Metabolism: How the metabolism works

Metabolism: How the metabolism works

Written by

Anna Roell
26 September, 2023

Medically checked article All HOMED-IQ content is reviewed by medical specialists

Human metabolism is a complex network of chemical reactions that convert food and drinks into energy your body can use. Even while at rest, your metabolism works constantly to provide energy for essential body functions such as breathing, digestion, cell growth, and circulation. Understanding how metabolism works is important for maintaining your health and preventing diseases related to this important body process. Read on to learn more about what metabolism is, how it works in our bodies, and what can happen when metabolism is disrupted.

What is metabolism?

Metabolism is a continuous process that takes place constantly in every living thing, including humans. It involves a variety of chemical reactions that keep our bodies alive by allowing them to absorb nutrients, produce energy, eliminate waste, and produce new cells. A healthy metabolism refers to the efficiency and speed of these processes. A well-functioning metabolism ensures you have sufficient energy for daily activities and a lower risk of certain diseases (De Nava & Raja, 2022).

What is the difference between catabolism and anabolism?

Human metabolism can be divided into two categories:

  1. Catabolism: In the catabolic state, the body breaks down molecules to produce energy. This process involves the breakdown of complex substances into simpler ones. One example of a catabolic process is glycolysis, in which glucose (sugar) is broken down into energy that can be used by the body.
  2. Anabolism: In the anabolic state, the body builds cells and tissues and stores energy for later. These metabolic building processes involve the formation of complex molecules from simple precursors. One example is protein synthesis, in which amino acids are combined to form proteins.

As these two processes work hand in hand, disruptions in either anabolism or catabolism can negatively affect health (Britannica, 2023).

What are the main functions of metabolism?

Metabolism plays a variety of essential roles in our bodies. Its primary functions are:

  1. Energy production: Metabolism converts the nutrients from the food and drinks we eat into energy that our bodies need to function. Even when at rest, your body requires energy for processes like breathing and digestion.
  2. Cell growth and repair: Metabolism is responsible for producing proteins and other substances necessary for cell growth, repairing damaged cells and tissues, and creating new cells.
  3. Maintenance of body functions: Metabolism helps maintain the functions of our organs and body systems, including the cardiovascular system, nervous system, digestive system, and more.
  4. Detoxification: Metabolism plays a key role in removing toxins and waste products from our bodies, a process that is critical to our overall health and well-being.

Source: Britannica, 2023

Where in the body does metabolism take place?

Our bodies are made up of trillions of cells, and each of these cells is a venue for countless metabolic processes. Inside each cell are specialized structures that contribute to various aspects of metabolism. For example, mitochondria play a central role in cellular respiration, a process by which nutrients are converted into energy. On the other hand, the endoplasmic reticulum is involved in the assembly and degradation of proteins and lipids. Although each cell type in our body is capable of performing basic metabolic processes, specialized cells in different organs have functions specific to metabolism. Some examples are:

  • Gastrointestinal tract: The digestive system is responsible for breaking down food into its component parts (protein, carbohydrate, and fat metabolism). These components are then used for the various metabolic processes in cells and organs.
  • Bones: A constant exchange of minerals takes place in the bones, contributing to a cycle where old bone is removed and new bone tissue is formed. This process is regulated by three hormones: parathyroid hormone, calcitonin, and vitamin D.
  • Liver: The liver also plays a crucial role in metabolism. It helps process nutrients from food and is responsible for detoxifying the body by breaking down and removing toxic substances from the blood.
  • Muscles: Our muscles are also an important site for metabolic processes. During physical activity, the muscles use glucose and fat for energy.
  • Adipose tissue: Also known as body fat, adipose tissue is responsible for heat production by burning fats and sugars.

Each organ and tissue has its own role and function in metabolism, and together they keep our bodies functioning and healthy (De Nava & Raja, 2022; Britannica, 2023).

What is a slow and fast metabolism?

Metabolic rate is the rate at which your body burns calories or uses energy to maintain basic body functions. There are three types of metabolic rates:

  • Basal Metabolic Rate (BMR) is the amount of energy your body needs to maintain basic body functions such as breathing, heartbeat, and cell regeneration at complete rest. Even at rest, your body still requires calories equal to the BMR to stay alive.
  • Resting Energy Expenditure (REE) is the amount of energy your body uses in a resting but awake state. It is generally slightly higher than BMR because it includes energy for light movement and digestion.
  • Total Energy Expenditure (TEE) is the total energy expended in a given period of time, including energy needed for physical activity and digestion of food.

Metabolic rate can be influenced by a number of factors, including age, gender, genetics, body size, hormone levels, physical activity, and diet (Fletcher, 2020). Consequently, everyone’s metabolism is different- some people have a faster metabolism, while others have a slower metabolism.

  1. A fast metabolism means that the body burns more calories, even at rest. This can lead to weight loss and may make it harder to gain weight. While a fast metabolism can be beneficial, an excessively fast metabolism (such as in people with hyperthyroidism) can cause health problems, including unwanted weight loss, increased blood pressure, heart palpitations, and sweating.
  2. A slow metabolism burns fewer calories, which can lead to weight gain, even with moderate calorie consumption. While a slower metabolism can be normal (especially as we age), a severe slowing of metabolism, such as in people with hypothyroidism, can lead to persistent fatigue, constipation, and weight gain, despite balanced eating and regular exercise.

It is important to note that both an extremely fast or slow metabolism may indicate an underlying medical condition. If you notice unexplained weight loss or gain, fatigue, or other unusual symptoms, you should seek medical attention (Arnarson, 2023).

