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At What Weight Is a Person Considered Underweight?

Although there is an increasing public emphasis on the health risks of obesity, being underweight is also a serious concern. Being underweight is different than being slender. Those who are underweight are putting their health, and potentially their lives, in danger. The healthiest weight for you depends on your height and body type, and varies from person to person. The body mass index can be used to determine whether or not you are underweight.

BMI Overview

Your body mass index, or BMI for short, is calculated based on your height and weight. It determines how much of your total body mass is fat. The formula for a BMI is not exact, but it provides a good estimation of a person's body fat percentage. A normal BMI is between 18.5 and 24.9. Those with a BMI of 25 or higher are considered overweight, while those with a BMI below 18.5 are considered underweight.

Significance and Causes

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When your BMI is less than 18.5 percent fat, you have a potentially dangerous low amount of body fat. Body fat is necessary for organ insulation and proper body function. A BMI that indicates someone is underweight can be the result of an eating disorder, illness, or disease. Certain cancers, hyperthyroidism and drug or alcohol abuse can also cause a low BMI.


There are significant health risks associated with being underweight. According to the "Journal of Bone and Mineral Research," there is a direct relationship between low body mass and low bone density. A study reviewed by the University of Pennsylvania School of Medical found that pregnant women whose BMIs are below 18.5 had a risk ratio of 1.39 for preterm birth. A low BMI can also put a person at risk for cardiovascular complications, anemia and anemorrhea.

Raising Your BMI

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Raising your BMI to a normal level requires healthy eating, attention to caloric intake and consistent exercise. While it might seem counterproductive to exercise while gaining weight, 30 minutes of exercise helps maintain joint and bone health and will increase your appetite. Eat between 500 to 1,000 calories more than you burn each day to gain one to two pounds per week. Eat high-calorie, nutrient-dense foods to boost your daily caloric intake instead of junk food high in sugar and unhealthy fat.