Body mass index, or BMI, is a function of your weight and height. BMI is used as a measure of body fatness with high BMIs indicating overweight or obesity, and low BMIs indicating possible underweight malnutrition. A low BMI is one that is less than a value of 18.5. A person standing 5 feet, 9 inches would need to weigh 124 lbs. or less to have a BMI this low.
BMI Calculation
Your BMI is based on your height and weight. To calculate your BMI using pounds and inches, divide your weight in pounds by your height in inches squared and then multiply by a conversion factor of 703. To calculate your BMI from kilograms and centimeters, you can divide your weight in kilograms by your height in square meters. Since height is commonly measured in centimeters, divide your height in centimeters by 100 to convert it to meters and then square it.
BMI Categories
According to the U.S. Centers for Disease Control and Prevention, a low BMI is less than 18.5. A normal BMI is between 18.5 and 24.9. A BMI of 25 to 29.9 is considered overweight, and a BMI of 30 or more is considered obese. Athletes may have a high BMI but a low percentage of body fat and in this case BMI may not be a reliable indicator of body fatness.
Basis for Low BMI
A person may have a low BMI for a number of reasons. Always ask your physician if you have questions or concerns about your BMI. Someone who has experienced recent, unintentional weight loss due to a number of medical conditions may have a low BMI which needs to be addressed by nutritional therapy. People with a naturally slight build may have a low BMI in spite of eating a nutritious diet. Men and women with eating disorders may have a low BMI; a low BMI is one criterion for diagnosing certain eating disorders.
Considerations
BMI can be calculated for children and teens the same way it is calculated for adults; however, BMI may be a less reliable indicator of body fatness for younger age groups because boys and girls grow at different rates. Health care providers often use height-weight percentiles by age to evaluate weight status.
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