What Is Metabolism?

Medically reviewed by Kierra Brown, RD

Metabolism is a term used to describe the reactions that take place within your cells to provide your body with energy.

Your body depends on energy from foods and drinks to support growth, development, reproduction, and other life-sustaining functions.

There are different types of metabolism, and several factors that can impact how your metabolism works.

How Does Metabolism Work?

Derived from the Greek word, metabolē, which means “to change”, metabolism refers to the processes that occur within your cells that provide your body with energy. Metabolism is also used to describe physical and chemical processes that use energy, such as breathing and digestion.

Metabolic reactions are organized into metabolic pathways, which are linked reactions triggered by enzymes, or proteins that act as catalysts for reactions in the body. For example, glycolysis is the metabolic pathway that breaks down glucose (sugar) into energy using enzymes.

These reactions occur constantly, even when you’re sleeping, in order to supply your body with a continuous source of energy and to sustain functions like heartbeat.

Metabolic reactions are controlled by enzymes and hormones like insulin, adrenaline, and thyroid hormones. For example, thyroid hormones increase your basal metabolic rate (BMR), which is the amount of energy your body uses while at rest to perform its most basic functions. Thyroid hormones also stimulate the metabolism of carbohydrates and the building of proteins.

Your metabolism is affected by factors like your age, diet, gender, activity levels, genetics, and health conditions, such as hypothyroidism and cancer.

The Different Types of Metabolism

Metabolic reactions are divided into two categories:

  • Catabolic reactions: Catabolic reactions involve the breakdown of larger molecules into smaller molecules. These types of reactions usually release energy. An example of a catabolic reaction is the digestion of food, which gives your body energy.

  • Anabolic reactions: Anabolic reactions create larger molecules from smaller molecules. Unlike catabolic reactions, anabolic reactions use energy. Building tissue, such as building muscle or bone mass, is an example of anabolism.

In order to sustain life and power bodily functions like heartbeat and cellular repair, you must provide your body with a steady supply of energy by consuming calories from foods and beverages.

Most of the energy or calories your body expends on a daily basis comes from your basal metabolic rate (BMR). BMR refers to the energy your body requires for critical functions, such as breathing, blood circulation, and keeping your heart beating while at rest. BMR accounts for 50-80% of your daily energy use.

The two other factors that contribute to your total energy expenditure are the thermal effect of food (TEF) and the energy you use during physical activity.

TEF, also known as thermogenesis, is the energy your body uses to digest and process foods and drinks. TEF accounts for around 10% of total daily energy expenditure.

Certain foods, such as high-protein foods, require more energy to digest than others, so your dietary choices can impact your metabolic rate. Proteins have the highest TEF out of the three macronutrients (protein, fats, and carbohydrates).

The energy required for physical movements, such as exercise, varies depending on your activity levels. Someone who’s very active and exercises daily or has a physically-demanding job will need more calories in order to maintain their weight compared to someone with low activity levels.

Likewise, someone trying to gain muscle mass, which is an anabolic state, will require more energy than someone trying to maintain their weight or lose weight.

Factors That Affect Metabolism

Your metabolic rate is impacted by a number of factors, some of which aren’t within your control. For example, your metabolic rate is partially determined by your genetics, meaning some people naturally require fewer calories than others in order to maintain their body weight.

Other factors that influence metabolism include:

  • Body size: Larger individuals generally require more energy compared to smaller people. For example, a person who’s six feet tall will generally require more calories to sustain their body weight compared to someone who’s five feet tall. However, this isn’t always the case, as certain factors, such as physical activity levels, impact energy needs.

  • Gender: Men typically require more calories than women because they’re usually larger and have more muscle mass.

  • Body composition: People who have more muscle mass have a faster metabolic rate than people with low muscle mass and high body fat levels. This is because muscle cells burn energy more efficiently than fat cells.

