From fast metabolism to slow metabolism to a broken 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 and that provide the body with energy (1). Energy consumed 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, 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 on a daily basis is used to keep catabolic and anabolic reactions operating in the body, while a smaller amount of the energy (calories) we consume is utilized to fuel activity.
Components of Metabolism
There are four main components of metabolism (3)(4):
- Basal Metabolic Rate (BMR)
- Thermic Effect of Food (TEF)
- Exercise Activity Thermogenesis (EAT)
- 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), also known as your metabolism.
What is Basal Metabolic Rate?
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?
The thermic effect of food (TEF), also known as diet-induced thermogenesis, is the energy expended to digest, metabolise, 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 (TEF), 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 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 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 (calories) your body expends per day. Given certain components of metabolism, namely 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” and “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.
How does metabolism impact weight?
Metabolism has a direct impact on weight loss and weight gain. When more energy is consumed than is needed for metabolism and physical activity the excess is stored primarily as adipose tissue, also known as body fat (17). Conversely, when less energy is consumed than is needed for metabolism and physical activity the body will utilize the energy it has previously stored. This relationship between “energy in” vs. “energy out” is known as energy balance, which is defined by the laws of thermodynamics, and dictates whether weight is lost, gained, or remains the same.
Does diet impact metabolism?
Given that the thermic effect of food (TEF) is a component of your total daily energy expenditure (TDEE), your diet does impact your metabolism. The macronutrient composition of your meal has a direct effect on how much energy your body expends to digest it. Protein has the highest thermic effect of food, 10% to 30% higher than carbohydrates or fat, therefore, protein-rich foods can help to increase your metabolism, and, 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, in addition to having a higher thermic effect of food, a high-protein diet helps to support the development and maintenance of lean muscle mass, which has a direct impact on basal metabolic rate.
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 (18)(19).
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 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 (20). Interestingly, while aerobic activities such as running, cycling or swimming burn more calories during exercise, anaerobic activities such as weight lifting or interval training can help you expend more energy after exercising as they have a higher EPOC (21). 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 (22):
- From infancy until age 1, when calorie burning is at its peak, metabolic rate surges until it was 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 change in weight cannot be attributed to a slowing metabolism. While metabolic rate does very gradually decline over the age of 60, prior to 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), there are several aspects that 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 you 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 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 (23)(24)(25).
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 can I speed up my metabolism?
While many of the factors that influence metabolism are outside of our control, such as age, height, and genetics, there are many factors within our control that 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. Increase your NEAT.
Think of ways, outside of the gym and intentional exercise, to move your body more on a regular basis. 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.
2. 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. Aim to consume, at least, one palm-sized portion of protein at every meal.
3. Exercise regularly.
Be it walking, cycling, dancing, skiing or swimming, engaging in intentional exercise on a regular basis 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 exercise or 75 minutes per week.
4. Focus on building muscle.
Prioritizing resistance and strength training will help you build and maintain more lean muscle mass, which directly increases your basal metabolic rate (BMR), exercise activity thermogenesis, and overall metabolism.
5. Get good sleep.
Rest has a direct impact on your energy levels, hormone balance, and hunger cues. 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.
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