What Is Metabolism (Really), and Why It Might Be Holding You Back

Supplementing for a Sluggish Metabolism — Part 1

You’re eating clean, training hard, even skipping that cheeky midweek pint — but the scale still isn’t budging. Sound familiar? It might not be a willpower problem — it could be your metabolism working against you.

Let’s clear something up: metabolism isn’t just a vague excuse for muffin tops or midday slumps. It’s your body’s internal engine — converting food into energy, powering every breath, thought, workout, and repair process. But when that engine starts running low, your progress stalls, your energy dips, and your health goals get stuck in idle.

In this blog, we’re diving deep into what metabolism actually is, why it sometimes slows down, and most importantly — which supplements are actually worth your money (and which are better left on the shelf). Think of it as your no-nonsense, science-backed guide to giving your metabolism a bit of a nudge — or in some cases, a full-on kick up the arse.

What Is Metabolism, Anyway?

Metabolism is your body’s internal power station — the complex network of chemical processes that keep you alive, functioning, and (ideally) thriving. Every time you breathe, think, digest food, repair tissue, or balance a hormone, your body is “spending” energy. That energy comes from the calories you eat — and how efficiently your body converts those calories into fuel is what we call metabolism.

In simpler terms, metabolism is the process of turning food into usable energy — but it’s far more than just calorie burn.

Quick Note:
When most people talk about metabolism, they’re referring to how the body burns energy or maintains weight. But scientifically, metabolism refers to all the chemical reactions happening in your body — including processes like bone metabolism, hormone metabolism, and even detoxification through the liver.
In this series, we're focusing on energy metabolism — the kind that affects your mood, energy, weight, workouts, and hormones.

The Core of Your Metabolism: Basal Metabolic Rate (BMR)

A large portion of your metabolism is accounted for by your Basal Metabolic Rate (BMR) — the amount of energy your body needs at complete rest to carry out essential functions like breathing, pumping blood, regulating body temperature, and regenerating cells.

According to research (Mifflin et al., 1990; Hall et al., 2011), BMR accounts for 60–75% of your total daily calorie burn. So even when you’re not moving, your body is working hard to keep you alive — and that work takes fuel.

Factors That Influence Your BMR:

• Age: Metabolic rate generally declines over time, largely due to hormonal shifts and loss of lean muscle.
  
• Genetics: Some people are simply born with a higher or lower metabolic baseline.
  
• Hormones: Thyroid hormones (T3 and T4), cortisol, insulin, and sex hormones all influence metabolic speed and efficiency.
  
• Muscle Mass: Muscle is metabolically active tissue. The more you have, the more energy your body burns — even at rest.
  

BMR Is the Foundation — But It’s Not the Full Picture

To get a complete view of how many calories your body burns each day, we look at Total Daily Energy Expenditure (TDEE), which includes:

• BMR – Your baseline energy usage at rest
  
• Physical Activity – All intentional exercise
  
• Thermic Effect of Food (TEF) – The energy it takes to digest and process food (usually about 10% of your daily burn)
  
• Non-Exercise Activity Thermogenesis (NEAT) – All the spontaneous movement you do throughout the day: walking, fidgeting, cleaning, pacing, typing
  

Why This Matters

When people say they have a “slow metabolism,” they’re often only thinking about how many calories they burn. But your metabolism is an incredibly adaptive system. It speeds up, slows down, and adjusts based on signals from your lifestyle — including how much (or how little) you're eating, how well you're sleeping, how stressed you are, and how much muscle you carry.

The takeaway? Your metabolism isn’t static. You can influence it — for better or worse — through your daily choices.

BMR vs RMR — What’s the Difference (and Why It Matters)

Before we dive into calorie targets and metabolism-boosting strategies, let’s clear up one of the most common confusions in nutrition: Basal Metabolic Rate (BMR) vs Resting Metabolic Rate (RMR).
They’re often used interchangeably, but they’re not quite the same.

• BMR is your body’s bare minimum energy requirement — the calories you’d burn lying completely still for 24 hours in a lab, after 12 hours of fasting.
  
