The Science of Metabolism

Metabolism is how your body generates and uses energy for everything it does, and influences your health in many ways. Learn how metabolism works and how it might play a role in your ability to shed pounds.

Metabolism is the term for all the chemical reactions that happen in your body to keep you alive and functioning. It affects many aspects of your health, such as your energy levels, your strength, your weight, and your risk of chronic disease. Let’s find out what metabolism is, how it converts the food you eat into energy, and how it can relate to fluctuations in weight.

What is metabolism?

Put simply, metabolism is the process by which living cells use food to generate energy for their survival, growth, and function. Metabolism is not a single process, but a balance between two types: anabolism and catabolism. Anabolism is the set of metabolic pathways that construct larger molecules from smaller units. For example, your body can make a protein from many small amino acids. To do this, your body needs energy. To generate energy, your body breaks down larger molecules into smaller units. This process is called catabolism. The best example of a catabolic process is cellular respiration, which is when your cells break down sugar to make adenosine triphosphate (ATP) that gives them energy. In humans, under normal conditions, the main source of energy is from breaking down glucose, a simple sugar, which we get from the carbohydrates in our food.

There are three steps required to turn food into energy:

Digesting food into smaller pieces. In the stomach, your body breaks down complex carbohydrates into small sugars, and big fats into small fats, and big proteins into small pieces called amino acids.
Converting all small sugars into glucose which gets absorbed from the intestines into the bloodstream, along with all the fats and amino acids.
Delivering all the glucose into the cells of every organ in the body, where it gets metabolized to generate energy. This step requires insulin, a hormone secreted from the pancreas in response to rising blood sugar. Any excess glucose then gets packaged into a larger molecule called glycogen, to be stored in the liver and muscles to be used later when there is no more glucose coming from the intestines (between meals and at night).

Metabolizing carbohydrates, protein, and fat for energy

Now let's dig a bit deeper and understand how metabolism breaks down the body’s primary sources of energy found in food: carbohydrates, fats, and amino acids.

As mentioned above, carbohydrates are the body’s preferred fuel source and include sugars, starches, and fiber in food that can be broken down and mixed with water to travel easily in the blood.

Fats are generally divided into cholesterol and triglycerides. Cholesterol is essential for building new tissues during growth and replacing old tissues, and is required for making important hormones such as cortisol, estrogen, and testosterone, as well as vitamin D. Excess cholesterol, however, can build plaques that block the arteries and cause heart disease and strokes. Triglycerides get broken down into fatty acids and glycerol in the intestines. Fatty acids are then transported to the liver or other tissues that need them. Fatty acids can be metabolized to produce ketones which in turn generate ATP, when the body runs out of glucose and/or insulin.

Protein is made of amino acids. Some amino acids come from food, and some your body can make on its own. Protein is metabolized by breaking it down into amino acids, which have a nitrogen component and a sugar component. The sugar component can be converted into glucose under conditions of extreme starvation, which causes a loss of muscle mass.

Understanding energy expenditure

Energy expenditure is the amount of energy your body uses to perform essential physical functions and movements. You can measure energy expenditure in calories, which are units of heat that indicate how much energy is available in food or used by the body. An example of energy expenditure in everyday life is walking. Walking is a physical activity that requires your muscles to contract and relax, your heart to pump blood, your lungs to breathe oxygen, and your brain to coordinate your movements. All these processes use energy and burn calories.

Your metabolism is the sum of all the energy your body uses to operate. This is also called your total energy expenditure (TEE). It is helpful to know what your TEE is to help you estimate how many calories you need to provide your body on a daily basis to keep it in balance. Consuming fewer calories than your TEE can result in weight loss, while consuming more calories than you need causes weight gain.

You can think of your TEE as three main parts:

Basal Metabolic Rate (BMR): Your BMR is the minimum amount of energy that your body needs to keep all its vital functions going at rest. BMR is measured in very specific conditions, such as in the morning after fasting and sleeping, in a comfortable temperature, and without any stress. BMR makes up the largest part - as much as 66% - of the total energy that you use in a day.
Thermic Effect of Food (TEF): This is the amount of energy that your body uses to digest, absorb, and store the food. TEF usually makes up about 8-15% of the total energy and it can vary depending on what kind of food and how much food you eat.
Energy expended for physical activity: This is the energy your body uses to move around. This includes both exercise and non-exercise activities, such as walking, playing, or doing chores. This part of your total energy expenditure can vary a lot depending on how active you are.

The link between energy balance and weight

BMR might make up the largest part of the total energy that the body uses in a day, but it turns out that BMR is not very influential when it comes to weight. The part of metabolism that is closely linked with weight is what scientists call the non-exercise activity thermogenesis (NEAT). NEAT is the energy the body uses for spontaneous physical activity that is not planned exercise, such as standing, walking, fidgeting, and doing household chores.

NEAT is variable and adjusts energy expenditure in response to changes in energy balance. Energy balance is the balance between how much energy you get from the calories in the food you eat versus how much energy you use up. This balance is important for health because it influences body weight and composition.

Energy balance can be positive, negative, or neutral depending on whether we consume more, less, or the same amount of energy as we expend.

Positive energy balance: This means you eat more calories than you use. This can make you gain weight over time. This can happen if you eat too much or move too little, or both.
Negative energy balance: This means you use more calories than you eat. This can make you lose weight over time. This can happen if you eat too little or move too much, or both.
Perfect energy balance: This means you eat the same amount of calories as you use. This can keep your weight the same over time.

You can change your energy balance by changing how much you eat and how much you move. You can eat more or less food to change how much energy you take in. You can also move more or less to change how much energy you use.

Because NEAT can change depending on how much food intake there is and can help adjust the energy balance, it is important for regulating body weight.

What explains the differences in weight loss or gain?

A timeless question seems to be, given the same lifestyle, why do some people gain weight while others remain slim? Some people are more likely to gain weight than others depending on their metabolic responses to their diet. For example, some experts say that TEE can vary for each person. Also, some people may naturally have a better sense of how much energy they need and eat accordingly, while others may overeat or undereat for their metabolism - leading to weight gain or loss over time.

The bottom line

Metabolism is the process of turning food into energy for the body's functions by breaking down carbohydrates, fats, and proteins into smaller units and using them for energy. It refers to the sum of all the energy the body needs to perform its vital functions, such as breathing, thinking, digesting, and moving. The sum of the energy the body needs consists of three main components: the basal metabolic rate, the thermic effect of food, and the energy expended for physical activity. Energy balance is the difference between the calories you consume from food and the calories you burn from metabolism and physical activity, and it helps determine whether you gain, lose, or maintain your weight.