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Underweight refers to a human who is considered to be under a healthy weight. The definition is usually made with reference to the body mass index (BMI). Most consider individuals under 18.5 to be underweight - although some authorities use a higher value of 20. It is important to note that the BMI is a statistical estimate and some indivduals classified as underweight may be perfectly healthy.


Underweight individuals generally have poor physical stamina and a weak immune system - leaving them open to infection. They are also at higher risk from heart disease, blood vessel disease, and osteoporosis.

In women, being underweight can result in amenorrhoea (absence of periods) and possible complications during pregnancy.


The causes of being underweight can be:

* Poor diet or starvation (malnutrition)
* Psychological problems (such as Anorexia nervosa or Body dysmorphic disorder)
* Weight loss due to chronic illnesses (e.g. tuberculosis or cancer)
* Hyperthyroidism
* Drug use (e.g. Cocaine)


The treatment for an underweight individual is to increase the calorie intake so that more calories are consumed than are being used as energy. It is usually suggested weight training is also undertaken to increase muscle mass.

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