What are the causes of being underweight

Underweight - general information

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Where normal weight ends and underweight begins cannot be precisely defined, as there are many individual biological parameters must be taken into account. For the sake of standardization, the BMI (Body Mass Index) classification speaks of underweight with a BMI of 18.5 kg / m2 and below.

However, a distinction must be made between underweight and malnutrition. Underweight does not have to mean that there is a disease-causing lack of energy and nutrients, while undernourishment always shows deficiency symptoms.

When it comes to malnutrition, one differentiates primary and secondary To shape. While insufficient amounts of food and essential nutrients are available in the primary form, illness or medication are the cause in the secondary form.


Compared to developing countries where partially almost 50% of the adult population are underweight, the occurrence is relatively rare in the western industrialized nations. In the US, 3.5% of the population is underweight and in France 4.9%. It looks similar in the Federal Republic of Germany. Although the incidence of obesity is increasing in our part of the world and is a serious nutritional problem of our time, the problem of underweight should not be underestimated.


In the western industrialized countries, a disease or metabolic disorder is the cause in the majority of cases of underweight, and less often only diet.

Insufficient intake of nutrients

Loss of appetite

This can be a side effect of radiation therapy, pharmacotherapy or painful conditions. It can also occur with poisoning and with the eating disorders bulimia and anorexia.


Malnutrition can be present in:

  • Cancers
  • insufficient intake during pregnancy
  • Performance-related increased energy and nutrient requirements (e.g. through sport)
  • Fast

Injuries / inflammations in the head area

Insufficient food intake can be caused by chronic inflammation of the mouth, throat or jaw or by injuries or narrowing in the head area.

Other causes

  • No food after operations
  • Obstructions in the gastrointestinal tract

Nutrient losses

Insufficient utilization of food

The fact that the ingested food is poorly utilized and underweight is the result can have the following causes:

  • too little breakdown of nutrients in the stomach, especially after part of the stomach has been removed
  • decreased absorption of nutrients in the intestine
  • Biliary obstruction
  • Crohn's disease
  • HIV infection
  • Hereditary enzyme disorders, e.g. lactose intolerance
  • Radiation or pharmacotherapy
  • Parasites
  • Metabolic diseases such as hyperthyroidism

Protein loss syndrome

Causes of abnormally high protein loss:

  • decreased production of protein in the body
  • Liver disease
  • Blood poisoning
  • Gastrointestinal diseases such as stomach cancer, colon polyps, food poisoning
  • Kidney disease (nephrotic syndrome)

Increased catabolism

Increased catabolism (breakdown metabolism) can be the result of an increased need for nutrients (due to infections, malignant tumors or certain medications) or metabolic disorders such as hyperthyroidism, adrenal insufficiency, diabetes mellitus or Simmonds disease.

Idiopathic anorexia

In so-called idiopathic anorexia (without recognizable causes), there is a hereditary anomaly in which, despite an increased energy intake, usually no significant weight gain can be achieved.


Age in itself is not an independent cause, but malnutrition and the resulting underweight are particularly important senior citizens reinforced on. The food intake that is far too low can have various causes:

  • Loss of appetite
  • physical disabilities
  • mental limitations
  • mental problems
  • social problems
  • financial problems
  • excessive alcohol and / or drug use
  • acute or chronic diseases

Risks / Complications


Underweight is usually associated with an insufficient supply of essential nutrients and / or trace elements. This primarily messes up the water and electrolyte balance. Especially in the eating disorder bulimia, misuse of laxatives can lead to imbalances. Usually there is not enough potassium in the blood (hypokalaemia). A deficiency in calcium, magnesium, iron, phosphate and chloride can also occur.

Underweight pregnant women have an increased risk of infant mortality and a low birth weight of the newborn.

Being underweight promotes the development of osteoporosis, even in younger people. This is a particularly insidious consequence, because the people affected do not feel any of it, they can feel completely fit and, for example, be excellent endurance athletes. After the first spontaneous fractures have occurred, the damage is often irreversible.

Other risks

  • Impairment of muscle function
  • increased susceptibility to infection
  • poor wound healing
  • slowed recovery after acute illness
  • increased risk of complications in the course of the disease
  • decreased life expectancy

For more information on the effects on the body of being underweight, see anorexia nervosa


Therapy for underweight corresponds to nutritional therapy for anorexia nervosa. Of course, if you are underweight, you don't necessarily need psychological support.

The average recommended energy intake for the therapy of underweight people is 2500-3000 kcal per day.