Metabolism ABCs: The Essentials

Metabolism is the sum of all the chemical reactions in the body that convert energy and substances from food into building blocks, heat, and movement. It's not a single process, but a vast network of pathways: some reactions break down substances, others synthesize them, and still others process and eliminate waste. It's through metabolism that cells respire, divide, contract, transmit nerve impulses, and renew their structures. [1]

The main principle of metabolism is energy balance. The body obtains energy from food (carbohydrates, fats, proteins, alcohol) and expends it on basic life functions, muscle function, maintaining body temperature, and digesting food itself. When energy intake equals energy expenditure, weight is stable. If energy intake exceeds energy expenditure, the excess is stored primarily in adipose tissue. When energy is insufficient, the body is forced to use its reserves—glycogen and fat—and in severe energy deficiencies, muscle tissue also suffers. [2]

Metabolism is conventionally divided into two pathways. Catabolic processes break down large molecules (glycogen, fats, proteins) into simpler ones, releasing energy. Anabolic reactions, conversely, use energy to synthesize proteins, glycogen, hormones, components of the immune system, and other structures. In a healthy body, these two pathways constantly balance each other: active catabolism occurs in some areas (for example, in working muscles), while anabolism occurs in others (in growing tissues, during recovery after exercise). [3]

Metabolic rate is influenced by a variety of factors: gender, age, height, body weight, muscle-to-fat ratio, hormonal levels, sleep, stress levels, chronic illnesses, and even the time of day. Two people of the same weight can differ in their daily energy expenditure by hundreds of kilocalories simply due to differences in body composition and activity levels. Recent reviews emphasize that individual differences in metabolism and hormonal responses greatly influence how a person responds to the same diet. [4]

It's important to understand: a "slow" or "fast" metabolism isn't a stigma or a death sentence, but a combination of biological characteristics and habits. Many of these can be influenced through diet, exercise, sleep, and treatment of medical issues. The goal isn't to "break" metabolism with extreme diets, but to create conditions in which the body uses energy efficiently and tissues receive sufficient building blocks and protective factors. [5]

Table 1. Metabolism in simple terms

Concept	What does this mean in everyday language?
Metabolism	All chemical reactions in the body
Energy balance	The difference between energy received and energy spent
Catabolism	Disassembly of large molecules with the release of energy
Anabolism	"Assembling" the desired structures from simpler molecules
Metabolic rate	How much energy does the body spend per day under current conditions?

Where does energy consumption come from: the main components

Total daily energy expenditure consists of several major components. The most significant component is basal metabolic rate, or the level of energy expended at rest on breathing, heart function, maintaining body temperature, and functioning of the brain, liver, kidneys, and other organs. Basal metabolic rate accounts for approximately 60% to 70% of daily energy expenditure for most people. [6]

The second component is energy expended on physical activity. This includes both targeted training and all everyday movements: walking, housework, climbing stairs, gesturing, fidgeting. This so-called non-sports daily activity can vary significantly among individuals and contribute anywhere from 10% to 30% or more to daily energy expenditure. Some people spend almost all their work sedentary, while others experience movement throughout the day. [7]

The third component is the energy expenditure on digestion, absorption, and processing of food. This process is called the "thermic effect of food." After eating, energy expenditure temporarily increases, as the intestines, liver, pancreas, and other organs become more active, and some energy is spent on enzyme synthesis and nutrient transport. Carbohydrates, fats, and proteins have different thermic effects: protein requires more energy to process than fat and carbohydrates. [8]

All these components together form the total daily energy expenditure. However, they are not rigidly fixed and can adapt. For example, with severe calorie restriction, the body strives to conserve energy: basal metabolic rate decreases slightly, spontaneous activity is reduced, and hormonal signals are altered. Conversely, with increased movement and strength training, muscle mass increases and energy expenditure increases, at least partially, even at rest. [9]

