The Truth About Metabolism

You’ve probably heard the term “metabolism” tossed around, and might even have a general idea of what it means.

But when it comes to its impact on your health, especially in terms of weight loss, there are a lot of myths and misconceptions.

There’s a ton of misleading advice about metabolism that isn’t based on actual evidence, which creates plenty of confusion around the subject.

In today’s issue, we’re breaking it all down—what metabolism really is, how it works, and which claims simply don’t hold up to science.

What is metabolism?

In simple terms, metabolism is the process by which your body generates and uses energy to sustain itself.

This energy comes from two primary sources: the food and drinks you consume, and your body’s internal energy reserves, such as glycogen and fat.

Energy is measured in calories, and every food and beverage contains a certain number of calories.

Macronutrients—carbohydrates, proteins, and fats—are the primary sources of calories.

During digestion, carbohydrates are broken down into glucose, fats into fatty acids and proteins into amino acids.

Once broken down, glucose and fatty acids—the body’s main energy sources—are absorbed into cells, where they undergo further reactions to produce energy.

This entire process of converting food and stored nutrients into usable energy is what we call metabolism.

It’s the engine that’s always running to ensure your body has all the energy it needs.

How the body uses energy

You’ve probably heard the phrase “burning calories”. This simply refers to how the body uses up the energy it obtains from food.

Your body requires energy for everything it does. As such, your metabolic processes never stop—even when you’re resting or sleeping.

The body uses energy in three main ways:

1. Basal Metabolic Rate (BMR): This is the energy your body needs to maintain basic functions like breathing, circulating blood, and repairing cells while at complete rest. It’s the largest component of your daily energy use, accounting for about 60%.

2. Physical Activity: This includes everything from structured exercise like walking or lifting weights to small, everyday movements like typing, cleaning or fidgeting (non-exercise activity thermogenesis or NEAT). It typically accounts for around 15–30% of the total energy you use daily, depending on how active you are.

3. Thermic Effect of Food (TEF): Digesting, absorbing, and transporting nutrients from your meals also requires energy.

The sum of these components forms your total daily energy expenditure.

Total daily energy expenditure (TDEE) = BMR + Physical Activity + NEAT + TEF

E.g., A person with a BMR of 1,500 calories who burns 500 calories through physical activity and 200 calories through digestion has a TDEE of 2,200 calories.

Understanding energy balance

Energy balance is a critical concept in nutrition and metabolism. It reflects the relationship between:

• Energy intake - Total number of calories you consume

• Energy expenditure - Total number of calories you burn in a day

• Energy storage - Excess calories stored as fat when intake exceeds expenditure

Energy intake > Energy expenditure = Excess energy stored as fat = Weight gain

Energy intake < Energy expenditure = Stored energy (fat) utilized = Weight loss

Understanding how your body uses energy can help you make informed choices to support your health and wellness goals.

Slow vs Fast metabolism: What it means

We’ve already established that metabolism is how your body coverts the energy from food (calories) into energy that it can use.

The basal metabolic rate (or BMR) mentioned earlier—the number of calories your body burns at rest to maintain basic functions—is a key player in metabolism.

Since BMR accounts for majority of your total daily energy expenditure, it plays a significant role in defining whether someone has a "slow" or "fast" metabolism.

A slow metabolism means that your body:

• Uses less energy to maintain these basic functions, and therefore requires fewer calories daily to sustain itself.

• Burns calories at a slower rate at rest, resulting in a lower BMR.

And the opposite is true for a fast metabolism.

What affects your metabolism speed?

While your metabolism is largely influenced by your genetics, several factors contribute to individual differences:

• Age: Metabolism naturally slows down as you age, partly due to a decline in muscle mass and hormonal changes.

• Body Composition: More lean muscle mass = faster metabolism. Muscle burns more calories compared to fat, even when you’re not moving.

• Sex: On average, men tend to have faster metabolisms than women because they generally have more lean muscle mass and less body fat.

• Hormones: Thyroid hormones, in particular, are critical for regulating metabolism. An overactive or underactive thyroid can speed up or slow down metabolism.

Now that we’ve covered the basics of metabolism, let’s dive into some common myths and separate fact from fiction.

Common misconceptions

Myth: Having a slow metabolism means you’re unhealthy

Truth: Metabolic health isn’t determined solely by the speed of your metabolism

Having a naturally slow metabolism simply reflects how your body generates and uses energy, influenced by factors like genetics, age, and muscle mass.

It doesn’t mean that your metabolism is “bad” or that you’re automatically unhealthy or at a disadvantage.

A slow metabolism only becomes a problem if it leads to an imbalance in energy intake and expenditure, or is the result of an underlying health issue, such as an underactive thyroid.

For instance, consistently consuming more calories than your slow metabolism can burn leads to excess energy being stored as fat, particularly visceral fat (fat around your organs).

This increases your risk of weight gain and other metabolic disorders like diabetes, especially if also combined with physical inactivity.

But here’s the important part: the risk isn’t a direct consequence of a slow metabolism itself. It’s primarily tied to lifestyle choices, like overeating and insufficient movement.

So, can you be healthy with a slow metabolism? Absolutely!

What matters more is how you manage your overall lifestyle to support metabolic health and energy balance:

• Ensure your diet contains more nutrient-dense (whole) foods than calorie-dense (highly processed) foods.

• Eat enough to match your energy needs (not too much, not too little).

• Stay active. Regular movement boosts your energy expenditure and supports overall metabolic health.

• Practice habits that support metabolic processes like managing stress, getting enough sleep and staying well hydrated.

By focusing on these healthy habits, you can thrive no matter your metabolism’s speed.

Myth: A slow metabolism is why you can’t lose weight

Truth: Weight management is often more about energy balance (calories in vs. calories out) than metabolism alone

Metabolism does play a role in weight regulation, but it’s not the only factor.

Diet, physical activity, hormones, sleep, stress, and genetics all contribute.

While you can’t change your genetic makeup, you can control all these other factors.

Weight loss happens when you’re in a calorie deficit, which can be achieved in two ways:

1. Eating fewer calories than your body needs, so it uses stored energy (fat) to make up the difference.

2. Increasing physical activity to burn more calories.

You can have a slow metabolism and still maintain a healthy weight by balancing your calorie intake with expenditure.

True metabolic disorders like hypothyroidism or other hormonal issues can slow metabolism and cause issues with weight management, but they’re relatively rare.

For most people, weight loss struggles are more often caused by factors like overeating, or lack of activity, rather than a "broken" metabolism.

The Bottom line

Having a fast or slow metabolism isn’t an inherently good or bad thing.

What truly matters is the state of your metabolic health—your body’s ability to:

• Efficiently process and use energy.

• Maintain stable levels of blood sugar, cholesterol, and blood pressure.

• Avoid the risk of metabolic diseases like type 2 diabetes, heart disease, and obesity.

And while your genetics influence your baseline metabolism and predisposition to conditions like diabetes or obesity, lifestyle choices—like staying active, eating well, and managing stress—have the greatest impact on improving and maintaining metabolic health.

Unless it’s part of a larger health problem, how fast or slow your metabolism is on its own isn’t the most significant factor for overall health.

So, rather than fixating on the speed of your metabolism, focus on creating sustainable, healthy habits that support your body’s ability to function at its best.

Thanks for reading! I hope you found something valuable :)

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