Unique Energy Profile Associated with Persistent Thin Phenotype

The concept of constitutional thinness first emerged in the scientific literature at the beginning of the 20th century. Yet, this concept of a physiologically determined thinness, which clearly differentiated it from anorexia nervosa and challenged the basic notions of metabolism, intrigued the medical community. How could a young, healthy person with normal food intake present such an underweight state that resists body-weight gain? Due to either skepticism or disinterest, the fact remains that fewer than 50 clinical studies have focused on constitutional thinness in the last 90 years, contrasting with the hundreds of thousands conducted on the other end of the weight status spectrum (obesity). The aim of this review is to discuss current evidence of various aspects of this persistent thin phenotype, from its definition and diagnosis to its metabolic and physiological mechanisms. A deeper understanding of constitutional thinness would facilitate medical/societal recognition and clinical management of this condition. Greater insight into constitutional thinness could also help identify metabolic patterns that may be protective in the current context of the global obesity pandemic and its related critical health implications. Constitutional thinness stands out as a unique model questioning basic dogmatic knowledge about energy balance and metabolism.