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), with a value under 18.5. It is important to note that the BMI is a statistical estimate, and some individuals classified as underweight may be perfectly healthy.


The most common cause of a person being underweight is primarily malnutrition caused by the unavailability of adequate food, which can be as high as 50% in some parts of the world.

In the presence of adequate food resources, being underweight can sometimes be the result of mental or physical disease. There are hundreds of possible medical causes for excessive weight loss. Some of the more prevalent include:

  • Anorexia Nervosa
  • Bulimia Nervosa
  • Over-training (endurance sports)
  • Hyperthyroidism
  • Chronic liver and kidney disease
  • Malabsortpion due to bowel diseases
  • Cancer and the treatment
  • Tuberculosis
  • Diabetes
  • Anxiety and depressive disorders
  • Drug abuse

Problems Of Underweight

The most immediate problem with underweight is that it might be secondary to an underlying disease. Unexplained weight loss requires professional medical diagnosis.

Underweight can also be a primary causative condition. Severely underweight individuals may have poor physical stamina and a weak immune system, leaving them open to infection. People who are malnutrative underweight may also lack intake and absorption of other vital nutrients, especially essential amino acids, vitamins and minerals.

In women, being grossly underweight can result in absence of menstruation and possible complications during pregnancy. It can also cause anaemia and hair loss.

It is an established risk factor for impaired bone metabolism and may lead to osteoporosis, even for young people.


If an individual is severely underweight to the point where problems with his or her health develop, it may be necessary for the person to make a concentrated effort to gain weight. The treatment for an underweight individual is to increase the nutritional energy intake so that more is consumed than is being used as work. It is usually suggested that weight training is also to be undertaken to increase muscle mass.

If weight loss results from a disease, resolving the illness and consuming adequate calories can bring many underweight individuals to a healthy body weight.

Anorexia nervosa and bulimia are eating disorders associated with a negative body image. Anorexia nervosa is a disorder in which people extremely limit their food intake. This results in dangerously quick weight loss, to the point of starvation. This disorder is most commonly found in adolescent females.

Bulimia is binge eating followed by self-induced vomiting. It's often associated with anorexia nervosa. Many people with bulimia do not lose a lot of weight.

These conditions require the help of psychological counselling and psychiatric intervention.

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