Metabolism 101: How Do You Burn Calories?

From fast metabolism to slow metabolism, there are a lot of buzzwords in the fitness and nutrition industry when it comes to metabolism but what exactly do they mean, if anything at all? Here is everything that you need to know about your metabolism and how it works.

What is metabolism?

Metabolism is the sum of all reactions that occur throughout the body within each cell that provide the body with energy (1). The energy we consume from food and beverages is converted into energy that fuels all of the vital processes that are continuously occurring inside the body that allow for life and normal functioning. The body’s rate of energy production, measured in calories, is affected by factors such as sex, height, age, exercise, diet, hormone health, and disease.

How does metabolism work?

There are two primary chemical reactions involved in metabolism: catabolism and anabolism. Catabolic reactions govern the breakdown of food to obtain energy, while anabolic reactions use the energy produced by catabolic reactions to synthesize larger molecules (2). Both catabolic and anabolic reactions are critical to maintaining life.

• Catabolism: The breakdown of molecules to obtain energy. This includes the breakdown of macronutrients (proteins, carbohydrates, and fats) into their simpler forms to provide the body with energy and the basic building blocks required for growth.
• Anabolism: The synthesis of all compounds required by the cells. This includes growth and repair, which utilize energy that comes from our food.

The majority of the energy (calories) we consume daily is used to keep catabolic and anabolic reactions operating in the body, while a smaller portion of the energy (calories) we consume is utilized to fuel activity.

Components of Metabolism

There are four main components of metabolism (3, 4):

1. Basal Metabolic Rate (BMR)
2. Thermic Effect of Food (TEF)
3. Exercise Activity Thermogenesis (EAT)
4. Non-Exercise Activity Thermogenesis (NEAT)

TDEE = BMR + TEF + EAT + NEAT

Each component of metabolism utilizes energy and makes up our total daily energy expenditure (TDEE).

What is Basal Metabolic Rate (BMR)?

Your basal metabolic rate (BMR) is the amount of energy used by the body at rest or for your most basic life-sustaining functions. Even when you’re resting, your body requires energy for breathing, circulation, nutrient processing, adjusting hormone levels, and cell production. The body’s BMR accounts for the largest amount of energy expended daily, approximately 70% of total daily energy expenditure (3, 5). BMR is influenced by various factors including sex, age, height, fat mass, fat-free mass, and hormones (6, 7, 8).

Basal metabolic rate (BMR) is often used interchangeably with resting metabolic rate (RMR), however, there is a small difference between the two. BMR and RMR both measure the amount of energy – in calories – the body needs to stay alive and function properly, however, RMR accounts for additional low-effort daily activities on top of basic body functions. These low-effort activities include things such as eating or using the bathroom (9). For this reason, there is roughly a 10% difference between your BMR and your RMR, since RMR accounts for slightly more energy expended each day (10).

What is the Thermic Effect of Food (TEF)?

The thermic effect of food (TEF), also known as diet-induced thermogenesis, is the energy expended to digest, metabolize, absorb, and store the food that you eat (11). TEF represents approximately 10% of the energy you expend each day, however, it is influenced by age, meal timing, and the macronutrient composition of your meal (3).

Each macronutrient – protein, carbohydrate, and fat – requires a certain amount of energy to be digested by the body, which can be expressed as a percentage of the energy that they contain (12):

• Thermic Effect of Fats = 0-3%
• Thermic Effect of Carbohydrates = 5-10%
• Thermic Effect of Protein = 20-30%

Of all three macronutrients, protein has the highest thermic effect of food, meaning that the body requires more energy to break down 1 gram of protein compared to 1 gram of carbohydrate or fat.

What is Exercise Activity Thermogenesis (EAT)?

Exercise activity thermogenesis (EAT) is the energy expended by the body for physical movement. EAT accounts for periods of intentional exercise, such as going for a run, lifting weights, going for a swim, or doing a workout. Of all components of metabolism, EAT varies the most as it is dependent upon how active you are each day and can account for anywhere from 5% to 30% of your total daily expenditure depending on the individual (3, 13).

What is Non-Exercise Activity Thermogenesis (NEAT)?

Non-exercise activity thermogenesis (NEAT) is the energy expended for everything we do that is not sleeping, eating, or sports-like exercise (14). This ranges from the energy expended walking to the bathroom, standing, cooking, cleaning, and fidgeting. Your NEAT accounts for approximately 15% of your total daily energy expenditure (3).

What is a “slow” metabolism or “fast” metabolism?

Since metabolism is the sum of all the energy your body expends throughout the day, the terms “slow metabolism” and “fast metabolism” generally refer to how little or how much energy your body expends per day. Given certain components of metabolism, namely basal metabolic rate (BMR), are influenced by factors outside of our control, including age, sex, height, and genetics, metabolism is sometimes classified as “slow” or “fast” based on these factors.

