Several causes or factors may be responsible for the undernutrition of an individual.
However the 2 most frequent are malnutrition of the sick subject and undernutrition of the elderly.
In a sick person, significant weight loss is related to an acceleration of metabolism. This is the case of infectious diseases (AIDS, for example) and cancer, this acceleration being linked to the fact that cancer cells take a lot of energy to multiply. This intense cellular activity leads to a decrease in resources and fatigue, which is why it is necessary to favor very nutritious foods in this case.
With age, people often suffer from a loss of appetite. An elderly person who feels isolated and loses his autonomy can easily slip into undernutrition and progressively weaken. We must react quickly and not let the complications settle. Another reason: the elderly assimilate less well food, which again promotes undernutrition.
For the particular case of undernutrition in adolescents, the cause may be a disorder of eating behavior, and in particular anorexia. It is a specialist in eating disorders who will have to take care of the young victim of this problem.