due to having a higher muscle mass.
For a long time, researchers could calculate how many calories a person burns by measuring the amount of carbon dioxide exhaled. However, study subjects typically had to be in a laboratory and undergo necessary tests. Therefore, the results only reflect metabolic rates at rest and at a specific point in time. It wasn’t until the 1980s that a method for measuring metabolism in everyday life, known as the doubly labeled water technique, began to be implemented. Volunteers drink water containing two stable, non-radioactive isotopes: deuterium (a stable isotope of hydrogen) and oxygen-18. The dose is absorbed and mixed with water in the body.
Deuterium is expelled from the body only through body water, such as urine or sweat. Meanwhile, oxygen-18 is expelled more quickly through breathing. Scientists collect urine samples before and during the 7 to 14 days following the volunteer’s consumption. They analyze the samples to see how quickly the body eliminates deuterium compared to oxygen-18. This is how the rate of carbon dioxide production in the body is measured. Carbon dioxide is produced when the body generates energy. Therefore, scientists’ measurements of carbon dioxide allow them to calculate and understand the amount of energy a person expends.
Using the doubly labeled water method is very expensive. Only about 9 laboratories in the world regularly apply this technique, according to Jennifer Rood, executive deputy director at the Pennington Biomedical Research Center and co-author of the article. A study conducted using this method often includes fewer than 100 volunteers, which is not enough to identify trends in the population.
However, in 2014, laboratories using the doubly labeled water method came up with the idea of creating a database to compare as many measurement results as possible over the past 40 years. This growing database contains samples from dozens of countries and cultures, from Tanzania to the United States.
The scale and diversity of the samples allowed the research team to identify common trends in how metabolism changes with age. However, there are still significant differences in the metabolic rates of study subjects, demonstrating the important role of many other factors such as genetics and lifestyle. These factors likely contribute to why individuals with similar body weights and habits have very different daily energy expenditures.
The article also raises many questions. For example, how does faster metabolism in children and slower metabolism in the elderly affect nutritional recommendations and medication dosages? What is the relationship between the slowdown in metabolic rate from age 60 and the increase in chronic diseases?
In fact, many medications currently in use affect metabolism in humans and increase lifespan in mice. The research team hypothesizes that aging cells use less energy because they perform fewer activities that help prevent disease. Understanding when metabolic rates change will help researchers gain a clearer understanding of the concept of health at every age.
