Loss of Appetite, Malnutrition, and Undernutrition

Loss of appetite is part of the Sickness behavior or the fight-or-flight response, a positively selected program for conserving energy when an acute inflammation or an acute stressful life situation must be managed. Since foraging is energetically expensive, it is reduced to make the energy released available to the immune system or the brain/muscles. Then problems such as malnutrition and undernutrition, but also loss of muscle mass (protein loss) with simultaneous increase in relative fat mass can be the result. In chronic inflammation, this program is used again, and in old age there is also the phenomenon of “anorexia of aging.” The anorexia of aging is based on various factors, including reduced physical and mental activity. In both cases, the reduced appetite program is unfavorable.