The Complete Guide to Metabolism

Spend a few minutes scrolling through fitness social media, and you will find the word 'metabolism' being tossed around everywhere. People blame a 'slow metabolism' for stubborn body fat, credit a 'fast metabolism' for rapid progress, and see it used as a selling point for everything from diet plans to supplements. But for all the attention it gets, metabolism is still often misunderstood and, honestly, oversimplified. Most people talk about metabolism without really knowing what it is or how it works. So, let's clear up the confusion!

What is Metabolism?

Metabolism is the process your body uses to convert the food you eat into usable energy for every single function - from moving your muscles to keeping your brain active and even maintaining your heartbeat while you sleep. In the simplest terms, it's your body's 'engine', running constantly, whether you're sprinting on the treadmill or just binge-watching Netflix.

In fitness and nutrition circles, you'll often hear people talk about metabolism in two main ways: Basal Metabolic Rate (BMR) and Total Daily Energy Expenditure (TDEE). It's important to understand that BMR is actually just one part of your TDEE. Your TDEE is the total number of calories your body burns in a day, and your BMR is the largest contributor to that total - but it's not the only factor.

Basal Metabolic Rate: Your Body's Baseline Energy Burn

Your Basal Metabolic Rate is the number of calories your body needs to keep you alive and functioning at rest. Imagine you lay in bed all day doing nothing: your body would still use energy to power your heart, lungs, brains, and every cell. That energy requirement is your BMR.

BMR is usually responsible for about 60-70% of the calories you burn each day (Hall & Guyton, 2015). For most healthy adults, BMR ranges from about 1,200 to 2,000 calories per day, depending on factors like body size, muscle mass, age, and sex.

Several things determine your BMR:

1. Muscle Mass: Muscle is metabolically active, meaning it burns more calories at rest than fat. Each kilogram of muscle burns about 13 calories per day, compared to just 4.5 calories per kilogram of fat (Wolfe, 2006)

2. Body Size: Larger bodies require more energy to function, so people with higher boy mass have higher BMRs.

3. Age: Contrary to popular belief, metabolism does not decline steadily from your twenties. When scientists control for muscle mass and body size, metabolism stays stable from age 20 to 60. It is only after age 60 that BMR starts to increase, and even then, the drop is usually about 0.7% per year (Pontzer et al, 2021).

   
   

Genetics, 'Slow' vs 'Fast' Metabolism, and Why You're Not Doomed

When it comes to metabolism, genetics play a significnat role in how many calories your body naturally burns at rest and throughout the day. Most of this influence comes down to your Basal Metabolic Rate. People often say they have a 'slow' or fast' metabolism, but what does that actually mean?

A 'fast metabolism' means your body burns more calories at rest and throughout the day. This can happen because you have a naturally higher amount of muscle mass, a larger body size, or simply genetic factors that make your cells use energy more quickly. Some people inherit genes that make their thyroid more active or their mitochondria burn through energy a little faster. This is why they can sometimes eat more without gaining weight.

A 'slow metabolism' means your body burns fewer calories at rest and in daily life. This is often due to a smaller frame, lower muscle mass, or genes that make your cells a bit more energy- efficient. For example, you may have inherited a thyroid that is less active, or your body might just naturally 'run' on fewer calories per day.

Genetic research shows that between 25-40% of the differences in resting metabolic rate between people are due to genetics (Pontzer et al, 2021). That means, if you put two people of the same sex, age, and size side by side, one might naturally burn 200-500 more calories per day at rest just because of inherited factors (Müller et al. 2004). Over time, this difference adds up - but it doesn't mean you're stuck.

It's also important to realise that 'slow' and 'fast' metabolism are not medical conditions - they're just points on a normal spectrum of human variation. And while your genetic starting point can influence how easy or difficult it feels to manage your weight, your habits and daily choices - like movement, nutrition, and sleep - still have a huge impact on your overall energy expenditure and results.

So, while you can't change the genes you were born with, you can absolutely make changes that help your metabolism work for you.

TDEE: What Else Impacts Your Total Daily Energy Expenditure?

Remember, your BMR is just the starting point. The rest of your Total Daily Energy Expenditure (TDEE) is made up of factors you can influence more directly - often with bigger day-to-day impact than most people realise.

Thermic Effect of Food (TEF): Calories Burned Just by Eating

Every time you eat, your body spends energy digesting, absorbing, and processing your food. This is known as the thermic effect of food, and it typically makes up about 5-10% of your total daily calorie burn (Westerterp, 2004).

Not all foods are equal when it comes to TEF:

• Protein has the highest thermic effect - about 20-30% of protein calories are burned off just to process and use them

• Carbohydrates require 5-10% of their calories for digestion

• Fats use the least, at only 0-3%

This means that, for example, eating 100 calories of chicken breast might only deliver about 70-80 calories of usable energy to your body, because the rest is spent breaking it down. That's one reason higher-protein diets can give your metabolism a subtle but real boost.