• RMR is more forgiving — it includes basic bodily functions at rest, and is usually about 10% higher than BMR.
  

How to Work Out Your Daily Calorie Needs

Use the Mifflin-St Jeor Equation (Mifflin et al., 1990):

Men:
(10 × weight in kg) + (6.25 × height in cm) – (5 × age) + 5
Women:
(10 × weight in kg) + (6.25 × height in cm) – (5 × age) – 161

Activity Multipliers:

• Sedentary: 1.2
  
• Lightly Active: 1.375
  
• Moderately Active: 1.55
  
• Very Active: 1.725
  
• Super Active: 1.9
  

Example:
30-year-old woman, 65 kg, 170 cm, training 4x/week:
RMR ≈ 1401.5 kcal → TDEE ≈ 2172 kcal
To lose fat: aim for a 15–20% deficit.
To restore metabolism: eat at maintenance or slightly above.

Macro Split – Getting Your Fuel Mix Right

• Protein: 1.6–2.2 g per kg of body weight
  
• Fats: 20–30% of total calories
  
• Carbs: Adjust based on energy demands
  (figures for general health and fitness)

Ultra-low carb can be useful short-term, but often tanks thyroid and mood if overdone. For most people: smart carbs + resistance training = metabolic momentum.

Quick Note on Oxidiser Types
Some people feel and perform better on higher carbs; others thrive with more fat and protein. This may relate to individual differences in how quickly they oxidise (burn) carbs vs fats.
While this isn’t fully mapped in science, tracking how your body responds to different macro ratios — energy, hunger, focus — can guide you to a better fuel mix.
Pay attention to how meals make you feel, not just how they fit your macros.

What Is a Calorie, and Why It Still Matters

A calorie is simply a unit of energy — specifically, the amount of energy required to raise the temperature of one gram of water by one degree Celsius. In nutrition, calories are a way of measuring the energy potential in food and the energy expenditure of your body. But they’re not just numbers on a label — they’re your body’s fuel.

Every biological function, from blinking your eyes to repairing muscle tissue after a workout, requires calories. And when you consistently consume fewer calories than your body needs — especially for extended periods — your metabolism doesn’t just “pause fat burning.” It actively down-regulates.

Here's how chronic calorie restriction affects your body:

• Thyroid hormone drops: Levels of T3, your active thyroid hormone, decline (Müller & Bosy-Westphal, 2013). This slows your metabolic rate to conserve energy.
  
• Leptin decreases: The hormone responsible for satiety and energy expenditure drops, making you hungrier and less energetic.
  
• Cortisol increases: Stress hormones often rise during restriction, promoting fat storage — especially around the abdomen.
  
• Body temperature may drop: Feeling cold all the time is a classic sign that your metabolic engine has dialled down.
  
• Digestive slowdown: Constipation or sluggish digestion can occur as your body conserves energy in non-vital systems.
  

You might also notice:

• Fatigue despite adequate sleep
  
• Mood swings or irritability
  
• Poor workout recovery
  
• Loss of menstrual cycle in women (a sign of energy imbalance) (Williams, N. I., et al. 2015)
  

Stay Tuned for Part 2:
In the next part of this series, we’ll break down why eating more — not less — might be the fix your metabolism needs, and how reverse dieting and nutrient-dense foods can help you rebound from restriction the right way.

References

1. Mifflin, M. D., et al. (1990). A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 51(2), 241–247.
   
2. Hall, K. D., et al. (2011). Quantification of the effect of energy imbalance on bodyweight. The Lancet, 378(9793), 826–837.
   
3. Müller, M. J., & Bosy-Westphal, A. (2013). Adaptive thermogenesis with weight loss in humans. Obesity, 21(2), 218–228.
   
4. Williams, N. I., et al. (2015). Magnitude of daily energy deficit predicts frequency of menstrual disturbances in exercising women. American Journal of Physiology-Endocrinology and Metabolism, 308(1), E29–E39.