Body images to illustrate obesity: Images 1 to 5 show people who are underweight.

Image of an uderweight woman

The term underweight refers to humans that have much less weight than they should. Being underweight is bad for the overall health of the person affected. It is normal that a person's weights changes over time. Being underweight is often defined using a measure called the body mass index which calculates a score, based on height and weight. If this score is below 18.5, a person is generally said to be underweight. This is only a general indication, though as the scores must not be calculated for everyone. It does not apply to children and adolescents, for example.