Due to the difficulty for survival, especially in the past, and numerous years of evolution, the human body is an almost perfect “machine”, able to use few resources to get great results, to adapt to the circumstances with the main objective of survive:
a) It takes advantage of energy to the maximum: it spends the minimum and saves the excess as adipose tissue (body fat). Energy is the main resource for survival, so losing fat is very difficult.
b) It does not create nor maintain tissues, but eliminate them, if they are not necessary. Muscle tissue is very expensive energetically, so gaining muscle is very difficult too.
c) It adapts to the circumstances: to the age, body composition, intake, physical activity, solar radiation, environmental temperature, amount of oxygen in the air, etc.
d) It metabolizes (transforms) or excretes (eliminates by the urine) harmful, unnecessary,… substances, or nutrients that are in excess.
e) It repairs or improves organs, tissues, etc.
f) It produces signs such as hunger, pain, sleep, cold, heat, fear, anger,… to give rise to an action that protects it.
g) It defends itself from infections or other diseases: the immune system (the body’s defenses) attacks harmful agents (for example, microorganisms such as bacteria or viruses), fever kills them by increasing the body temperature,… mucus, sneezing or coughing expel them from the respiratory tract, diarrhea expels them from the intestine, etc.