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How Metabolism Changes as we Age
A slower metabolism may not be causing your middle-age spread.

How fast is your metabolism?
How fast is your metabolism?

Metabolism is a confusing, highly misunderstood thing. To know how it impacts you, it’s important to understand precisely what it is. According to the Oxford Dictionaries, it’s “The chemical processes that occur within a living organism in order to maintain life:”

Your body does things like converting food into energy, proteins and nucleic acids. Your body also has to eliminate waste. Those chemical processes burn calories. The more you got going on, the more calories your body burns and the higher your metabolism is.

In the traditional view, it was believed that metabolism continued to increase until you reached your mid-20s, after which it would begin a slow decline. A groundbreaking new study shows a lot of our long-held assumptions were wrong.

Eighty-two researchers in 29 countries collected data on 6,421 subjects over 40 years. The study is called “Daily energy expenditure through the human life course.” It was released on August 13 of 2021, in the journal Science.

Researchers defined four distinct metabolic periods.

Phase One is Newborns up to the age of 1. Pound for pound, infants were discovered to have the highest metabolic rate of any age group. They burn through calories at a rate 50% faster for their body size than adults. This helps explain why infants run into difficulty so quickly, if they have what adults might consider a minor nutritional lapse or interruption.

Phase Two is Juveniles from 1 to 20 years of age. After the initial first year burst, metabolism continues to increase each year. However, children and teenagers put on a lot of body mass as they grow up. So their metabolism, after taking body size into consideration, drops by about 2.8 % a year, until they reach the age of 20.

Researchers also found that when teenagers experienced growth spurts, they didn’t have increases in metabolism that weren’t tied to their greater body mass. Even puberty, evident in subjects between 10 and 15 years of age, failed to lead to “increases in adjusted [metabolism].”

Phase Three is adulthood, from 20 to 60 years of age. This was the most stable period, with few changes happening. Surprisingly, pregnancy failed to increase “basal expenditures” or metabolism beyond what would be expected from the mother’s gain in weight.

Before this study, it was believed that metabolism would begin a slow decline after peaking in the mid-20s. People often blamed middle-age spread on this problem.

Your metabolism may not be making you fat. It could be the extra crispy chicken.
Your metabolism may not be making you fat. It could be the extra crispy chicken.

The reality is more complicated. Several factors contribute to expanding waistlines that have nothing to do with metabolism. Injuries sustained while someone is young prevents them from being as active as they age. Even without injuries, older people tend to move less, so they burn fewer calories. As income levels rise, access to food increases, becoming another potential source of weight gain.

It’s also a cumulative problem. If you gain just 2 pounds a year, that’s 60 pounds by the time you’re 50. Each pound might seem insignificant, but over time the final result can be dramatic.

Your metabolism isn’t failing you. If you’re putting on weight between your 20s and 60s, you’re overeating and moving too little.

Phase Four is Adults over the age of 60. At this point, researchers found metabolism did start a gradual decline. It wasn’t much, just 0.7% a year. But over time, that trend can lead to severe outcomes. By the time you’re 80, you can expect a 14% decline in metabolism. Subjects who were 90 years old or more saw their metabolism drop “26% below that of middle-aged adults.”

Another surprise came as researchers compared men and women. After factoring in body size and muscle mass, they found no noticeable differences in the metabolisms between men and women. Researchers didn’t find racial differences either.

Muscle loss in people over the age of 60 was thought to be part of the cause. However, researchers calculated muscle mass (and the loss of muscle) into their equations. A principal reason for the decline is simply that people’s cells slow down as we age.

With this study, we now know that it’s not metabolism that’s a problem for most people. It’s what we eat, drink and how much we move.

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