underweight

physiology

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July 22, 2026

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Underweight is a physiological condition characterized by having too little weight for one's height. It is often defined by a Body Mass Index (BMI) below 18.5 for adults. This condition can result from insufficient food intake, inadequate nutrient absorption, or certain metabolic defects.12

Consequences of being underweight can include weight loss, failure to thrive, wasting of body fat and muscle, and a weakened immune system. In children, chronic underweight can lead to impaired growth and development. While a BMI of 18.5 to 24.9 is considered healthy for adults, a BMI below 18.5 indicates an underweight status.321

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body mass index
…weight status groupings, such as underweight, healthy weight, overweight, and obese, that are adjusted for age and sex. For all adults over age 20, BMI numbers correlate to the same weight status designations. For example, a BMI for adult women and men between 18.5 and 24.9 is considered healthy. A…
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…weight status groupings, such as underweight, healthy weight, overweight, and obese, that are adjusted for age and sex. For all adults over age 20, BMI numbers correlate to the same weight status designations. For example, a BMI for adult women and men between 18.5 and 24.9 is considered healthy. A…
Read More