The Dangers of Being Underweight

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According to the National Institutes of Health, a person is considered underweight with a Body Mass Index (BMI) of 18.5 or lower. BMI is calculated using a table that compares a person's height and weight to determine the percentage of body fat. While some people are naturally underweight because they have fast metabolisms, having a low percentage of body fat due to low caloric intake can lead to serious health complications.

Heart Irregularities

Inadequate caloric intake can cause heart irregularities. According to the National Eating Disorders Association, when a person is not consuming enough calories and is underweight, the body begins to conserve its energy by altering its normal processes. The heart may begin to beat slower, which can cause blood pressure to lower. If a person continues to remain underweight, the slowed heartbeat and lowered blood pressure can permanently damage the heart and eventually lead to heart failure.

Weakened Immune System

The United Nations Children's Fund reports that undernourished children are more likely to die from diseases. If the body doesn't receive enough nutrients and is underweight, it can weaken the immune system. The immune system is the body's defense mechanism against disease. People who are underweight are more likely to be afflicted with diseases and respond not as well to treatment due to their weakened immune systems. According to the United Nations Children's Fund, developing children who are underweight may not ever be able to form a healthy immune system.


According to the National Institutes of Health, women who are underweight may stop menstruating, a condition referred to as amenorrhea. Amenorrhea has two types: primary and secondary. Primary amenorrhea is when a female has not started menstruating by age 16, while secondary amenorrhea is when a female's regular menstrual period stops. The National Institutes of Health reports that regular menstrual periods are one of the body's indicators of good health. The absence of menstrual periods can be a sign that a female is not consuming enough calories for the body to function properly. Amenorrhea due to being underweight can cause ovulation irregularities and affect fertility.


The National Osteoporosis Foundation reports that underweight people are at a higher risk for developing osteoporosis. Osteoporosis is a condition in which the bones have a lower mass and become weakened and brittle, making them more likely to break. Being underweight makes a person more susceptible to osteoporosis because the person may not consume enough bone-strengthening nutrients, especially calcium and vitamin D. Low sex hormones also contribute to osteoporosis, so if a woman stops menstruating due to her weight, her risk for osteoporosis is even higher.


Anemia is a condition in which the blood lacks red blood cells. Red blood cells contain hemoglobin, a protein that helps the lungs supply oxygen to the rest of the body. According to the National Heart Lung and Blood Institute, those who don't consume enough nutrients, especially iron, are at a higher risk for developing anemia. If a person is underweight and not eating enough, the person can develop anemia. Underweight people who are anemic often feel fatigued because their bodies are not getting enough oxygen from their blood.

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