A slowing metabolism is often blamed for weight-gain, especially by those in their twenties and thirties (and forties…and fifties). By contrast, naturally thin people are often said to have a fast metabolism. But what even is a ‘metabolism’? And what does it have to do with controlling your weight?


Simply put, the metabolism is the organic and chemical process that occur inside organisms which support the maintenance of life. In the context of weight loss, the metabolic rate is the pace at which our bodies burn calories (as calories are a measure of energy). However, it’s not a simple measure of ‘X calories per hour’ or ‘X calories per day’. Our TDEE (total daily energy expenditure) is influenced by 3 factors:

1. BMR – basal metabolic rate
2. TEF – Thermic effect of food
3. TEPA – Thermic effect of physical activity


BMR, also known as resting metabolic rate, is the amount of energy the body uses to maintain and support the functions of the organs and physiological system when fasting and completely at rest. In essence, it’s the minimum energy necessary to stay alive!
Our BMR accounts for about 60-75% of TDEE. The liver, brain and skeletal muscles are heavy calorie users and are responsible for burning the most energy when we are at rest. BMRs vary from person-to-person and are affected by variables including height, weight and gender. To increase our BMR, so that we burn more calories while at rest, we need to build more muscle mass through resistance training.

The importance of muscle mass to BMR is at the centre of the claim that our metabolism slows as we age. The majority of this slowdown is due to changes in body composition – we lose 0.5-1% muscle mass every year after the age of 35. On top of this natural loss, lifestyle factors impact our bodies too, as we tend to lead more sedentary lifestyles and therefore use less energy. And let’s not forget the role of our diet. Often people change their activity levels but don’t adjust their food intake to match. Or they get into bad eating habits that snowball over time.

A combination of these controllable factors is why we gain weight as we get older. This is good news! It means that weight training, good nutrition and living an overall healthy and active lifestyle will remedy that expanding waistline. It’s the lack of exercise and calorie surplus that make you fat, not the number of candles on your birthday cake!


The calories content of food is a measure of the energy that it contains; however, this energy is not readily available. Your body must digest food and convert it into a useable product before this can be absorbed and used. This process uses energy and is known as the TEF. It varies depending on the macronutrient composition of the food.
The macronutrients are fats, carbohydrates and protein. Fats have the lowest TEF, at around 5% of their total energy content; carbs come in at 5-15%; and protein uses the most, at 20-35%. In addition to having a higher TEF, the longer a macronutrient takes to be digested into a useable form of energy, the longer you feel full. This is why a protein rich meal satisfies you longer than a carb-heavy one.

The TEF explains why we should look past the calorie label when making food choices. Being mindful of what we eat is important because this can have a major impact on our net calorie intake at the end of the day.


Given that the TEF measures how much energy you need to convert food into useable energy, it should be easy to understand that the TEPA is the energy expended when we engage in physical activity. This isn’t limited to time spent on the treadmill, it includes all forms of movement and even periods of inactivity! Why does ‘doing nothing’ burn calories? Because, even when you’re perfectly still, there’s a whirlwind of processes occurring inside of your body that require energy to keep going.
The TEPA is linked to body size, as obviously it takes more energy to move a heavier load.

TEPA can be divided into two different types of activities: planned exercise and spontaneous non-exercise activities. These spontaneous non-exercise activities help us increase our NEAT (non exercise activity thermogenesis). NEAT is the energy expended for everything we do that is not sleeping, eating or sports-like exercise. It ranges from the energy expended shivering, fidgeting, standing up from a seated position, running to catch the bus or cleaning our home.

These activities help us burn extra calories throughout the day without us realising. Simple easy ways to increase our NEAT can be getting more steps in by walking work, use cycling as your transportation, play with your kids, get more house work done – gardening, cleaning, hoovering or simply stand more!

Planned trips to the gym can burn a high amount of calories in one go, but never underestimate the power of those spontaneous non-exercise activities which can play a significant role in helping to maximize the total amount of calories burned in a single day.


So, there you have it. Your metabolism isn’t a simplistic measure of calories burnt per hour. It’s a complex matrix that’s affected by multiple factors. The good news is that most of these can be influenced by the lifestyle decisions you make, meaning that there is no cursed ‘slow metabolism’.

If you want to kick your metabolism into gear, increase your strength training to build muscle mass, address any calorie surplus in your diet, look at the types of foods you’re eating, and really move your body regularly.

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