Physical Activity: The True Metabolism Multiplier

Physical activity is where you have the greatest ability to boost your daily calorie burn. It actually breaks down into two important parts: intentional exercise and Non-Exercise Activity Thermogenesis (NEAT)

• Exercise and Intentional movement

  
  

  Intentional movement includes your gym workouts, running, cycling, fitness classes, team sports, or any other planned physical activity. The calories burn from exercise adds up. For example, a 70kg person burns about 112 calories during 30 minutes of moderate weightlifting and about 298 calories running at a steady pace for 30 minutes. Resistance training offers an extra advantage: building muscle increases your BMR, meaning you'll burn more calories even when resting.

  
  

  However, your body is hardwired to defend its current weight and maintain balance - a principle known as homeostasis. When you increase your calorie burn through exercise, your body naturally tries to hold onto its energy stores. This lead to a phenomenon called metabolic compensation. Metabolic compensation means that after you add more exercise, your body may unconsciously adjust in ways that limit weight loss:

• You might start moving less during the rest of the day - fidgeting less, taking fewer steps, or spending more time sitting - to conserve energy.

• You may also experience increased hunger and eat more without realising it, which can replace the calories you burned during your workout

• These changes can reduce the net calorie deficit you create with exercise, making progress slower than you'd expect.

  
  

  It's important to note that fitness trackers and exercise calculators only measure the calories burned during your workout itself - they do not account for the metabolic compensation that happens afterwards. So, if you try to 'eat back' all the calories your device says you've burned, you'll likely overshoot your true calorie needs and slow your progress.

  
  

  Exercise is absolutely fantastic—not only does it support heart health, build muscle, improve mood, and reduce disease risk, but it really does help you burn more calories than doing nothing. Even with your body’s natural compensation, about 50 percent of the calories you burn through formal exercise still “stay burned” and contribute to your calorie deficit and results. It’s just important to remember that the real calorie impact is often less than what trackers or exercise machines suggest, and less than some people might expect. So, keep moving and make exercise a regular part of your life—not just for calorie burning, but for all the physical and mental benefits it brings. Just know that consistency, an overall active lifestyle, and realistic expectations will always serve you best.

• NEAT: Non-Exercise Activity Thermogenesis (NEAT)

  While formal exercise is powerful, one of the most underrates contributors to your total daily calorie burn is Non-Exercise Activity Thermogenesis, or NEAT. This is all the energy you use for everything that isn't intentional exercise, eating, or sleeping.

  
  

  NEAT includes activities like walking around your home, or workplace, doing chores, grocery shopping, standing instead of sitting, playing with your kids, gardening, or even fidgeting and gesturing while you talk. Research shows that differences in NEAT between people with similar body sizes and ages can easily reach 300-500 calories per day, and in some cases even more, depending on how active someone is outside the gym. For many people, NEAT can actually contribute more to their daily calorie burn than time spent at the gym, especially for those with active jobs or lifestyles.

  
  

  What does this look like in real life?

• Taking the stairs instead of the elevator, parking further away and walking, doing yardwork, or just standing up and stretching more frequently can all increase your NEAT.

• Even increasing your step count by 2000 steps per day can burn an extra 80-100 calories. Hitting 10,000 steps per day can add up to 200-400 extra calories burned - without needing an official workout.

  
  

  NEAT is often the 'secret weapon' for people who maintain a healthy weight or lose fat without hours in the gym. Small choices really do add up. Over the course of a week, consistently moving more can create a series deficit!

But Why Is It So Hard to Lose Weight in My 30s and 40s?

Many people notice that losing weight feels a lot tougher as they move through their thirties and forties—and some start gaining weight even if they haven’t changed much about how they eat or move. It’s easy to assume, “My metabolism must just be slow now.” As we've seen, your metabolism doesn't genetically slow down in your thirties and fourties.

What does change for many people is lifestyle. We tend to move less, lose muscle gradually, and face more time sitting due to work or family. Even small reductions in daily movement or strength can quietly lower your calorie burn over time.

Combine that with the fact that eating habits or portion sizes often don’t adjust as the activity drops, and it’s easy for a subtle calorie surplus to creep in, leading to slow weight gain. The good news? Focusing on building or maintaining muscle, moving more throughout the day, and staying consistent with your healthy habits can go a long way to keeping your metabolism working for you—no matter your age.

Conclusion: The Real 'Secret' to a Good Metabolism

If there’s one thing to take away, it’s that your metabolism is not just a number you’re stuck with for life. While genetics set your baseline, your daily habits are what truly shape your results—at any age.

The “secret formula” is not really a secret at all:

• Build and keep muscle: Muscle mass is your metabolic engine. Lifting weights and staying strong helps you burn more calories, even at rest.

• Eat high in protein: Prioritizing protein in your meals not only supports muscle growth and repair but also boosts the calories you burn through digestion.

• Move as much as possible throughout the day: Don’t rely on exercise alone. Stay active with small movements, daily walks, and regular breaks from sitting—every step counts and adds up.

  
  

Forget crash diets and shortcuts. Consistency with these three habits is what keeps your metabolism healthy, your energy high, and your results sustainable—no matter your age or starting point.