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Your metabolism is not “broken” you’ve just had some lifestyle changes

In the world of fitness numbers don’t lie. Whether you're trying to lose some weight, increase the width of your chest or add a plate to your back squat, quantitative measures of performance are the best indicators of progress, period. Secondly, despite what we might like, progress in fitness isn’t linear. Changes in external factors happening in a client's life as well as the change in rate of physiological adaptations can lead to weeks of lesser improvements and even a temporary loss of progress. When you combine these two realities of the fitness industry it becomes apparent that over the course of a program, graphed progress is more likely to resemble a dodgy staircase than a Westfield travelator. All jokes aside, while this makes perfect sense it doesn’t detract from the feelings of inadequacy that can occur when our performance indicators are not aligned with our ambitions.


As a PT, I expect clients to have good weeks and bad weeks and do my best to communicate this to them if ever I see disappointment on their face. Try as I might, I have noticed that people default to putting the blame on a factor outside of their control, in an attempt to absolve themselves from the perceived unsatisfactory result. These excuses - for lack of a better word - often lack strong scientific evidence in support of their legitimacy and are often based on outdated thinking that had once circulated fitness communities but have since been discredited.


To help dispel one these misunderstandings and make a case that often the progress fluctuations are actually the result of factors within an athletes control, this blog is going to unpack the concept of a ‘broken metabolism’ that body recomposition clients like to suggest as the root cause of their progress plateaus.


What is your metabolism?

Your metabolism is the general term used to describe the chemical process that your body uses to convert energy from the foods you ingest. Your metabolism involves two processes, catabolism and anabolism, that are carefully regulated to remain in balance:

  • Catabolism is the process of breaking down food into simpler forms, releasing energy.

  • Anabolism is the process of using this energy to grow and repair cells in the body.


The metabolic processes themselves, as well as non-discretionary bodily functions such as circulating blood, breathing and creating cells require energy. The sum of these functions are known as the basal metabolic rate (BRM), on average the BMR - also known as the resting energy expenditure - accounts for a staggering 70% of your daily energy expenditure. When someone refers to their metabolism as ‘slowing down’ or speeding up’ what they are suggesting is that the metabolic rate of their body has changed and therefore, so has their total energy requirements.


Importantly, metabolic rate is a highly individualised process. Research does not yet understand why individuals with the same body composition and size can have vastly different metabolic rates. This is why it's crucial for everyone to determine their own BMR through trial and error without relying on generic figures.


So is there any merit to thinking that your metabolism is slowing down?

It shouldn’t come as a surprise to learn that your metabolism is variable and will fluctuate throughout your life. If you take a step back and think about how much food your body required when you were a toddler compared to the peak of your pubescent years this is logical. In fact there are four distinct variables that will impact your metabolism over the course of your life and also lead to day to day fluctuations.

  • Body size and composition - larger people, and specifically those with more muscle mass have greater energy demands both during exercise and resting. This is why bodybuilders who do very little cardiovascular training that require larger amounts of energy still have to eat incredible quantities of food especially when in a weight gaining phase.

  • Gender - men usually burn more kilojoules than women of the same age and weight, this is because of the physiological differences between genders with men often having a larger muscle to fat ratio.

  • Age - It’s a tough pill to swallow but as you get older, your muscle mass begins to slowly decline leading to a decrease in energy requirements. The severity of this is often grossly overestimated though with evidence suggesting the metabolism doesn't significantly decline again until after age 60. This slowdown is a little less than 1 percent a year, with the compounding effects of this amounting to approximately a 25% differential by the time someone reaches their 90’s

  • Diet - The fitness community remains divided on all the ways diet can marginally impact your BMR. What is certain is that different macronutrients have a different thermic effect, meaning that the energy required to undergo the metabolic effects vary between fats, carbs and protein. Protein provides the greatest at approximately 30% compared meaning your net metabolised energy from 100g of protein is less than the same quantity of fat or carbs.

Based on these four factors it's clear that metabolism does change over the course of your life, however you’ll notice that most of these factors change very gradually, with the exception of your diet. Hence, the idea that there can be dramatic changes in your BMR week to week is unrealistic which is why this excuse has been discredited by researchers.


Lifestyle changes are more likely the culprit for weight gain.

In addition to the resting energy expenditure or the BMR each of us also has a non-resting energy expenditure that sums to give our daily total expenditure. It may come as a surprise but a significant component of this is the non-exercise energy thermogenesis that - as the name suggests - accounts for the energy required to complete tasks that don’t constitute exercise such as doing the groceries and playing with your kids. Because these tasks don’t constitute exercises it’s easy to underestimate their importance in the grand scheme of energy requirements when the reality is they account for approximately 20% of the volume. Hence when people have lifestyle changes that affect their routine they can expect to have a different caloric requirement even if they are maintaining regular physical activity.


A timely example that can help illustrate this is the return to a company office across Australia as we reach vaccination targets. For most corporate workers the office life is far more sedentary than working from home with time spent commuting, less incentive to move from your desk in the day and the inevitable social events after work all impacting the non-exercise energy thermogenesis. For this reason it’s important to be mindful of changes in your routine and the likely implications that this can have on your caloric needs.


If you’d like to learn more about energy expenditure and how you can adjust your diet to best align with your training goals, pick up a copy of my nutritional guidelines here or reach out to me for personal training consultation.




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