Episode 50 – Does Your Metabolism Slow Down As You Age?

Key Topics Covered

This is a pretty complex topic actually with a lot of variables to account for so let’s start with defining ‘metabolism’. 

Metabolism Defined– TDEE vs BMR  

  • Metabolism is often referring to our BMR – Which is the energy required for all basic bodily functions before adding on things like activity and exercise.
  • On the other hand, TDEE – Is how much energy we burn with all things accounted for.
    • As we age, both our BMR and TDEE tend to slow down. BUT, there is actually a lot more nuance to this.

The Argument Against Metabolism Changing Much 

  • The Herman Pontzer study from 2021 explained this well.
  • They found TDEE was stable between ages 20 and 60 when fat-free mass was equated for, with a slow decline after. The decline post 60 was 0.7% annually – likely due to the energy metabolism of organs changing.  
  • Prior to the age of 20, it is also a bit higher due to growth.  
  • It also found men and women had similar TDEE when this was accounted for.  
  • But, the big issue with this study is that it is obviously not representative of what happens when fat-free mass is not equal. Most people lose muscle as they age. Men also will on average have more muscle than women. 

What Does Other Research Show in Regards to a Change? 

There is research looking at the average TDEE in different age groups and it ALSO compares this to the averages once things like muscle mass and activity are accounted for.  

  •  One study compared the RMR of three groups of people: those aged 20–34, 60–74, and over 90. 
    • Compared to the youngest group, people aged 60–74 burned roughly 122 fewer calories at rest, while people over 90 burned around 422 fewer calories.  
    • However, after accounting for differences in gender, muscle, and fat, they found that the people aged 60–74 burned only 24 fewer calories, while those over 90 burned 53 fewer calories on average daily.  
    • These small changes can be accounted for by a decrease in calories burned through internal organs and mitochondrial efficiency. 
  • Another study followed 516 older adults aged 60 plus for 12 years to see how much their metabolism fell per decade.  
    • After accounting for muscle and fat differences, per decade, women burned 20 fewer calories at rest, while men burned 70 fewer calories.  
    • Interestingly, both men and women were also less active and burned 115 fewer calories through activity per decade.  
    • This adds to the importance of staying active.  
  • So based on that – maintaining muscle mass and activity levels is super important if you want to maintain your metabolism and TDEE as you age  

Why Do Calorie Requirements Decrease even When Muscle Mass is Accounted For?

  • There are multiple answers to this question.
  • However, one of the main ones is a reduction in mitochondrial efficiency.
  • Meaning, that if organs aren’t functioning as well, they’re probably using fewer calories.

Other Thoughts 

How Does Menopause Affect Things?

  • Firstly, changes in body fat distribution.
  • Prior to menopause, body fat may have been more evenly distributed around the body. Then as estrogen decreases with menopause, body fat may be more central to the stomach. Now individuals probably don’t feel as lean.
  • Likely to be exercising less.
  • Sleep – Those who get poor sleep tend to crave higher-calorie foods.
  • Women’s TDEE tends to be lower than males anyway.

Test Your Metabolism

  • You can test your REE if interested in confirming whether it is lower.
  • The machine is called indirect calorimetry.
  • This helps you establish your base rate, this can ensure clear and specific measures can be taken to help you achieve your body composition goal.

A lot of changes occur throughout life 

  • Work + stress + sedentary nature vs walking everywhere + literally being able to eat any time you are hungry + alcohol + lack of sleep + caffeinated beverages containing calories + body composition e.g. likely less muscle 
  • Staying/keeping active + building/maintaining muscle is important for keeping TDEE high 

Relevant Links/Resources

Studies Mentioned

Useful Resources

Related Blog Posts