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Underweight: symptoms, consequences, causes, therapy

by Josephine Andrews
Published: Last Updated on 398 views

Being underweight can have many causes. Sometimes low body weight just runs in the family. In other cases, stress, illness or eating disorders are behind it. Too low a weight becomes medically relevant if it is associated with deficiency symptoms. Read more about: When is underweight? What are the possible causes and signs of being underweight? What are the consequences? And how do you treat being underweight?

ICD codes for this disease:

ICD codes are internationally valid codes for medical diagnoses. They can be found, for example, in doctor’s letters or on certificates of incapacity for work.


quick overview

  • Definition : According to the WHO, underweight is defined as having a body mass index (BMI) of less than 18.5. However, being underweight is only medically relevant if there is a proven nutrient deficiency.
  • Symptoms : Tiredness and listlessness with a slight underweight. If you are severely underweight, for example, poor concentration , slow wound healing , susceptibility to infections , osteoporosis , menstrual cycle disorders, muscle wasting. Life-threatening underweight is a BMI below 14.5.
  • Causes : eg genetic predisposition, swallowing disorders, stress, depression, cancer, hyperthyroidism , drug or alcohol abuse, eating disorders.
  • Diagnosis : based on BMI. In addition, cause research in the doctor-patient conversation and, if necessary, with the help of examinations such as ultrasound and blood tests. The latter also help to identify deficiencies.
  • Treatment : If you are slightly underweight, you should change your diet and receive nutritional advice. If you are severely underweight: nutrient supply via nasal tube or PEG tube, if necessary infusions with nutrient solution. Supplementary gentle training for muscle building. Also: Treatment of the underlying causes.

Underweight: definition

Being slim is considered a beauty ideal, at least in the western world. But when does being slim become underweight? Doctors use the Body Mass Index (BMI) as a guide for this question.as a guide when answering this question. Accordingly, underweight is a BMI below 18.5. The BMI is calculated using a simple formula: body weight in kg divided by height in m2.

Example: If someone with a height of 1.80 m weighs 75 kg, the BMI is 75:1.802 = 23.15

In Germany, being underweight is comparatively rare. Only about two percent of those over the age of 18 are affected. Things are different in poorer regions of the world like India or Africa. Mainly because of the widespread lack of food, a broad section of the population is affected by underweight and deficiency symptoms.

Underweight: symptoms and consequences

If you have a BMI of less than 18.5 and are therefore underweight, your health is not automatically at risk. Being underweight is only harmful if there is malnutrition . Then the body gets too few nutrients such as proteins, fats, carbohydrates , vitamins and minerals .

The extent to which underweight triggers symptoms depends largely on its severity. The main signs of being slightly underweight are tiredness and listlessness.

Severe underweight shows other symptoms if it persists for a long time and is associated with serious nutrient deficiencies. These include:

  • limited physical and mental performance
  • lack of concentration
  • low muscle strength to muscle wasting
  • bad condition, rapid breathlessness during physical exertion
  • slow wound healing
  • bone loss (osteoporosis)
  • Water retention in the tissue (edema)
  • increased susceptibility to infection
  • in women: menstrual disorders up to the complete absence of menstruation and infertility
  • in men: infertility
  • in children: restricted growth

Life-threatening underweight threatens with a BMI below 14.5, because then the brain metabolism is impaired. Being so severely underweight is dangerous and requires immediate medical attention.

Underweight: causes

The reasons for being underweight vary greatly from person to person. Some people are genetically very slim: they have a fast metabolism, so they use the calories they eat particularly quickly and therefore hardly or not at all gain weight.

In other underweight people, certain illnesses or physical or mental stress are responsible for the low weight. Common causes of being underweight include:

  • Loss of appetite : Especially in old age, many people have less appetite and therefore eat less food.
  • Swallowing disorders, eg in diseases of the throat or esophagus
  • Stress: Persistent negative stress can literally “hit your stomach ” and spoil your appetite.
  • Depression: Depressed people often have less appetite in the acute phases of their illness.
  • Narrowing of the gastrointestinal tract, eg in the case of esophageal or stomach cancer
  • Crohn’s disease , ulcerative colitis : These chronic inflammatory bowel diseases can reduce the absorption of nutrients in the intestine .
  • Cancer diseases: tumor diseases increase energy consumption; at the same time, those affected usually feel less appetite.
  • Hyperthyroidism: Energy consumption increases due to excess thyroid hormones. People who have to take thyroid hormones because of hypothyroidism , for example, can also increase their energy requirements and become underweight as a result.
  • Drugs and alcohol abuse: Illegal substances such as cocaine, but also the legal drug alcohol can also cause underweight in the long term.
  • Eating disorders: Eating disorders such as anorexia nervosa or binge eating ( bulimia ) occur particularly in young people. Depending on the severity of the disease, the associated weight loss can be life-threatening. Professional treatment of the underlying eating disorder is therefore very important.

Underweight: Diagnosis

Underweight is diagnosed using the BMI. If it is below 18.5, according to the criteria of the World Health Organization (WHO), you are underweight.

Once the diagnosis has been established, the next step is to determine the underlying causes in order to derive the appropriate therapy. For this purpose, the doctor can use various examination methods after an initial consultation ( anamnesis ):

Imaging methods such as ultrasound or magnetic resonance imaging (magnetic resonance imaging, MRT) can be used, for example, to detect pathological changes in the gastrointestinal tract or in the esophagus. Blood tests provide information about the nutritional status and nutrient supply.

Underweight: treatment

The treatment of underweight depends on the one hand on the causes of the reduced body weight. For example, if there is a disease behind it, it must be treated. If stress triggers excessive weight loss, relaxation techniques and/or psychological treatment can help to develop more composure and to practice dealing with stressors in a more positive way .

On the other hand, the treatment depends on the severity of the underweight.

Slightly underweight

Slightly underweight can often be treated at home with the help of a professionally supervised change in diet. A nutrition plan that can be drawn up by a nutritionist can help.

Tips for slightly underweight :

  • Eat what you like. But make sure you eat a wholesome diet according to the recommendations of the German Society for Nutrition (DGE).
  • It’s easier to eat five small meals than three large ones.
  • Eat energy- and nutrient-dense foods like nuts and bananas.

Severely underweight

Those affected can hardly get a grip on being severely underweight on their own. Treatment under medical supervision in the hospital is usually necessary. Those affected are supplied with nutrients via a nasal tube, for example, in order to slowly increase the weight.

Clinical treatment is indicated for severe underweight, not least in view of the so-called refeeding syndrome: people who are severely underweight can develop electrolyte imbalances and avitamin B1 deficiency if the calorie increase is too rapid . They also excrete too little water and sodium while the ammonia level rises. All of this can severely disrupt the functionality of the organs – up to and including organ failure.

Refeeding syndrome can be avoided through gentle treatment with a slow increase in daily calorie intake.

If there are problems with eating through the mouth or nose , a so-called PEG tube can be used, which directs nutrients directly into the stomach. Infusions with nutrient solutions are also suitable in these cases.

In addition, moderate exercise can help build muscle if you are underweight.

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