Think Your Metabolism Runs Fast? The Science May Surprise You

Have you ever heard someone say they’ve got a fast metabolism-meaning they can eat pretty much anything and never seem to put on weight?

On the other hand, plenty of people point to a slow metabolism as the reason they struggle to lose weight.

So is a “fast” or “slow” metabolism really a thing? Here’s what research suggests.

What is metabolism, exactly?

Metabolism covers every chemical process that keeps your body alive and functioning. That spans everything from breathing and pumping blood to repairing and renewing cells.

When metabolism is discussed in relation to body weight, people are usually talking about metabolic rate-how quickly your body turns food (and stored energy) into fuel you can use.

To make sense of how metabolic rate works, it helps to understand four key terms:

• Basal metabolic rate (BMR): the energy your body uses to keep going while you’re completely at rest. This typically makes up about 60% to 75% of your daily energy use. It’s largely driven by body size, although factors such as age, sex, race and height can also play a part.
• Diet-induced thermogenesis: the energy used to digest and process food. This usually accounts for around 10% to 15% of daily energy expenditure.
• Non-exercise activity thermogenesis (NEAT): the energy spent on day-to-day movement such as fidgeting, standing and walking. This generally contributes roughly 20% to 30% of the energy you use each day.
• Exercise activity thermogenesis: the energy used during structured physical activity-for example, running or weight training at the gym. This can make up 10% to 50% of daily energy use, depending on how active you are.

Can you genuinely have a “slow metabolism” or a “fast metabolism”?

It isn’t a straightforward yes-or-no.

In some cases, a fast metabolism can exist in a technical sense. If you have hypermetabolism, your resting energy expenditure (the energy you burn while at rest) is at least 10% higher than average. Hypermetabolism is most often linked to medical issues such as hyperthyroidism, diabetes, and some genetic disorders.

By contrast, two conditions can reduce resting energy expenditure and may therefore be described as causing a slow metabolism:

• Hypothyroidism, where the thyroid gland produces fewer hormones than normal
  
• Polycystic ovary syndrome (PCOS), which affects how the ovaries function

Both conditions can contribute to weight gain because they lower how much energy the body uses at rest.

That said, these situations generally reflect metabolism not functioning as it should. If you’re otherwise healthy, your metabolic rate typically stays within a normal range rather than swinging dramatically high or low.

What affects your metabolic rate (and your metabolism)?

A wide range of influences can shift how much energy you use each day.

Genetics and metabolism

The impact of genetics shows up clearly in research on weight loss involving identical twins.

In one study, pairs of identical female twins followed an energy-restricted diet. The twins tended to lose a similar amount of weight to each other. Meanwhile, there was substantial variation in weight loss among non-twins under the same conditions-suggesting inherited factors play a meaningful role.

Eating habits

What you eat-and how often-shapes how much energy you take in over the course of a day, which is why diet choices can affect body weight and the way metabolic rate is discussed.

However, some popular claims need clearing up, including the idea that eating small, frequent meals automatically “boosts” metabolism.

Narrowing your daily eating window may help with weight loss for some people. But overall, timing tends to matter less than the total amount of food you eat. And if you do lose weight, your body may respond by burning fewer kilocalories. This effect is known as adaptive thermogenesis, and it can make further weight loss harder.

Exercise and metabolic rate

Consider two people of a similar body weight: one works at a desk, the other has a physically active job. Even if neither person does structured exercise, the person in the active role may burn up to 1,000 kilocalories more per day (about 4,200 kJ) than their sedentary colleague.

That difference is before you add planned workouts, such as running. Biologically, muscle tissue uses more energy than fat tissue, so resistance training (aimed at building muscle) may increase metabolic rate.

Sleep

Current evidence suggests that sleep deprivation does not directly lower metabolic rate. However, it may encourage the body to produce more appetite-stimulating hormones such as ghrelin, which signals the brain to eat. More research is still needed in this area.

Two related points people often miss about metabolism

Metabolism gets talked about a lot, but two practical realities can be overlooked.

First, measuring energy use accurately is difficult outside laboratory settings. The most reliable approaches (such as indirect calorimetry) require specialist equipment and controlled conditions, which makes everyday “metabolism” claims hard to verify.

Second, metabolic rate is heavily influenced by body size and body composition. Because muscle tissue is more metabolically active than fat tissue, changes in lean mass can shift resting energy expenditure over time-even if the scales don’t move much.

Why are metabolism myths still so common?

They haven’t disappeared, and there are good reasons for that.

1) They’re simple

If you’re finding it difficult to lose weight or maintain a healthy weight, it can feel easier to blame a slow metabolism than to untangle the many interacting influences on body weight.

2) They’re woven into diet culture

Many products claim they can “boost metabolism” without solid scientific evidence. Some weight-loss supplements may raise metabolic rate, but typically only for a few hours at most.

3) They’re hard to disprove

It’s genuinely challenging to measure how the body uses energy with precision, partly because most people eat and burn a different number of kilocalories from one day to the next. Methods that assess energy expenditure well can also be expensive and time-consuming.

The bottom line

Metabolic rate is shaped by many factors. To understand how the body works, it helps to move past the idea that people are simply born with either a fast metabolism or a slow metabolism. Human biology is more complex-and more interesting-than that.

Hayley O’Neill, Assistant Professor, Faculty of Health Sciences and Medicine, Bond University

This article is republished from The Conversation under a Creative Commons licence.