The changes that occur in nutrients when they reach the tissues are known by the name of metabolism, while cellular metabolism is the set of reactions that occur inside cells and which lead to obtain matter to grow and energy to perform vital functions.

Energy metabolism.

The main nutrients (carbohydrates, proteins and fats) have the function of supplying energy, the problem is that the tissues cannot use it directly, but they need certain cellular energy transformations to produce a single molecule, adenosine triphosphate (ATP). It is a nucleotide of great importance in metabolism, it acts as an energetic molecule being able to store or transfer energy very easily.
When it hydrolyzes (interacts with water) it dephosphorylates, producing on the one hand ADP and on the other a molecule of phosphoric acid.

• The carbohydrates that are ingested are mainly composed of starch, some disaccharides and glucose. Therefore, it is from these molecules that most of the energy is obtained.
• The energy in fat comes mainly from fatty acids.
• In proteins, the energy is obtained through starting from the amino acids that form it.

Intermediate metabolism.

It covers catabolism and anabolism. The catabolic routes refer to routes in which large molecules are degraded to obtain simpler ones, while anabolic routes obtain large molecules from simpler ones with energy expenditure. However, some pathways can be both degradative and biosynthetic, and are called amphibole pathways.

Regarding the role of vitamins and minerals in metabolism, they act mainly as coenzymes (molecules that are responsible for transporting chemical groups between enzymes) in the process of obtaining enzymes, which are responsible for catalyzing the metabolic reactions, that is, to increase the reaction rate. For this reason, it is necessary to avoid the deficit of these micronutrients so that the biochemical pathways are not altered.

For all these biochemical reactions to take place without any type of alterations, it is necessary to eat a diet
balanced that provides the necessary amounts of both macronutrients and micronutrients, since diets based on highly refined foods practically lack the minimum contribution of micronutrients, originating only
empty calories.

Basal metabolism.

Basal metabolism is defined as the energy expenditure necessary for the body to maintain the physiological functions essential for life.
Basal metabolism consumes the Higher percentage of the total energy that the body needs, up to 60 70%.

This expenditure refers to the energy consumed by each vital organ, each tissue … The heat that is produced as a consequence of the expenditure in those tissues it allows ue body temperature is ideal for life, which makes the human being a homeotherm.

Basal metabolism is considered as a global indicator of physiological normality, so two important aspects must be taken into account:

• An alteration in metabolism indicates that there is a problem related to the individual’s energy and body performance.
• Except for some indication that shows that there is a problem, the basal metabolic rate should not be modified.

Measurements of basal metabolism.

• Physical rest: the subject must be awake, without making muscular movements, lying on his back and without having previously performed any activity. During sleep, the basal metabolism decreases by 10%.
• Mental rest: the individual should be with a relaxed mind, without stress and without noise.
• Room temperature, 25 oC with light clothing: at this temperature the Basal metabolism remains constant, neither increases nor decreases.
• Fasting state: when you want to measure the basal metabolism of an individual, you must take at least twelve hours without having eaten anything, since the process of digestion, absorption and metabolizing nutrients carries with it an energy expenditure.
• Others: you should not ingest stimulant drinks, have not smoked or consumed any type of drug at least twelve hours before the test.

Energy expenditure due to physical activity,

The second major component of energy expenditure is physical activity due to the thermogenic effect of exercise. This expense can represent between 20 and 40% of the total expense.
This type of activity should be taken into account, because the human being is rarely in a state of absolute rest, except when sleeping.
The caloric consumption for physical activity is the most variable. Not only does it vary from one individual to another, but within the same
individual, since the physical exercise that he performs may vary depending on the day, time, etc.