When is being underweight dangerous?


Being underweight has a big impact on your health. But when does it become critical? Facts about being underweight you should know.

Every person has an individual body structure. Where and how much fat you put on, whether you gain weight quickly or not – all of this varies from person to person and depends heavily on genes. This is normal and nothing to worry about.

However, body weight not only affects your appearance, but also your health . Therefore, from a health perspective, it still makes sense to know when you are considered underweight and what consequences this can have. Some methods can help you assess whether you are already in a critical weight range.

Normal weight versus ideal weight

Depending on gender, body type and size, every person has an individual range in which their weight is considered healthy. In this case, science speaks of normal weight. For example, it can be determined using the body mass index (BMI).

Normal weight should not be confused with ideal weight. This term is not scientifically defined and usually follows personal, aesthetic goals. The healthy normal weight can definitely differ from your personally defined ideal weight. This can be the case, especially if the underweight is due to an eating disorder.

Since every person can define their own ideal weight, this article is about the difference between normal weight and underweight. So, when do low body weight and body fat become bad for your health?

How can underweight be measured?

First of all: Underweight is when your body weight is below normal for a certain height. Body weight alone is therefore not, or only to a limited extent, meaningful. There are various methods to find out more reliably whether you are normal or underweight.

Body mass index

Body mass index (BMI) is the most commonly used measure to define normal, overweight or underweight. The values ​​are based on empirical studies that have determined at what point a weight that is too high or too low is associated with risks for the body.

It is calculated by relating body weight (in kilograms) and height (in meters squared):

  • Body weight (kilograms) : Height (meters x meters) = BMI

You can then use the result to see whether your body weight is appropriate for your height, too high or too low:

  • Underweight: under 18.5
  • Normal weight: 18.5 to 24.9
  • Overweight: 25 to 29.9
  • Obesity: from 30

With the BMI calculator you can quickly and easily determine your BMI.

A BMI between 18.5 and 25 is considered a healthy weight for adults. As soon as the weight falls below this value, doctors speak of being underweight. Values ​​below 17.5 are considered anorexic. This means that the person affected has a weight that is typical for anorexia (you can find out what this means for your health below). If the BMI is even lower, it becomes life-threatening. From a BMI of 14.5, the brain’s metabolism is already impaired.

The BMI is therefore relatively easy to record, but should only be used as a guideline for initial orientation. Because the BMI does not indicate what your body composition is, i.e. how much body fat and muscle mass you have.

Evaluate underweight: Parameters apart from BMI

Blood sample (symbolic image): The urinary gland value can be determined with a blood test.
Blood sample (symbolic image): A nutrient deficiency can be determined with a blood test. (Source: Westend61/imago-images-bilder)

Being underweight – i.e. a BMI around 18 – is not always acutely dangerous. Low body weight can also be a predisposition. However, it becomes dangerous when, in addition to being underweight, there is so-called malnutrition, i.e. when important food components (such as calories, vitamins or protein) are not sufficiently absorbed.

In order to find out whether you are sufficiently supplied with nutrients despite a low BMI and whether your ratio of fat to muscle mass is in a healthy range, it makes sense to take the following methods and parameters into account in addition to the BMI:

  • Bioelectrical Impedance Analysis (BIA): Measurement of the ratio and distribution of fat and muscle mass
  • Blood analysis : measurement of clinical parameters, for example iron or albumin
  • Recording your weight progression : the faster you lose weight, the greater the need for action
  • Nutritional history : Recording nutritional behavior (also to determine whether an eating disorder is the cause of the underweight)

    The consequences of being underweight

    Ultimately, being underweight is always an expression of an energy deficit. However, the body relies on a minimum amount of energy (calories) to function. If, in addition to being underweight, there is also malnutrition – whether in calories or individual nutrients – the immune system and the entire organism are weakened. Those affected are then more susceptible to infections and certain illnesses.

    It clearly signals that the body has too little energy. People who are underweight often suffer from:

    • Lack of concentration and tiredness
    • Limited physical performance
    • Lack of motivation
    • Irritability
    • Dry skin
    • Frequent freezing
    • Low blood pressure

    Severe underweight (BMI 17.5 or lower) is usually associated with a severe nutrient deficiency. This reduces physical and mental performance and has serious consequences:

    • Muscle loss
    • Hair loss
    • Memory disorders
    • Impaired digestion
    • Edema
    • Fertility disorders and even infertility
    • Organ damage
    • Weakened immune system

    A weakened immune system increases the risk of becoming infected with infectious diseases. In addition, people who are underweight generally suffer more severely from illnesses and complications are more common. They also recover more slowly after illnesses and wounds heal more slowly.

    However, which symptoms appear when and to what extent varies from patient to patient and always depends on the degree of underweight and the speed of weight loss.

    Underweight in women

    In women, being underweight can also affect hormonal levels. So the menstrual cycle can become irregular; If you are severely underweight, your period may even stop completely ( amenorrhea ). In this way, being underweight can be a contributing cause of an unfulfilled desire to have children .

    When being underweight over a long period of time, women also often develop osteoporosis .

    Causes of being underweight

    Being underweight can have many causes. However, they have one thing in common: the body has less energy (calories) available than it needs to fulfill its functions.

    The causes can be divided into three groups:

    Insufficient food intake , for example due to

    • Loss of appetite (e.g. due to stress or depression)
    • Swallowing disorders
    • Injuries (e.g. to the brain or digestive organs)
    • Malnutrition and diets

    Decreased nutrient absorption in the intestine , for example due to

    • Inflammatory bowel diseases (such as Crohn’s disease, ulcerative colitis, celiac disease)
    • Infections with parasites (such as tapeworms)

    Increased energy expenditure , for example due to

    • Chronic diseases that cost a lot of energy (such as cancer, AIDS, chronic obstructive pulmonary disease (COPD))
    • Certain medications (such as thyroid hormones)
    • Metabolic diseases that increase energy turnover (such as hyperthyroidism).

    eating disorder

    • Anorexia nervosa
    • Binge eating addiction (bulimia)

    Psychology plays an important role in anorexia

    An extremely thin girl stands on the scales.
    Anorexia and bulimia: People with these eating disorders feel too fat despite being extremely underweight. (Source: Zinkevych/Getty Images)

    It is important to know that not every underweight person automatically has an eating disorder. In eating disorders such as anorexia, thoughts and behavior constantly revolve around the topic of food and weight. This includes, among other things, restricting food, abusing laxatives or exercising excessively . In addition, the relationship with one’s own body is impaired.

    • Conclusion

      You are considered underweight if your body mass index (BMI) is below 18.5. However, being underweight as defined in this way does not always have to be acutely dangerous, because the BMI reveals nothing about your body composition or nutrient supply. However, if you are severely underweight (BMI below 17.5) and have malnutrition – i.e. if you do not consume enough calories, protein or vitamins – this can weaken your immune system and the entire organism. If your BMI is 14.5 or lower, your life is in acute danger.


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