Human Metabolism: How It Works, Rate, and Key Facts

Author: Disabled World (DW)
Updated/Revised Date: 4 Aug 2026

Table of Contents:
Synopsis - Definition - About This Section - FAQs - Publications - Subtopics

Synopsis

Understand human metabolism, including catabolism and anabolism, metabolic rate, basal metabolic rate, and the factors that affect how your body uses energy.

At a Glance

• 1 - Metabolism combines catabolism, which breaks molecules down, and anabolism, which builds them up.
• 2 - Basal metabolic rate can account for up to 70 percent of the total calories a person burns.
• 3 - For every liter of oxygen a person breathes, the body uses about 4.82 kcal of energy from glycogen or fat.
• 4 - After age 20, basal metabolic rate falls by roughly 2 percent each decade, and severe dieting can cut it up to 30 percent.

Topic Definition

Metabolism

Metabolism is the full set of life-sustaining chemical reactions that take place within the cells of a living organism, allowing the body to grow, repair itself, respond to its surroundings, and turn food into usable energy. It works through two complementary processes: catabolism, in which larger molecules such as proteins and carbohydrates are broken down into smaller units to release energy, and anabolism, in which smaller molecules are assembled into larger ones and stored for later use. The pace of these reactions is described as the metabolic rate, measured in calories, and the largest share of it, the basal metabolic rate, is the energy the body needs simply to stay alive at rest. A person's metabolism is shaped by many factors, including age, body composition, hormones such as thyroxine, activity level, diet, and genetics, which together influence how efficiently the body uses and stores energy.

Overview

Defining Metabolism

Metabolism is defined as the set of life-sustaining chemical transformations within the cells of living organisms. These enzyme-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. The word metabolism can also refer to all chemical reactions that occur in living organisms, including digestion and the transport of substances into and between different cells, in which case the set of reactions within the cells is called intermediary metabolism or intermediate metabolism. Metabolism is something that consists of both, 'Catabolism,' and 'Anabolism;' which are the buildup and breakdown of substances. In the field of Biology, Metabolism refers to all the body's chemical processes, the digestion of food, and the elimination of waste.

Cell Metabolism

Every living cell in a person's body has a metabolism, referred to as, Cell Metabolism. Multicellular organisms such as animals and plants do as well. People have an overall metabolism that differs from the metabolism of individual cells. There are metabolic pathways which form a two-part process; the first part is the one mentioned called, 'Catabolism,' during which the body processes food to use for energy. The other part is called, 'Anabolism,' where the person's body uses food in order to either repair or build cells. The metabolic process ceases only when a person dies.

Catabolism

The term, 'Catabolism,' comes from the Greek word, 'Cata,' which means, 'down.' Catabolism is a process consisting of all the reactions during which larger molecules are broken down into smaller ones, releasing energy. This process is exemplified by the digestion of protein which is then broken down into amino acids that a person's body can absorb and use through the metabolic process, storing glycogen in their liver for energy. Chemically, this process is known as an 'Oxidation Reaction.'

Anabolism

The term, 'Anabolism,' comes from the Greek word, 'Ana,' which means, 'up.' Anabolism is a process consisting of all the reactions during which the assembly of small molecules are created into larger ones and then stored as energy in newly formed chemical bonds. This is evidenced by the assembly of amino acids into larger proteins and the subsequent synthesis of fat and glycogen for the person to use as energy. Chemically, this synthetic process is known as a 'Reduction Reaction.'

Defining Metabolic Rate

The term, 'Metabolic Rate,' refers to the amount of chemical energy a person frees from their body per unit time. Chemical energy is measured in calories, or the amount of energy that will heat one gram of water by one degree Celsius. It is easier to measure calories by kilo-calories, or, 'kcal's'. One kcal is 1,000 calories; something that both food labels and Dietitians refer to as a calorie with a capital, 'C.'. A person's metabolic rate is commonly expressed in terms of kcal's per hour or day. One way to measure someone's metabolic rate is through the use of a 'Spirometer,' which is a device that measures their rate of oxygen consumption. For every liter of oxygen a person breathes, they use about 4.82 kcal of energy from glycogen or fat.

A person's metabolic rate is dependent on certain variables such as whether they have been fasting, their hormone levels, physical activity, their mental state, and their thyroid hormone in particular. A person's Total Metabolic Rate (TMR) involves their Basic Metabolic Rate (discussed below) in addition to their energy expenditures for other activities. A person's metabolic rate rises due to physical activity, anxiety, eating, pregnancy, fever or other factors. There are factors that can reduce a person's total metabolic rate as well, such as apathy, depression, or prolonged starvation.

Children have a higher TMR than adults. As people near middle-age, they gain weight many times, even though they may not change their eating habits. People who pursue diets can become frustrated, in part because initial weight lost is from water that is rapidly regained, but also because their TMR declines over time. As their diet process progresses, they burn fewer calories and begin to synthesize more fat, even with a stable intake of calories.

Defining Metabolic States

There are two metabolic states defined as, 'Absorptive,' and, 'Post-absorptive,' which are defined by the time that has elapsed since the person has taken in food and the changes in their body's energy processing. The, 'Absorptive,' state lasts around four hours both during and after a person has eaten a meal. During the absorptive state, the person's body absorbs nutrients they have consumed, uses some of them to meet their immediate needs, and converts excess nutrients into energy that is stored. The absorptive state is regulated largely through a hormone called, 'Insulin,' that promotes cellular uptake of glucose, or blood sugar, as well as amino acids, glucose oxidation, the synthesis of fat and glycogen. Due to quick cellular uptake of glucose, a person's blood sugar level falls off because of insulin.