Understanding your daily energy expenditure allows you to take a sensible approach to weight management. Instead of endlessly counting every calorie, it's helpful to look at the big picture: maintain a sufficiently high basal metabolic rate through muscle mass and adequate nutrition, increase daily activity, and consciously select your diet to ensure the thermic effect of food is not minimal. [10]

Table 2. Main components of daily energy expenditure

Component	Approximate share of total consumption	What does it include?
Basal metabolism	About 60-70%	The work of the heart, lungs, brain, internal organs
Physical activity	About 15-30%	Exercise, walking, everyday movements
Thermic effect of food	About 10%	Digestion, absorption, processing of nutrients

How carbohydrates, fats, and proteins are processed

The main sources of energy are carbohydrates and fats. Carbohydrates (starch, sugar, and natural sugars in fruits) are broken down into simple sugars, primarily glucose. Glucose readily enters the bloodstream and is used by muscle cells, the brain, and other organs as a quick source of energy. When carbohydrates are in excess, some of the glucose is converted into glycogen in the liver and muscles, and when carbohydrate stores are full, it is converted into fat. [11]

Fats are a concentrated energy reserve. They consist of fatty acids and glycerol and, when broken down, yield approximately twice as much energy per gram as carbohydrates or proteins. Adipose tissue serves as a reservoir in times of energy deficit and also performs hormonal and protective functions. When energy intake decreases or during prolonged exercise, fatty acids become the primary fuel for many tissues, especially muscles at rest and during moderate activity. [12]

Proteins are primarily a building block. They consist of amino acids, which the body uses to construct enzymes, hormones, immune system components, muscle fibers, and other structures. With a normal diet, proteins are rarely used as a primary energy source, but with severe deficiencies or serious illnesses, the body is forced to break down protein reserves, which can lead to muscle loss and weakened immunity. [13]

Different types of food affect metabolism differently. Foods high in fiber and complex carbohydrates provide a smoother rise in blood glucose and greater satiety than refined sugars. Protein produces a pronounced thermic effect and promotes muscle mass preservation while reducing calories. Fats are important for hormonal balance, vitamin absorption, and cell membrane structure, but when consumed in excess, they can easily lead to excess energy. [14]

Current data emphasizes that there is no "ideal" ratio of carbohydrates, fats, and proteins for everyone. Overall energy balance, food quality, and individual tolerance are more important. Protein deficiency, excess refined sugars, and extreme restrictions on one food group most often worsen metabolic health and interfere with sustainable weight management. [15]

Table 3. Main functions of macronutrients in metabolism

Nutrient	Leading role	Additional effects
Carbohydrates	Fast fuel	Glycogen stores and their effects on glycemia and insulin
Fats	Concentrated fuel and reserve	Hormonal regulation, membrane structure, thermal insulation
Squirrels	Building material	Enzymes, hormones, immunity, muscle mass support

Hormones that control metabolism

Hormones are chemical "messengers" that coordinate the work of various metabolic processes. One of the key ones is insulin, which is secreted by the pancreas in response to elevated blood glucose. Insulin helps glucose enter cells and stimulates its storage as glycogen and fat. Sustained high insulin levels and decreased sensitivity to it (insulin resistance) disrupt energy balance and increase the risk of type 2 diabetes and weight gain. [16]

Glucagon acts as a counterweight to insulin. When blood glucose levels drop, this hormone stimulates the breakdown of glycogen in the liver and the release of glucose into the blood. Normally, insulin and glucagon maintain relatively stable blood sugar levels, adjusting to mealtimes and periods of fasting. An imbalance in this balance leads to more pronounced glucose fluctuations and additional stress on blood vessels and organs. [17]

Leptin and ghrelin regulate appetite and energy balance. Leptin is produced by adipose tissue and signals the brain about energy reserves: the more fat, the higher the leptin level and the stronger the physiological suppression of appetite. However, in obesity, leptin resistance often develops, and the satiety signal weakens. Ghrelin, on the other hand, is produced primarily in the stomach and increases before meals, increasing hunger. [18]

