Have you ever heard someone claim they have a “fast metabolism”? This typically means they can eat whatever they want without gaining weight.

Meanwhile, others blame their inability to lose weight on having a “slow metabolism”.

But can you actually have a fast or slow metabolism? Let’s see what the science says.

Remind me, what’s metabolism?

Metabolism refers to all the chemical processes which allow your body to function. This includes everything from breathing to circulating blood and repairing cells.

When we talk about metabolism in the context of weight, we’re usually referring to metabolic rate. This is a measure of how quickly your body converts food and stored energy into usable fuel.

To understand how your metabolism works, it’s helpful to know these four terms:

• basal metabolic rate, which is the amount of energy your body uses to keep itself running when at rest. It usually accounts for about 60% to 75% of your daily energy use. It is largely determined by body size, but factors such as age, sex, race, and height may also contribute
• diet-induced thermogenesis, which is the amount of energy you use while digesting and processing food. It usually accounts for between 10% and 15% of your daily energy use
• non-exercise activity thermogenesis, which is the amount of energy you use for everyday movements such as fidgeting, standing and walking. It generally accounts for between 20 and 30% of the energy you use each day
• exercise activity thermogenesis, which is the amount of energy you use while doing structured physical activity, such as going for a run or lifting weights at the gym. It usually represents 10 to 50% of your daily energy use, but this varies depending on how active you are.

So, can I have a ‘slow’ or ‘fast’ metabolism?

The answer is: it’s complicated.

If you have a condition called hypermetabolism, you could technically say you have a fast metabolism. Hypermetabolism occurs where your resting energy expenditure, or the amount of energy you use while your body is at rest, is at least 10% higher than average. Hypermetabolism is mainly associated with medical conditions such as hyperthyroidism, diabetes and certain genetic disorders.

In contrast, there are two conditions which may slow your metabolism. These are hypothyroidism (where your thyroid gland releases fewer hormones than normal) and polycystic ovary syndrome (which affects how the ovaries work). Both conditions can cause you to gain weight because they reduce how much energy your body uses while at rest. In this way, they could be said to give you a slow metabolism.

However, these three conditions tend to arise when your metabolism isn’t working as it should. So if you are generally healthy, your metabolic rate should stay within a normal range without significant highs and lows.

What actually does affect your metabolism?

There are many different factors. These include:

Genetics

We can observe the effect of genetics on metabolism in studies examining weight loss in identical twins. One study looked at pairs of identical female twins who were put on a calorie-restricted diet. It found these twins lost a similar amount of weight. In comparison, the researchers recorded significant variation in how much weight non-twins lost under the same conditions.

Eating habits

What and how often we eat shapes how much energy we consume each day. This is why dietary choices can affect your metabolic rate. However, there are some misconceptions to clear up. These include the idea that eating small, frequent meals boosts your metabolism. Shortening your feeding window may help you lose weight. But on the whole, timing matters less than how much food you actually eat. If you do lose weight, your body may respond by burning fewer calories. This process, known as adaptive thermogenesis, can make losing more weight difficult.

Exercise

Let’s compare two people of a similar weight: one who works at a desk and one who has an active job. Even if neither does structured exercise, the latter may use up to 1,000 calories more per day than her sedentary colleague.

And that’s before you add formal exercise, such as going for a run, into the mix. On a biological level, muscle tissue burns more energy compared to fat tissue. This means doing resistance training, which is designed to build muscle, may increase your metabolic rate.

Sleep

Current research suggests sleep deprivation does not reduce metabolic rate. However, it may cause your body to produce more hunger-inducing hormones such as ghrelin, which tells your brain to eat. But we need more research in this space.

But these ‘metabolism myths’ are still around today?

Yes. Here are three reasons why.

1. They’re easy to understand

If you struggle with losing or maintaining a healthy weight, it’s easier to say you have a slow metabolism than to unpack the many interacting factors that influence weight.

2. They’re embedded in diet culture

Many products claim to boost metabolism without providing any scientific evidence. Some weight loss supplements may increase your metabolic rate, but only for a few hours at most.

3. They’re difficult to disprove

It’s difficult to accurately measure how your body uses energy. This is because you generally consume and use a different number of calories each day. Current methods of measuring energy use can be expensive and time-consuming to run.

The bottom line

Many different factors influence your metabolic rate. So to understand how our bodies work, we need to debunk the idea that people are born with either a “fast” or “slow” metabolism. Our bodies are much more nuanced, and fascinating, 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 license. Read the original article.