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# How do you know if you are underweight?

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Calculating body mass index, or BMI, can tell a person if he is underweight. Usually, BMI calculations are used to determine if a person is overweight or obese, but being underweight is also a health risk, according to the United States Department of Agriculture.

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According to the National Health Service in the United Kingdom, being underweight causes health issues, such as low energy, an impaired immune system and brittle bones, and can affect a person's appearance with pale or discolored skin, thinning hair, and nails that break easily. Being underweight can be brought on by an underlying health condition, such as an overactive thyroid, or a diet that is insufficient in calories compared to the body's caloric output.

BMI is calculated by taking weight in pounds, dividing it by height in inches squared and multiplying it by a conversion factor of 703. A BMI that is below 18.5 signals that a person is underweight. BMI interprets a person's body "fatness," but that can vary even at the same BMI. For example, a woman generally has more fat than a man at the same BMI, and an older person generally has more fat than a younger person at the same BMI, according to the Centers for Disease Control.

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