Thyroid hormones (thyroxine and triiodothyronine) influence basal metabolic rate, or how much energy is expended at rest. A deficiency reduces mitochondrial energy production, reduces heat production, and slows down many processes, resulting in decreased body temperature, lethargy, and a tendency to gain weight. An excess, on the other hand, accelerates metabolism, leading to weight loss, palpitations, and heat intolerance. [19]

Glucocorticoids and other stress hormones also play a role. During acute stress, they help mobilize glucose and fatty acids to provide energy to muscles and the brain. During chronic stress, prolonged excess of these hormones contributes to insulin resistance, visceral fat accumulation, sleep disturbances, and appetite changes. These combined effects gradually alter metabolism for the worse. [20]

Table 4. The main metabolic hormones and their role

Hormone	The main effect on metabolism
Insulin	Reduces glucose, increases energy storage
Glucagon	Increases glucose, mobilizes reserves
Leptin	Signals fat reserves and reduces appetite
Ghrelin	Increases the feeling of hunger
Thyroxine and triiodothyronine	Speed up or slow down basal metabolism
Glucocorticoids	They increase glucose levels and, if in excess, promote fat accumulation.

Metabolism and weight: basal metabolic rate and adaptive changes

Basal metabolic rate is largely determined by the amount of lean mass: the more muscle and active organs you have, the higher your energy expenditure, even at rest. Modern research confirms that lean mass is the primary predictor of basal metabolic rate, while fat mass has an additional, but less pronounced, influence. This is why strength training, which increases muscle mass, helps slightly increase energy expenditure. [21]

When losing weight, the body doesn't simply "get lighter" and automatically expend less energy. An adaptive energy-saving mechanism called adaptive thermogenesis is activated. Energy expenditure decreases slightly more than would be expected from weight and muscle loss alone. A person becomes less active, spontaneous activity decreases, and heat production decreases slightly. This effect is especially pronounced with abrupt and significant calorie restriction. [22]

Adaptive thermogenesis serves a protective function: it helps us survive during energy deficits. However, in today's conditions of food oversupply, this mechanism hinders long-term weight loss maintenance. Reviews show that in some people, energy expenditure remains lower than expected after weight loss, which facilitates weight gain upon returning to a previous diet. [23]

With overly restrictive diets and prolonged calorie deficits, not only fat but also muscle tissue suffers. Muscle loss further reduces basal metabolic rate, impairing strength, endurance, and quality of life. Research emphasizes that maintaining or even slightly increasing muscle mass during weight loss is an important goal, which can be achieved through adequate protein intake and strength training. [24]

This means that the goals of "losing weight at any cost" and "maintaining a healthy metabolism" are not the same. It is far more beneficial to approach weight loss slowly and systematically, combining moderate calorie restriction, physical activity, and behavioral modification, than to try to "jump-start" your metabolism with extreme methods that only enhance adaptive energy conservation. [25]

Table 5. What happens to metabolism during weight loss

Change	Typical effect
Less body weight	Energy expenditure on carrying one's own weight is reduced
Muscle loss	Additional decrease in basal metabolism
Adaptive thermogenesis	Energy savings exceeded expectations
Improving insulin sensitivity	Better utilization of glucose and fats
Moderate weight loss	Improved metabolic parameters

Age, gender, sleep, and stress: additional influences on metabolism

Age is one of the most well-known factors influencing metabolism. With age, muscle mass and physical activity gradually decrease, and basal metabolic rate declines along with them. Current data show that a significant portion of the "age-related slowdown" in metabolism is associated with a decrease in lean mass and lifestyle changes, rather than just age. [26]