  • Age: As we age, our basal metabolic rate tends to decline. This is related to muscle tissue loss, lower activity levels, and other age-related metabolic changes. Research shows BMR remains mostly stable from ages 20-60 and then starts to decline after age 60.

  • Physical activity: People who are more active require more energy than people with low activity levels. Regular physical activity increases muscle mass and increases the calories you burn while you’re at rest, or BMR.

  • Diet: Your diet is another factor that influences your metabolism. Protein digestion requires 10-30% of the energy content of the ingested protein, while carbohydrates and fats require 5-10% and 0-3%, respectively. This means your body burns more calories digesting protein than it does digesting carbs or fats.

Stimulant drugs, illnesses, and infections are additional factors that can influence your metabolic rate.

Health Conditions That Can Affect Metabolism

Some health conditions can impact your metabolism, which can lead to weight loss or weight gain as well as other complications.

Thyroid conditions can either speed up or slow down your metabolism, depending on the type. Hypothyroidism, or underactive thyroid, causes your body to burn fewer calories while hyperthyroidism, or overactive thyroid, increases your body’s energy demands. This is why hypothyroidism is commonly associated with weight gain, while hyperthyroidism can result in weight loss.

Other examples of health conditions that impact metabolism include cancer and Cushing’s syndrome. Cancer impacts energy needs through disease-associated changes in metabolism and increased inflammation, which can increase energy expenditure. This increase in energy expenditure is one of the reasons why people with cancer often lose weight.

People with Cushing’s syndrome, a condition caused by high cortisol levels, have issues maintaining a healthy body weight due to disruptions in the normal metabolic processes. For example, impaired glucose metabolism, which impacts how the body digests glucose (sugar), can cause weight gain.

Can You Boost Your Metabolism?

Certain factors that impact your metabolism are out of your control, but there are effective ways to increase your metabolic rate.

Following a nutritious diet and maintaining your optimal body composition can help boost your basal metabolic rate, or the calories you burn while at rest.

A study that included 304 women with obesity or overweight between the ages of 18 and 50 found women who closely followed an eating pattern consisting of vegetables, fruits, nuts, eggs, red and white meat, and legumes had significantly higher BMRs than women who followed diets high in processed foods high in sugars and fats.

Following a diet rich in whole foods, especially protein-rich foods, is an effective way to improve and maintain your metabolic rate. Protein has a higher TEF than carbs or fat, meaning it requires more energy to digest. Eating a protein-rich diet also helps your body maintain its lean mass, which is essential for a healthy BMR.

Staying physically active and supporting muscle mass with resistance training can also increase BMR and prevent the decline in BMR that's associated with aging. A research review found resistance exercise increased BMR by about 96 calories per day.

Another small study that included 67 people found nine months of resistance training increased BMR by 5% on average.

Getting enough sleep is also critical for maintaining a healthy BMR. Some research suggests sleep deprivation can alter metabolism and decrease BMR. A small study that included 36 people found sleep restriction decreased BMR by 2.6%.

While following a nutritious diet, staying physically fit, and getting enough sleep encourages a healthy metabolism, these habits also promote overall health and help reduce your risk of common conditions, such as heart disease.

If you have a health condition that impacts your metabolic rate, such as hypothyroidism or cancer, you may require medication along with lifestyle changes in order to improve your metabolic rate. Your healthcare provider may also recommend medications to prevent side effects associated with slowed or increased metabolism, like weight gain and weight loss.

A Quick Review

Metabolism refers to the reactions that occur within your body’s cells that provide your body with energy, and use energy to perform bodily functions like digestion and breathing.

Factors that impact your metabolic rate include age, gender, diet, and physical activity levels. Underlying health conditions, such as hypothyroidism, can impact metabolism, too.

Though some factors that influence metabolism, such as age and genetics, are out of your control, following a nutritious diet, staying physically active, and getting enough sleep are effective and evidence-based ways to maintain a healthy metabolism.

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Read the original article on Health.