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Body mass index chart.svg

The underweight range according to the Body Mass Index (BMI) is the white area on the chart.
ICD-10 R62.8
ICD-9 783.22
MeSH D013851

The term 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). A BMI of under 18.5 is usually referred to as underweight[1]. This medical definition of underweight may differ from other uses of the term, such as those based on attractiveness.



The most common cause of a person being underweight is primarily malnutrition caused by the unavailability of adequate food, which can run as high as 50% in parts of sub-Saharan Africa and south Asia. The effects of primary malnutrition may be amplified by disease; even easily treatable diseases such as diarrhea may lead to death.

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


The most immediate problem with underweight is that it might be secondary to, and/or symptomatic of, 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. According to Robert E. Black of the Johns Hopkins School of Public Health, "Underweight status ... and micronutrient deficiencies also cause decreases in immune and non-immune host defenses, and should be classified as underlying causes of death if followed by infectious diseases that are the terminal associated causes."[2] People who are malnutrative underweight raise special concerns, as not only gross caloric intake may be inadequate, but also intake and absorption of other vital nutrients, especially essential amino acids and micronutrients such as vitamins and minerals.

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

Underweight is an established[3] risk factor for osteoporosis, even for young people. This is a particular insidious consequence, because the affected persons do not notice the danger, they can feel fit and may be brilliant for example in endurance sports.[4][5] After the occurrence of first spontaneous fractures the damage is often already irreversible.

Weight gain

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 food energy intake so that more food energy 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.

See also


  1. ^ Calculate your Body Mass Index, National Institutes of Health,, retrieved 2009-04-27  
  2. ^ Black, Robert E.; Morris, Saul S.; Bryce, Jennifer (28 June 2003), "Where and Why are 10 Million Children Dying Every Year?", The Lancet 361: 2226–34, PMID 12842379  
  3. ^ Gjesdal (2008). "Impact of lean mass and fat mass on bone mineral density: the Hordaland Health Study". Maturitas 59 (2): 191–200. doi:10.1016/j.maturitas.2007.11.002. PMID 18221845.  
  4. ^ Nattiv (1994). "The female athlete triad. The inter-relatedness of disordered eating, amenorrhea, and osteoporosis". Clinics in sports medicine 13 (2): 405–18. PMID 8013041.  
  5. ^ Wilson (1994). "Osteoporosis and fracture complications in an amenorrhoeic athlete". British journal of rheumatology 33 (5): 480–1. doi:10.1093/rheumatology/33.5.480. PMID 8173855.  

External links

Simple English

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 what 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.

Being underweight can have different causes:

  • Malnutrition, not eating enough, or eating the wrong kinds of foods. The right kinds may not be available, or they may be too expensive to buy.
  • The weight of a person is also determined by genetic factors. One of these factors is that some people have a "lighter build" than others.
  • Stress, anxiety and depression can influence weight
  • There are eating disorders that cause people to become underweight. The most common of these are anorexia nervosa and bulimia nervosa
  • Sudden unexplained weight loss can point to a disease, such as cancer, tuberculosis, hyperthyroidism, Diabetes or Inflammatory bowel disease
  • Sometimes the use of drugs leads to weight loss
  • Organs of the digestive tract may not be working as expected
  • People who train sports too much may be using more energy than they get.


There are problems that are related to being underweight. Very often being underweight is not the first problem. Rather, it often is the effect of another condition or disease. Unexpected weight loss requires the attention of a doctor. There might be another disease. If that disease is treated, the weight problem also disappears.

Another problem underweight people face is that the weight problem affects their health: Their immune system is weakened, and they may get sick more often. People who are underweight because they cannot get or afford the right food need special attention. Women who are underweight may not have their menstruation and face difficult pregnancies.

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