Despite good intentions, many people find themselves gaining weight as they settle into adulthood. One common explanation is that a person's metabolic rate slows down as they move into middle age. However, a recent study published in the journal Science indicates that this isn't true.

Instead, the study found that metabolic activity increases rapidly during the first year of life, then gradually slows down during childhood and adolescence. Metabolic activity then remains surprisingly stable from about age 20 to age 60 and starts to slowly decline from there.

How Does Metabolism Work?

Metabolism includes all the chemical reactions that cells use to produce energy. This energy is required for a heart to beat, muscles to work, neurons to send signals, food to be digested and everything else your body does. When discussing diet and nutrition, people are usually concerned about the metabolic rate, which is how fast your body converts food into energy. People who consume more calories than they use for energy will gain weight, while people who consume fewer calories than they use for energy will lose weight.

While this seems simple, the details are more complex. For example, researchers are still learning about whether sugar makes a person feel hungry or full. At this point in time, scientists also don't have a very good idea of how metabolism changes over the course of a lifetime or even how the body's energy expenditure might change over the course of a day.

Typically, large metabolic studies have measured the metabolic rate at rest, which is done in a lab by analyzing the amount of oxygen and carbon dioxide in a person's breath. However, the basic bodily functions used "at rest" account for just 50 to 70% of the calories burned every day. While additional measurements can be made during exercise, these studies invariably involve a lot of estimates and rely on patients accurately reporting their daily activity.

What's So Special About This Recent Study?

This recent study is notable because it uses an extremely precise method for measuring metabolic activity involving "double-labeled water."

Study participants drank a precisely measured amount of water (H₂O) that had a heavy isotope of hydrogen in some water molecules and a heavy isotope of oxygen in other water molecules. Participants then went about their day as usual, only needing to collect urine samples for later analysis. Both isotopes are depleted through water loss, and heavy oxygen is also lost through the production of carbon dioxide, which is a very precise way of measuring energy expenditure (aka calories burned).

While this method has been used in humans since the 1980s, it is very expensive. Thus, studies involving double-labeled water tend to be small. To get around this problem, researchers from around the world joined forces. This recent study involved more than 80 scientists from 63 institutions in 16 countries across 4 continents. The data set included information on more than 6,600 people in 29 countries, ranging in age from 8 days to 95 years.

Researchers also incorporated published data from newborns and infants and from women before, during and after pregnancy. The data set included information about each person's age, height, weight, sex (male/female), physical activity and "fat-free mass" (which is mostly muscle but also includes bone and connective tissue).

What Was Discovered?

This large data set allowed researchers to tease apart differences among the recorded factors. As expected, adult men tended to burn more calories than adult women. However, when the researchers accounted for differences in body size (with men generally being larger than women) and fat-free mass (with women generally having more fat than men), these differences disappeared: On average, a pound of fat-free mass in an adult male burns the same number of calories per day as a pound of fat-free mass in an adult female.

This was a surprising result. Perhaps even more surprising was how consistent this number was through most of adulthood, from about age 20 to age 60. Thus, the common experience of "middle-age spread" can't be blamed on decreased metabolic activity. Changes in the amount of fat-free mass — often due to decreased physical activity and dietary choices — are more likely culprits.

What About Different Life Stages?

After accounting for differences in body size and fat-free mass, the study found four distinct life stages for metabolism:

Newborns (0-1 years old)
Juveniles (1-20 years old)
Adults (20-60 years old)
Older adults (over 60 years old)

Newborn infants start with a metabolic rate very similar to that of adults. That rate rapidly increases and is nearly 150% of the adult rate by age 1. From age 1 to age 20, the rate of metabolic activity declines by 2.8% per year, reaching adult levels around age 20. No metabolic changes were observed during puberty (age 10 to 15).

The metabolic rate remains stable from age 20 to 60, even through pregnancy and menopause in women. At about age 60, the metabolic rate starts to decline by 0.7% per year. This difference remains even after accounting for declines in fat-free mass and physical activity, which suggests that cells are slowing down as individuals age.

Why Are These Differences Important?

These findings highlight the importance of nutrition in infants and children. A child's brain is disproportionately larger than an adult brain and consumes more energy per pound than an adult brain, as the Proceedings of the National Academy of Sciences reports. Childhood is obviously a crucial time for learning and development.

In both children and older adults, the differences in metabolic rate can also affect how the body breaks down medication. In older adults, the decline in metabolic rate coincides with an increased risk for chronic diseases, including cancer, cardiovascular disease and neurodegenerative disease.

In commentary published alongside the original study in Science, two specialists in geriatric research wrote, "The decline from age 60 is thought to reflect a change in tissue-specific metabolism, the energy expended on maintenance. It cannot be a coincidence that the increase in incidence of noncommunicable diseases and disorders begins in this same time frame."

While the study identified patterns that apply to the general population, there was considerable variability among individuals. Some study participants had metabolic rates that were 25% higher than the average for their age, while others had metabolic rates that were 25% lower.

This study raises many new questions, which is quite exciting. It will ultimately help researchers answer the question "How does metabolism work over a lifetime?" and better understand individual differences.

Check out Northrop Grumman career opportunities to see how you can participate in this fascinating time of discovery in science, technology and engineering.

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