The 'Post-absorptive,' state usually occurs during the late morning, afternoon hours, and overnight when a person hasn't eaten for four or more hours. During the post-absorptive state, the person's stomach and small intestine are empty, and their metabolic requirements must be met from energy that has been stored.

Defining Basal Metabolic Rate

A person's Basal Metabolic Rate (BMR) (Basal Metabolic Rate Calculator) is the minimum calorie requirement they need to sustain life while resting. A person's BMR may be responsible for burning up to seventy-percent of their total calories they expend, although this figure varies according to various factors. Processes such as pumping blood, respiration, and maintaining body temperature burn calories. A person's BMR is the biggest factor in determining their overall metabolic rate, as well as how many calories they need to maintain, lose, or gain weight. A person's BMR is determined by a combination of environmental and genetic factors. These factors include:

• Age: A person's BMR decreases as they age; after twenty years, their BMR drops by approximately two-percent each decade.
• Body Fat Percentage: Persons with a lower body fat percentage have a higher BMR. (Body Fat Percentage Calculator)
• Body Surface Area: The greater a person's body surface area is, the higher their BMR is. People who are tall and thin have higher BMR's.
• Body Temperature: For each increase of 0.5C in a person's internal body temperature, their BMR increases by approximately seven-percent. Chemical reactions in a person's body occur more rapidly at higher temperatures. A person with a fever experiences an increase in their BMR.
• Diet: Abrupt calorie-reduction or starvation may radically reduce a person's BMR by up to thirty-percent. A restrictive, low-calorie diet can cause a person's BMR to decrease by as much as twenty-percent.
• Exercise: Exercise helps to raise a person's BMR through building additional lean tissue, and influences their body weight by burning calories.
• External Temperature: Temperature outside a person's body can also affect their BMR. Cold temperatures can cause an increase in a person's BMR, although short exposure to increased heat has little effect on the body's metabolism. Prolonged exposure to heat may raise a person's BMR.
• Gender: Men tend to have a greater muscle mass and lower body fat percentage than women, and therefore have a higher BMR.
• Genetics: Some persons are born with either slower or faster metabolisms.
• Glands: 'Thyroxine,' is a BMR regulator produced by the thyroid gland that speeds up a person's metabolic activity. The more thyroxine a person's thyroid gland produces, the higher that person's BMR will be. If the person's thyroid gland produces too much thyroxine, a condition referred to as, 'thrytoxicosis,' their BMR may double. Too little thyroxine production is referred to as, 'myxoedema,' and can cause the person's BMR to diminish to 30-40 percent below normal. Adrenaline may also increase a person's BMR, but to a lesser extent.
• Weight: The more a person weighs, the higher their BMR is.

Interesting Metabolic Facts

• Contrary to common belief, a slow metabolism is rarely the cause of excess weight gain.
• Anabolism is the set of constructive metabolic processes where the energy released by catabolism is used to synthesize complex molecules.
• Metabolism involves a vast array of chemical reactions, but most fall under a few basic types of reactions that involve the transfer of functional groups of atoms and their bonds within molecules.
• Skinny individuals almost invariably have slower resting metabolisms; there is literally less of them to burn while at rest.
• Carbohydrate catabolism is the breakdown of carbohydrates into smaller units.
• Metabolism refers to all the physical and chemical processes in the body that convert or use energy.
• One easy way to ramp up your metabolism is to build muscle through lifting weights.
• Metabolism can vary greatly. A 5'2" woman weighing 130 lbs. may have an entirely different metabolism than another woman who is the same height and weight.
• Most of the structures that make up animals, plants, and microbes are made from three basic classes of molecule: amino acids, carbohydrates, and lipids (often called fats).
• Men, who naturally have a higher ratio of muscle to fat, tend to burn up what they eat faster, although a fat man may have a slower metabolism than a slim woman with more muscle tissue.
• Your metabolism is regulated by a small, butterfly-shaped gland known as the thyroid.

Frequently Asked Questions

Can you speed up your metabolism

You can modestly raise your metabolic rate by building muscle through resistance exercise, staying active, and eating enough protein, though genetics and age still play a large role.

Does eating late at night slow your metabolism

Total daily calories and food quality matter more than timing for most people, and there is limited evidence that eating late by itself meaningfully slows metabolism.

Why does metabolism slow down with age

Metabolism tends to decline with age partly because people gradually lose lean muscle and become less active, both of which reduce the calories the body burns at rest.

What is the difference between metabolism and digestion

Digestion is the process of breaking food down in the gut, while metabolism is the broader set of chemical reactions that use those nutrients for energy, growth, and repair.

Do certain foods boost metabolism

Some foods and drinks may cause small, short-lived increases in energy use, but no single food dramatically raises metabolism, so overall diet and activity remain most important.

How does the thyroid affect metabolism

The thyroid gland produces hormones such as thyroxine that regulate metabolic rate, so an overactive thyroid can speed metabolism while an underactive one can slow it considerably.

Does drinking water affect metabolism

Staying well hydrated supports normal metabolic function, and drinking water may cause a brief, minor rise in calorie burning, though it is not a major weight loss method on its own.

Curated and edited by Ian C. Langtree, Founder & Editor-in-Chief, Disabled World. This section is maintained by the Disabled World editorial team.

Last updated: 4 Aug 2026