Research & Education

Health Effects of Being Underweight


Not a day passes in which the issue of excess body fat is not visibly evident here in the US reminding us of the sad trajectory we face. Weight loss programs obesity education corrective surgeries and diet foods mark this era. Understandably researchers are dedicating copious amounts of time and resources to understanding the nature of obesity. In the shadow of this current epidemic however lurks a smaller but equally concerning cluster of individuals. With our focus on obesity we have unintentionally turned a blind eye toward the underweight population.

Being underweight (defined as a body mass index less than 18.5 kg/m2) predisposes individuals to health risks equally as concerning as those associated with obesity. Underweight individuals often lack sufficient muscle mass an indicator of protein status. Without adequate protein stores the body struggles to maintain metabolic processes and hormone balance as well as basic healing repair growth and maintenance.


Those who are underweight rarely have a robust immune system and are more predisposed to infections compared to other weight groups. The immune response is energetic requiring adequate nutritional and energy stores in order to function at maximum capacity. Energy restriction alone does not depress the immune system but nutritional inadequacy and protein deficiency are linked to poor immune responses and susceptibility to infection. Protein energy malnutrition which is often seen in underweight individuals has been shown to decrease NK activity reduce lymphocyte proliferation decrease cytotoxic T lymphocyte activity alter cytokine production and reduce antibody response. Infections stimulate catabolic activity and specifically protein catabolism in order to generate an appropriate immune response. Compromised immunity has often been noted in underweight individuals who are hospitalized and has been associated with poor prognosis and increased mortality.


Underweight women are more likely to experience infertility and pregnancy-related complications. A large cohort study of 536098 women associated underweight pre-pregnancy status with increased risks for adverse perinatal outcomes including preterm birth low birth weight intrauterine growth restriction and spontaneous miscarriage. Underweight women are more likely to have significant nutritional deficiencies that may lead to a reduction of placental weight and surface area restricting fetal nutrient supply and subsequently fetal growth and development. Low birth weight and preterm deliveries are natural outcomes. Further nutritional deficiencies may be linked to childhood developmental and cognitive impairment.

Psychiatric Disorders

While both obesity and underweight status have been linked to various mood and psychiatric disorders (PDs) the nature of the PDs differs between weight groups. In a study of 563 participants underweight status was significantly associated with avoidant or dependent PDs. The underweight group also exhibited habitual anxiety fear and was excessively afraid of feeling out of control. Additionally they share “hypersensitivity to negative evaluation are easily hurt by criticism or disapproval and may look for a low body weight to prevent unwanted criticism.” While it is worth considering the fact that many of these same tendencies are strongly associated with anorexia or bulimia nervosa it does not eliminate the trend for underweight women without diagnosed eating disorders to exhibit the same characteristics. In light of the increasing prevalence of mood and psychiatric disorders leading to mismanaged drug protocols with potentially fatal side effects it is imperative that weight status be considered a potentiating factor in individuals suffering from mood and PDs. 


Similarly depression is associated with obesity but also affects underweight individuals. A study of 43534 individuals found a very significant U-shaped association between BMI categories (underweight normal overweight and obesity) and depression (p ≤ 0.001) even after accounting for socio-demographic variables. In fact the number of incidences of depression among underweight individuals was much higher than among obese individuals despite the fact that more research has been devoted to the association between obesity and depression. Adequate protein stores are critical for the production and management of neurotransmitters and hormones. Additionally key nutrients such as B vitamins omega-3 fatty acids and minerals play a significant role in neurotransmitter production affecting both the severity and duration of depression. As with PDs an alarming amount of pharmaceuticals are devoted to treating depression when weight status may be a large determinant in this epidemic condition.

In addition to fighting obesity and addressing the associated consequences it is paramount that we not forget the smaller but significant population of individuals who are experiencing poor health as a result of inadequate weight. It is relatively easy to disregard these individuals perhaps even with a sigh of relief that they are not participants of the rising obesity epidemic; however their health challenges require the same attention to weight and adequate nourishment for optimal health.