Signs of being underweight

Updated February 21, 2017

We hear a lot about the epidemic of obesity, but very little about the health dangers of being underweight. According to the National Heart, Lung, and Blood Institute, a healthy weight is one that falls between 18.5 and 24 on the body mass index (BMI) chart. A BMI of less than 18.5 is considered to be underweight.

Weakened Immune System

Our bodies need the nutrients and energy food provides to help fight off illness and infection. An underweight person may be more susceptible to getting sick, and when he or she does get sick, it can take longer to recover.

Hair Loss

Malnutrition and dehydration can lead to hair loss. Just like the body, the hair needs certain vitamins and nutrients, such as iron and vitamin B, to be healthy. When someone is underweight, he or she may experience hair loss from lack of proper nutrition.


Calcium and vitamin D are essential to keeping bones strong. Someone who is underweight may be deficient in these nutrients and develop osteoporosis, a disease in which the bones become weak and can break more easily. According to the National Osteoporosis Foundation, adults under the age of 50 need a total of 1,000 milligrams of calcium per day. Adults over 50 need 1,200 milligrams per day.

Hormonal Complications

In women, being underweight can cause problems with hormone production and lead to missed or irregular periods. Low body weight can also cause issues with fertility and make it difficult for a woman to become pregnant.

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About the Author

Based in Omaha, Neb., Amy Adkins has been a professional writer and editor since 2001. She writes primarily on the topic of health and health care and has experience in marketing communications, public relations, corporate communication and technical writing. She received her Master of Arts degree in communication from the University of Nebraska-Omaha.