Generally speaking, men tend to have a higher BMR than women; larger individuals tend to have a higher BMR than smaller individuals, and BMR increases rapidly before the age of one and gradually decreases after the age of 60 (15, 16).

While fast or slow metabolism are not technical terms, put simply, the more calories your body naturally burns at rest the faster your metabolism, and the fewer calories your body burns at rest the slower your metabolism.

Although less common, some individuals may suffer from a metabolic disorder which occurs when abnormal chemical reactions disrupt normal metabolic processes in the body. Examples of metabolic disorders include diabetes, Gaucher’s disease, and hemochromatosis (17).

How does metabolism impact weight?

Metabolism has a direct impact on weight loss and weight gain. Weight is fundamentally driven by energy balance; the relationship between “energy in” vs. “energy out”, which is defined by the laws of thermodynamics, and dictates whether weight is lost, gained, or remains the same. Put simply:

• To maintain weight, energy intake (calories consumed) must equal energy expenditure (calories burned).
• To lose weight, energy expenditure (calories burned) must exceed energy intake (calories consumed) to create a calorie deficit.
• To gain weight, energy intake (calories consumed) is greater than energy expenditure calories burned to create a calorie surplus.

When more calories are consumed than required for total daily energy expenditure (TDEE), the excess is stored primarily as adipose tissue, also known as body fat, as a form of stored energy (18). Conversely, when fewer calories are consumed than required for TDEE, the body will utilize the energy it has previously stored by burning body fat.

Does diet impact metabolism?

Yes, the amount and type of food you eat have a direct impact on your metabolism.

Given that the thermic effect of food (TEF) is a component of your total daily energy expenditure (TDEE), the macronutrient composition of your meal has a direct effect on how much energy your body expends to digest it. Since protein has the highest TEF, 10% to 30% higher than carbohydrates or fat, a protein-rich diet can help to increase your metabolism, as well as support the development and maintenance of lean muscle mass, which has a direct impact on BMR. Although there is limited evidence, some research suggests that high-carb meals produce a greater thermic effect when compared with high-fat meals (11).

Moreover, how many calories you choose to eat and for how long also has a direct impact on metabolism. Extended periods of undereating in a calorie deficit, extreme or not, can cause BMR to slow due to a phenomenon known as metabolic adaption. Also known as adaptive thermogenesis, metabolic adaption is a natural built-in protection mechanism the body uses to maintain homeostasis by causing the body to become more efficient at using energy and burn fewer calories in the process, a common cause of weight loss plateaus.

Does muscle increase metabolism?

Since basal metabolic rate (BMR) is influenced by body composition or an individual’s muscle-to-fat ratio, when you increase muscle mass you increase your metabolism. Total lean mass, or the total weight of your body minus the weight from fat mass, takes a lot of energy to maintain. Individuals with a higher degree of lean muscle will generally have a higher metabolic rate than individuals with lower levels of lean muscle mass (19, 20).

Generally speaking, less body fat and more muscle will result in a higher basal metabolic rate, while more body fat and less muscle will result in a slower higher basal metabolic rate. Therefore, if you build muscle, you will increase your BMR, however, if you lose muscle, you will decrease your BMR.

Does exercise increase metabolism?

Exercise has a direct impact on metabolism since exercise activity thermogenesis (EAT) influences the total daily energy expenditure (TDEE) or the total amount of calories expended per day. While basal metabolic rate (BMR) accounts for the majority of the energy expended by the body, exercise still plays a very important role in metabolism and overall health.

Not only does exercise directly increase energy expenditure during the activity itself but the increase in metabolic activity from exercise can outlast your workout. Depending on the duration and intensity of your physical activity, your metabolism can continue to burn calories at complete rest due to the result of rising oxygen consumption after workouts known as excess post-exercise oxygen consumption or EPOC (21).

Interestingly, while aerobic activities such as running, cycling, or swimming burn more calories during exercise, anaerobic activities such as weightlifting or interval training can help you expend more energy after exercising as they have a higher EPOC (22). After a resistance training workout, your body continues to burn calories as your muscles recover throughout the day.

Moreover, not only do anaerobic exercises that build muscle, such as resistance training, increase metabolism through increased EPOC but the maintenance of lean muscle mass itself increases total daily energy expenditure by increasing BMR and, therefore, overall metabolism.

Does metabolism slow with age?

