Underweight: causes, risks and therapy
Underweight people are often called “beanstalk”, “ironing board” or “asparagus tarzan”: It is not always easy for people who are extremely thin. From a medical point of view, “being thin” does not automatically mean “being healthy”. According to the classification of the World Health Organization (WHO), underweight is someone who has a body mass index  (BMI = body weight in kilograms divided by height in meters squared) of less than 18.5.
prevalence of underweight
Almost 2 million people in Germany are underweight, according to information from the Federal Statistical Office. Significantly more women than men are affected, especially in the age group between 14 and 29 years.
Similar values ​​apply to the other Western industrialized countries: in the USA around 3.5% of the population weighs too little, in France almost 5%. In contrast, in developing countries, almost 50% of adults are too light.Â
Causes: genetic predisposition and chronic diseases
The discussion about anorexic  models has brought the pathological BMI below 17 to the fore. This not only affects people with proven eating disorders, but above all those who simply do not gain weight, for example due to genetic predisposition, or suffer from chronic diseases. Inflammatory bowel diseases are often the reason for unintentional weight loss.
Other causes of being underweight can include:
In AIDS patients, sudden, unexplained weight loss can often be the first sign of the virus infection.
underweight and malnutrition
Being underweight does not automatically mean that you are malnourished. There are people who, due to their individual metabolism, do not gain weight and remain thin throughout their lives. In western industrialized nations in particular, such genetic factors or metabolic disorders are more often the reason for being underweight than, for example, malnutrition in developing countries.
When distinguishing between underweight and malnutrition, one must continue to distinguish between insufficient food intake on the one hand and nutrient losses due to missing or insufficient utilization on the other hand. Patients with chronic inflammation in the mouth and throat area, for example, can only consume a limited amount of food, while patients with lactose intolerance  have poor utilization of the food they eat.
underweight in the elderly
It is well known that eating disorders such as bulimia  and anorexia can lead to malnutrition and underweight and even death. What is much less known, however, is that older people are more likely to suffer from malnutrition and thus become underweight. Loss of appetite, physical and mental limitations, financial problems and acute illnesses are often a reason for the so-called “pudding seniors”.
health risk of being underweight
Just like being overweight, being underweight can pose specific health risks. Because “thin” doesn’t necessarily mean “healthy”. Anyone who is underweight is not only thin, but can also develop diabetes  (diabetes) like normal or overweight people. Underweight people are also significantly more susceptible to osteoporosis and infections than people of normal weight.
High susceptibility to infection due to lack of leptin
One reason for the increased susceptibility to infection is the reduced level of the protein “leptin” in the body cells of underweight people. This “satiation hormone” not only reduces appetite and thus ensures energy balance. Leptin  is a messenger substance and informs the body about the currently available energy in the body cells. The more fat is stored in the fat cells, the more leptin is released.
However, if the energy reserves are used up due to malnutrition or absorption disorders, the leptin level drops. As a result, all bodily functions that are not immediately vital are shut down. For the immune system  , this means that the immune cells are less active, more difficult to activate, and they mature more slowly.
Therapy for underweight
Only whipped cream does not help underweight people. The cause of the underweight must first be clarified and a diet initiated.
The aim of nutrition-based therapy must always be to compensate for any deficiencies without creating new health problems. In the long run, pure whipped cream is only good for raising cholesterol levels – but not for weight gain in the long term.
Being underweight can be treated with the means of nutritional therapy as with anorexia, whereby not all underweight people automatically have to be looked after psychologically. If high-calorie therapies do not work, astronaut food or even a feeding tube may have to be used.