Metabolism is the set of all biochemical reactions that occur inside our cells to keep us alive. It is the process by which your body converts nutrients from your diet into energy, which is crucial for basic functions such as respiration, blood circulation, and digestion.
Metabolism: how it works
Metabolism is divided into two main phases. The first is catabolism, the process by which complex molecules such as carbohydrates, fats, and proteins are broken down into simpler molecules releasing energy, in particular adenosine triphosphate (ATP). The second phase is anabolism, which can be seen as the constructive side of metabolism. During this phase, the body takes the energy produced by catabolism and uses it to build new molecules such as proteins, enzymes, and cell membranes. Anabolism is essential for growth, repair, and the maintenance of healthy tissues.
Basal metabolism
Basal metabolic rate (BMR) is the amount of energy your body uses when you are awake but completely at rest, both physically and mentally, and after fasting for at least 12 hours in a thermoneutral environment. It represents the minimum energy needed to keep you alive and awake, without taking into account extra calorie expenditure from activities such as exercise or daily movement.
BMR mainly comes from two sources: the energy required by internal organs to function, such as the calories burned by the heart muscle to keep it contracting, and the energy used by muscles at rest. This explains why, at the same weight and age, a healthy person with more lean mass generally has a faster metabolism than someone with less muscle.
Certain factors tend to raise basal metabolism. It is usually higher in men than in women, in younger people compared to older ones, in those who exercise regularly, in individuals with greater muscle mass, and in healthy people without conditions that slow down metabolism, such as hypothyroidism. BMR gradually declines as we age, beginning around the mid-20s. Between the ages of 60 and 90, it drops by about 8 percent per decade.This decline is not irreversible: consistent physical activity can slow it down. For instance, a lean, muscular, and well-trained 40-year-old man may have a higher metabolism than a 25-year-old woman who is obese, sedentary, and living with untreated hypothyroidism.
It is important to note that there is no universally “high” or “low” basal metabolism; what is considered optimal depends on factors like age, sex, diet, and lifestyle. The most effective strategy to lose weight combines three elements: increasing lean muscle mass (to raise BMR), moving more throughout the day, and following a calorie-controlled diet.
Note: Adding just one kilogram of muscle can raise basal metabolism by roughly 1.5 percent.
Daily Caloric Requirement (Total Metabolism)
When you add together your basal metabolic rate (BMR), the thermic effect of food (also called diet-induced thermogenesis, or the energy your body spends digesting and processing nutrients), and the calories burned through daily activities such as walking, studying, working, or exercising, you get your total daily energy expenditure (TDEE). This is sometimes referred to as your total metabolic rate (TMR). In simple terms, TDEE represents the daily caloric intake needed to balance the energy coming in from food with the energy going out through bodily functions and activity. By maintaining this balance, you will be able to keep a stable weight.
The daily caloric requirement is generally made up of three main components. Basal metabolism accounts for roughly 70 percent of total energy expenditure. Physical activity typically makes up around 20 percent, while the thermic effect of food contributes about 10 percent. If you eat more calories than your TDEE your body will store the excess as fat, while if you consume fewer calories than you expend, you will lose weight. Naturally, noticeable changes in body composition don’t happen in just one day. Long-term calorie surpluses or deficits are what cause visible fat gain or fat loss.
These percentages are not fixed; they vary considerably depending on a person’s lifestyle and level of physical activity. The basal metabolism of sedentary individuals may represent close to 70 percent of TDEE, while in athletes or bodybuilders who train for several hours a day, basal metabolism still remains high due to greater muscle mass, but the relative share of calories burned through activity increases significantly.
Therefore, it becomes clear that there are three main ways to raise your daily caloric needs. The first is to increase basal metabolism, primarily by building more muscle through regular strength training. The second is to boost physical activity levels, whether through structured workouts or simply by adding more movement into daily life (walking more, taking the stairs, or even doing household chores). The third is to take advantage of the thermic effect of food by prioritizing foods high in protein, which require more energy to digest than fats or carbohydrates.
Increasing physical activity, especially in a way that also promotes muscle growth, offers a double benefit. In the short term, it helps people who are overweight reduce fat or allows those at a healthy weight to eat a little more without gaining weight. In the long run, it raises basal metabolism itself, making it easier to maintain a healthy weight and body composition over time.