Contrary to popular belief, metabolism does not slow with age. In 2021, a large study – the most comprehensive on the topic to date – was released which demonstrated that until the age of 60, age has very little impact on our basal metabolic rate. Using data from nearly 6,500 people, ranging in age from 8 days to 95 years, researchers discovered that there are four distinct periods of metabolic life (23):

• From infancy until age 1, when calorie burning is at its peak, metabolic rate surges until it is about 50% higher than the adult rate,
• From age 1 to 20, metabolism gradually slows by about 3% a year,
• From age 20 to 60, metabolism holds steady,
• And from age 60 onward, metabolism declines by about 0.7% a year.

Although people gain a pound and a half per year during adulthood, on average, this weight change cannot be attributed to a slowing metabolism. While metabolic rate does very gradually decline over the age of 60, before this point, it is primarily dietary and lifestyle factors that are the largest contributing factors to changes in weight.

What causes slow metabolism?

Looking back at the components that influence metabolism (BMR, TEF, EAT, and NEAT), several aspects can contribute to slow metabolism, some controllable and some not.

• Height: Quite simply, the smaller you are the less energy you need so the slower your BMR will be.
• Sex: Generally speaking, women have a slightly lower BMR than men.
• Underrating Protein: Protein has the highest thermic effect of food (TEF); therefore, underrating protein will result in a slower metabolism.
• Skipping Resistance Training: More muscle mass = faster metabolic rate. While all forms of activity support overall health, resistance training has a direct impact on metabolism as it supports the development and maintenance of muscle mass.
• Not Engaging in Intentional Exercise: If you move your body but you’re not engaging in intentional exercise (run, workout, class, etc..) you’re leaving unexpended calories on the table.
• Limited Daily Movement: If you’re engaging in intentional exercise but don’t move your body regularly outside of the gym/run/class, you’re once again leaving unexpended calories on the table.
• Chronic Dieting/Undereating Calories: Chronic dieting and undereating can cause metabolic adaptation and BMR to decrease to ensure you burn fewer calories at rest to compensate for the lack of calories. Long-term consumption of fewer than 1,000 calories per day can have a significant impact on basal metabolic rate (24, 25, 26).

Fortunately, while some of the factors are outside of your control, several factors are within your control and there are plenty of ways to increase slow metabolism.

How to Speed Up Your Metabolism

While many of the factors that influence metabolism are outside of our control, such as age, height, and genetics, many factors within our control can help to increase metabolic rate. Although you most certainly can’t “boost” your metabolism, focusing on the factors within your control can help to increase your metabolic rate and total daily energy expenditure.

1. Eat more protein.

Due to the high thermic effect and other factors, a high protein intake tends to increase metabolism, while it also supports the development and maintenance of lean muscle, which helps keep BMR strong. While exactly how much protein you need to support your goals will vary from one individual to the next, as the recommended protein intake for weight loss and maximizing muscle growth can be as high as 0.7 to 1.0 grams per pound of body weight per day, aim to consume, at least, one palm-sized portion of protein at every meal.

Moreover, research has consistently shown that weight loss programs based on physical activity alone are less effective than programs that combine diet and exercise in both the short and long-term.

2. Resistance train to build muscle.

Prioritizing resistance and strength training will help you build and maintain lean muscle mass, which directly increases your basal metabolic rate (BMR), exercise activity thermogenesis, and overall metabolism. Our resting energy expenditure is largely dependent on our fat-free mass, therefore, the more muscle mass we build through resistance training, the higher our resting energy expenditure (27).

3. Exercise regularly. 

Be it walking, cycling, dancing, skiing, or swimming, engaging in intentional exercise regularly will help to increase your exercise activity thermogenesis (EAT), which is a factor in metabolism. Governing health bodies generally recommend a minimum of 150 minutes of moderate-intensity physical activity per week or 75 minutes of vigorous-intensity physical activity per week (28).

4. Increase your NEAT.

Think of ways, outside of the gym and intentional exercise, to move your body more regularly. This will help to increase your non-exercise activity thermogenesis (NEAT), which is arguably the most underrated component of metabolism. Everything from playing with your kids to pacing while talking on the phone to picking up a basket instead of using a shopping cart can help to increase your NEAT.

5. Get good sleep.

Rest has a direct impact on your energy levels, hormone balance, and hunger cues. Go to bed at a decent hour and aim for 7-9 hours of uninterrupted high-quality sleep (29). Managing stress and prioritizing quality sleep is a secret weight-loss tool as it can help to keep your hormones, energy levels, and metabolism strong.

The Bottom Line

Metabolism is the sum of all processes that occur in the body and that provide the body with energy. Your metabolism, or total daily energy expenditure, is made up of your basal metabolic rate, thermic effect of food, exercise, and non-exercise activity, and is impacted by age, sex, height, muscle mass, hormones, and genetics. Your metabolism can be supported by various diet and lifestyle factors including consuming adequate protein, participating in regular exercise, prioritizing resistance training, and getting adequate high-quality sleep.