What are metabolic disorders?

Any diseases or disorders that disrupt normal metabolism are called metabolic diseases. Some of these conditions are genetic, while others can occur due to environmental factors or lifestyle choices.

Below are some of the most common metabolic diseases:

  • Diabetes: Diabetes is a chronic disease in which glucose (sugar) metabolism is disturbed. This occurs if the body either does not produce enough insulin or cannot use insulin properly. Insulin is a hormone that helps move glucose from the bloodstream into cells, where it is used as energy. This process is disrupted in people with diabetes, resulting in glucose remaining in the blood. This leads to increased blood sugar levels (Cleveland Clinic, 2023).
  • Underactive thyroid (Hypothyroidism): The thyroid gland produces two hormones- T3 and T4– that control metabolism. In hypothyroidism, the thyroid gland does not produce enough of these hormones, resulting in a slowed metabolism. Symptoms can include fatigue, weight gain, and mood swings (NIH, 2021).
  • Overactive thyroid(Hyperthyroidism): In contrast to hypothyroidism, hyperthyroidism occurs when the thyroid gland produces too many hormones, resulting in a fast metabolism. Symptoms may include weight loss, high blood pressure, rapid heartbeat, and nervousness (NIH, 2021).
  • Phenylketonuria (PKU): This is a rare, inherited disorder in which the body cannot properly break down the amino acid phenylalanine. If individuals with PKU do not follow a special diet, phenylalanine can accumulate in the body and lead to serious health problems, such as intellectual disabilities, neurological problems, and delayed development (GARD, 2023).
  • Cushing’s syndrome: This rare condition is caused by an excess of cortisol, a steroid hormone that plays an important role in the boddy’s stress response. Excessively high cortisol can affect metabolism and cause symptoms such as weight gain, skin changes and fatigue. Cushing’s syndrome can be caused by long-term use of steroid medications or by tumors in the adrenal gland (NIH, 2018).

What is metabolic syndrome?

Metabolic syndrome is a group of conditions that occur together and increase the risk for heart disease, stroke, and type 2 diabetes. These conditions include elevated blood pressure, high blood sugar, excess body fat around the waist, and abnormal cholesterol or triglyceride levels.

Having just one of these conditions does not mean you have metabolic syndrome, but it puts you at increased risk of cardiovascular disease. Metabolic syndrome is linked to insulin resistance, a condition in which the body cells no longer respond effectively to insulin. Insulin plays a critical role in metabolism by helping glucose in the blood enter cells to be used as energy. In people with insulin resistance, glucose cannot enter cells and remains in the blood, meaning blood sugar levels rise while the body produces more and more insulin. In the long term, this can cause type 2 diabetes. While metabolic syndrome can put you at increased risk for serious health problems, a healthy lifestyle can help prevent it. This means maintaining a healthy weight, getting regular exercise, and eating a balanced diet. If you are diagnosed with conditions such as high blood pressure, diabetes, or high cholesterol, it is important to work with your doctor to keep them well-controlled. Adopting healthy habits and managing conditions that put your heart at risk can help prevent complications from metabolic syndrome (Mayo Clinic, 2021).

Want to get insight into your own health?

If you are interested in assessing the risk of metabolic syndrome, Homed-IQ offers a Blood Sugar Test and Cholesterol Test that can be used from home. The Blood Sugar Test measures HbA1c, or your average blood sugar level over the past three months. Elevated HbA1c can indicate prediabetes or diabetes. The Cholesterol Test provides important information about your blood lipid levels, including LDL (low-density lipoprotein), HDL (high-density lipoprotein) and triglycerides, which are essential markers for assessing cardiovascular risk. With Homed-IQ’s Thyroid Test, you can monitor thyroid function and check for signs of metabolic disorders such as hypo- or hyperthyroidism.

What can I do to speed up my metabolism?

The speed of metabolism largely depends on age, activity levels, genetics, and other factors. However, there are still several steps you can take to naturally speed up your metabolism. To support your metabolism, you should:

  • Eat regular meals: A balanced diet with regular meals can help keep your blood sugar levels stable. It’s also important to consume enough calories to ensure your body has enough energy to function.
  • Stay hydrated: Ensure adequate hydration, as this can help improve your metabolic rate.
  • Exercise: Get enough physical activity, especially strength training. Increased muscle mass can help increase energy expenditure and speed up metabolism.
  • Manage stress levels: Chronic stress leads to an increased output of the hormone cortisol, which can affect insulin function. This can increase blood sugar levels and disrupt metabolism. Stress management techniques such as mindfulness, yoga, meditation, breathing techniques, and exercise can lower cortisol levels, potentially positively affecting metabolism.
  • Treat hormone imbalances: Hormone imbalances can affect metabolism in different ways, depending on which hormones are affected. For example, an estrogen deficiency caused by menopause or certain health conditions can slow down metabolism.


Metabolism is a vital process that allows our bodies to convert nutrients into energy and create new cells and tissues. A variety of factors can affect our metabolic rate, including diet, exercise, sleep, and stress levels. Managing risk factors for metabolic disorder, treating conditions that affect metabolism, and making healthy lifestyle choices can help ensure our metabolism functions optimally.


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About the author

Anna Roell

Anna is a trained nurse and health economist specializing in epidemiology, combining her medical and scientific interests. Her goal is to improve others' understanding of medical information and to communicate it in an understandable way. Anna is originally from Germany and now lives in Amsterdam. What she appreciates most about Amsterdam is the open-minded, active attitude of the people, the markets, and the beautiful nature in the areas surrounding Amsterdam.