Gender also plays a role. Men typically have more muscle mass and a higher basal metabolic rate than women of the same age and height. Women's metabolism is sensitive to the phases of the menstrual cycle, pregnancy, and menopause, when sex hormone levels, fat distribution, and insulin sensitivity change. These hormonal fluctuations can affect appetite, food cravings, and subjective feelings of energy. [27]

Sleep is an often underrated regulator of metabolism. Chronic sleep deprivation and fragmented sleep alter leptin and ghrelin levels, increase hunger, especially cravings for sweets and fatty foods, impair insulin sensitivity, and increase the risk of weight gain. Reviews emphasize that normalizing sleep duration and quality is one of the simplest and most inexpensive steps to support a healthy metabolism. [28]

Chronic stress, as mentioned earlier, is accompanied by persistently elevated levels of hormones that stimulate glucose production while simultaneously impairing insulin sensitivity. Behavioral stress often leads to binge eating, decreased activity, and disrupted sleep patterns. Together, these factors create an unfavorable environment for metabolism and weight management. [29]

Finally, medications and chronic diseases have an impact. Some drugs used to treat mental disorders, epilepsy, hypertension, or endocrine disorders can increase appetite, alter insulin sensitivity, or affect thyroid hormone levels. Doctors are increasingly considering these effects when choosing treatment for people with excess weight and metabolic disorders. [30]

Table 6. Informal "regulators" of metabolism

Factor	How does it affect metabolism?
Age	Decreased muscle mass and activity, gradual slowing of basal metabolic rate
Floor	Differences in muscle mass and hormonal levels
Dream	With a deficiency - a violation of appetite hormones and insulin sensitivity
Stress	Increased stress hormones, increased cravings for high-calorie foods
Medicines and diseases	May increase or decrease metabolism and appetite

How to Maintain a Healthy Metabolism in Practice

Maintaining a healthy metabolism begins with nutrition. The body functions better when energy is distributed more or less evenly throughout the day, without prolonged periods of hunger alternating with overeating. It's beneficial to eat a diet rich in protein, fiber, vegetables, fruits, whole grains, healthy fats, and minimal amounts of refined sugars and ultra-processed foods. This approach improves insulin sensitivity and helps avoid sharp spikes in glucose and insulin levels. [31]

Regular physical activity is the second pillar. A combination of aerobic exercise (walking, cycling, swimming) and strength training helps increase or at least maintain muscle mass, improve cardiovascular function, enhance insulin sensitivity, and slightly increase overall energy expenditure. Even a moderate increase in step count and the introduction of two or three strength training sessions per week significantly improves metabolic rates. [32]

Sleep and stress management are equally important. Maintaining a regular sleep schedule, getting sufficient sleep, minimizing nighttime awakenings, and practicing relaxation techniques, breathing exercises, and psychotherapy when needed help reduce chronic stress hormone levels and stabilize appetite. This creates a more favorable hormonal balance for metabolism. [33]

Extreme diets that drastically cut calories, mono-diets, "detoxes," and aggressive "cutting" programs lead to rapid weight loss due to water and muscle loss, provoke strong adaptive thermogenesis, and damage the relationship with nutrition. In the long term, they most often slow basal metabolic rate and contribute to weight regain. A moderate calorie deficit, which a person can maintain for months and years, gradually reinforcing new habits, is much more beneficial. [34]

If you have chronic endocrine, heart, liver, or kidney diseases, it's advisable to discuss any major changes in diet and activity with your doctor. Working with your doctor and nutritionist can help you develop a plan that takes into account your metabolic characteristics, medications, and restrictions, rather than relying on one-size-fits-all approaches. This is especially important if you already have diabetes, severe obesity, suspected thyroid dysfunction, or severe sleep disturbances. [35]

Table 7. A simple “cheat sheet” for supporting metabolism

Direction	Practical step
Nutrition	More whole foods, enough protein and fiber
Activity	Daily walking and regular strength training
Dream	Stable mode and sufficient duration
Stress	Finding working stress management strategies
Health control	Regular check-ups and discussion of plans with your doctor