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Body Mass Index (BMI) – Definition & Calculation

The Body Mass Index (BMI) is a numerical value that relates a person´s body weight to their height and is used to assess weight status in adults and children.

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Things worth knowing about "Body Mass Index"

The Body Mass Index (BMI) is a numerical value that relates a person´s body weight to their height and is used to assess weight status in adults and children.

What is the Body Mass Index?

The Body Mass Index (BMI) is an internationally recognized measure that describes the relationship between a person´s body weight and height. It is widely used as a screening tool to classify individuals as underweight, normal weight, overweight, or obese (severely overweight). While the BMI does not replace a full medical assessment, it provides a quick and practical indicator of weight status.

How is the BMI Calculated?

The BMI is calculated using the following formula:

BMI = Body weight (kg) ÷ Height (m)²

Example: A person weighing 75 kg with a height of 1.75 m has a BMI of 75 ÷ (1.75 × 1.75) = 24.5 kg/m².

BMI Classification for Adults

The World Health Organization (WHO) categorizes BMI in adults as follows:

Underweight: BMI below 18.5 kg/m²
Normal weight: BMI 18.5 to 24.9 kg/m²
Overweight: BMI 25.0 to 29.9 kg/m²
Obesity Class I: BMI 30.0 to 34.9 kg/m²
Obesity Class II: BMI 35.0 to 39.9 kg/m²
Obesity Class III (severe obesity): BMI 40.0 kg/m² and above

Clinical Significance

An elevated BMI is associated with an increased risk of several chronic conditions, including:

Type 2 diabetes
Cardiovascular diseases
High blood pressure (hypertension)
Musculoskeletal disorders (e.g., osteoarthritis)
Certain types of cancer

A BMI that is too low (underweight) also carries health risks, such as malnutrition, bone loss (osteoporosis), and a weakened immune system.

Limitations of the BMI

Although the BMI is a simple and useful tool, it has important limitations:

It does not differentiate between muscle mass and fat mass. Highly muscular individuals may have a high BMI without being overweight in a clinical sense.
It does not account for fat distribution. Abdominal fat (visceral fat) poses particular health risks that the BMI alone cannot detect.
For children and adolescents, age- and sex-specific percentile charts are used instead of fixed cut-off values.
In older adults, the BMI may not accurately reflect body composition changes that occur with aging.

BMI in Children and Adolescents

For individuals under 18 years of age, BMI is not evaluated using fixed thresholds but is interpreted using age- and sex-specific percentile curves. A BMI above the 90th percentile is considered overweight, and above the 97th percentile is classified as obesity.

Complementary Assessment Methods

Health organizations recommend additional measurements to better assess individual health risks:

Waist circumference: A waist circumference above 88 cm in women and above 102 cm in men is considered a risk marker.
Waist-to-Hip Ratio (WHR): This ratio provides information about fat distribution patterns.
Bioelectrical Impedance Analysis (BIA): Directly measures body fat percentage.

References

World Health Organization (WHO): Obesity and overweight. Fact Sheet. Geneva, 2024. Available at: https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight
National Institutes of Health (NIH): Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. NIH Publication No. 98-4083, 1998 (updated guidance 2023).
Flegal K.M. et al.: Comparisons of percentage body fat, body mass index, waist circumference, and waist-stature ratio in adults. American Journal of Clinical Nutrition, 2009; 89(2): 500-508.

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