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Surprising findings about metabolism in the different stages of our lives

At what age does metabolism peak? When and how fast does it start to slow down? How does it affect our weight?

According to recent findings, metabolism peaks much earlier and starts to slow down much later than previously thought.


Before we go into the details, let’s clarify some terms.


What is metabolism?

Metabolism is the chemical processes in our bodies that convert food and drink into energy.

This energy fuels all the body’s functions – thinking, breathing, circulation, maintaining body temperature, growth and regeneration of cells, digestion…

The minimum amount of energy (calories) the body needs to support these functions is called the Basal Metabolic Rate (BMR). Basal metabolic rate is influenced by factors such as – age, sex, muscle mass and physical activity.

Basal metabolic rate (BMR) is often used as a synonym for Resting Metabolic Rate (RMR).

However, they are different:

  • BMR is the minimum number of calories required to perform basic functions at rest.


  • RMR is the number of calories the body actually burns at rest.

These two values usually differ by about 10%.

You can calculate your basal metabolic rate HERE.


How does metabolism affect our weight?

Slow metabolism is often cited as the main cause of weight gain.

However, metabolism naturally regulates to meet the body’s needs and is rarely the cause of weight gain or loss.

To lose weight, you need to take in fewer calories (energy) from food than your body uses. This has nothing to do with your metabolic rate – how much energy your body uses.


What is the difference between a fast and a slow metabolism?

A person with a fast metabolism or fast BMR burns a lot of calories, even at rest, but with a slow metabolism, the body needs fewer calories to sustain itself.

In other words, fast metabolisers may eat more to maintain a constant weight because their body needs more calories (energy).

A fast metabolism does not necessarily mean a slim body. In fact, research shows that overweight and obese people often have a fast metabolism.


And now about the study itself.


Daily energy consumption throughout a human life course

The study involved 6,421 people (64% women) aged between 8 days and 95 years from 29 countries.

Researchers calculated Total Energy Expenditure (TEE) for all participants by the doubly labelled water method, the gold standard for measuring energy consumption.

Body size, age and differences in reproductive status were also considered.

The results showed that we go through four phases of energy expenditure in our lifetime:

  1. Newborns (1 month to 1 year). In the first month of life, newborns had size-adjusted energy expenditure similar to that of adults. During the first year, energy expenditure increased rapidly, reaching a peak at 7 months of age. Between 9 and 15 months of age, adjusted energy consumption was almost 50% higher than for adults.
  1. Childhood and adolescence (1 to 20 years). While total and basal energy expenditure, as well as fat-free mass, continued to increase as the child grew, adjusted energy expenditure decreased steadily during this period. Gender did not affect the rate of decline, and at the age of 20.5 years, adjusted TEE and RMR reached a plateau similar to adult levels. It should be noted that there was no increase in adjusted total or basal energy expenditure during puberty between 10 and 15 years of age.
  1. Adulthood (20 to 60 years). Between the ages of 20 and 60 years, total and basal energy expenditure and fat-free mass were stable regardless of sex. Adjusted TEE and RMR remained stable even during pregnancy, and any increase in unadjusted energy expenditure during pregnancy was explained by an increase in body mass. Adjusted TEE started to decline at age 63 and adjusted BMR at age 46.5 years (although the researchers note that the small number of BMR measurements reduced their confidence in this estimate).
  1. Older adults (60+ years). TEE and BMR, as well as fat-free mass and fat mass, began to decline around age 60. However, the decrease in energy expenditure was greater than predicted (which would be expected from reduced body mass alone). Adjusted TEE and BMR decreased by 0.7% per year, and the adjusted total energy expenditure for people aged 90 years and older was about 26% lower than for middle-aged adults.

Other results

The study also looked at the effects of physical activity and tissue-specific metabolism on energy expenditure over the lifespan (some organs, such as the brain and liver, are thought to consume more energy than others and to account for a higher percentage of body mass in younger people).

Using different modelling scenarios, they determined that age-related changes in physical activity levels and tissue-specific metabolism contribute to TEE at different ages and specifically:

  • Increased tissue-specific metabolism in early life may be associated with growth or development.

In turn.

  • Reduced energy expenditure in later life may reflect a decrease in metabolism at the organ level.

What are the conclusions?

This study challenges the previously held view that metabolism is very high in infancy, childhood and adolescence and gradually declines in maturity and old age.

Instead, the authors observed that BMR was 30% higher than would be expected based on body and organ composition in children aged 1-20 years and 20% lower than would be expected in adults over 60 years of age.

These childhood and old-age abnormalities in TEE and BMR support the hypothesis that age-related metabolic changes may play a more significant role than we previously thought.

Moreover, these results strongly suggest we cannot blame a slowed metabolism for weight gain in middle age.


Key takeaways

It should be recognised that energy expenditure, or metabolic rate, is different for each of us, and this can affect how our bodies respond to various diets, etc.

In other words, weight management/weight loss strategies/diets designed for everyone may give good results for some and not for others.

The findings of the study do not contradict our current understanding of how to achieve and maintain a healthy weight throughout life. The evidence still overwhelmingly supports:

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