Underweight, also called underweightness, (synonyms: Abnormal weight loss; Abnormal weight loss; Dystrophy due to abnormal weight loss; Weight loss; Unusual weight loss; Unclear weight loss; ICD-10 R63.4: Abnormal weight loss) refers to a body weight with a BMI (body mass index) < 18.5 (according to the World Health Organization (WHO) classification).
Underweight is defined according to the ICD-11 criteria with a BMI < 18.5 kg/m2 in adults and falling below the fifth age percentile in children and adolescents.
You can calculate whether you are underweight by using BMI. BMI is calculated by dividing your body weight in kilograms by the square of your height.
The World Health Organization (WHO) has classified body weight by BMI for adults:
- Underweight: BMI < 18.5
- Normal weight: BMI 18.5-24.9
- Overweight: BMI 25.0-29.9
- Obesity grade I: BMI 30-34.9
- Obesity grade II: BMI 35-39.9
- Severe obesity grade III: BMI over 40
Age-dependent definition of a desirable BMI:
| Age group | BMI lower limit* * | BMI- Upper limit* * | Ideal weight (man) | Ideal weight (woman) |
| 19-24 years | 19 | 24 | 22 | 20 |
| 25-34 years | 20 | 25 | 22-22,5 | 20-21,5 |
| 35-44 years | 21 | 26 | 23-23,5 | 22-22,5 |
| 45-54 years | 22 | 27 | 24-24,5 | 23-23,5 |
| 55-64 years | 23 | 28 | 24,5-24,9 | 24-24,5 |
| ≥ 65 years | 24 | 29 | 24,9 | 24,9 |
* * National Research Council (NRC)
Frequency peak: adolescents, who are often eating disordered, and seniors are particularly likely to be underweight, and in some cases malnourished.
Sex ratio: Women are affected significantly more often than men.
The prevalence for people with a BMI < 18.5 is 2.3% of the population (in Germany). This corresponds to approximately 2 million people. If the above age-dependent definition of a desirable BMI were taken into account, the prevalence would be several times higher!
Course and prognosis: People with underweight have a lower life expectancy, since underweight causes the development of numerous secondary diseases, many of which accelerate aging processes. Thin people with a lot of abdominal fat are particularly at risk for cardiovascular (affecting the cardiovascular system) diseases. Thin women (BMI < 22.5) with particularly high abdominal fat (WHR* > 0.85) have a 2.4-fold higher mortality rate than women with the same BMI but little abdominal fat (WHR* < 0.77). With a BMI of 22.3 to 25.1, the mortality rate was 1.6-fold higher, and with a BMI above 25.2, it was 1.5-fold higher.
Underweight individuals (BMI: < 21) aged 40 years and older have a life shortened by approximately four years compared with normal-weight individuals of the same age (body mass index (BMI): 21-25 kg/m2): below a BMI of 25 kg/m2, the risk increased by 9% per 5 kg/m2 (hazard ratio: 0.81; 0.80-0.82 for the 5 kg/m2 increase); patients with a BMI of 25 kg/m2 had the lowest mortality risk (risk of death).
* WHI – waist-hip ratio = waist-hip index. You can determine for yourself whether you are a so-called apple or pear type by using the “waist-hip ratio” test.
