Body Mass Index (BMI)
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Body Mass Index (BMI) is one of the most commonly used anthropometric indicators to assess the relationship between body mass and height. In clinical practice, BMI is primarily used for the initial assessment of the risk of underweight, overweight, and obesity, as well as associated metabolic and cardiovascular diseases. Despite its simplicity, this indicator has for decades remained a fundamental epidemiological tool used by doctors, dietitians, and public health organizations, including the World Health Organization (WHO). However, BMI does not analyze body composition or body fat distribution, which is why its interpretation requires taking into account age, sex, body build, and physical activity level.
BMI - what it is
BMI is a mathematical index developed in the 19th century by the Belgian mathematician and statistician Adolphe Quetelet. Its purpose was to create a simple tool to assess body proportions in a population. Currently, BMI is a standard element of nutritional status assessment in adults.
This index is used primarily in:
- the diagnosis of overweight and obesity,
- the assessment of the risk of metabolic diseases,
- the prevention of cardiovascular diseases,
- monitoring the effects of dietary treatment,
- epidemiological studies.
A high BMI is associated with an increased risk of developing numerous conditions, such as:
- type 2 diabetes,
- arterial hypertension,
- atherosclerosis,
- ischemic heart disease,
- obstructive sleep apnea,
- joint degeneration,
- certain cancers.
At the same time, an excessively low BMI may indicate malnutrition, hormonal disorders, chronic diseases, or eating disorders. In clinical practice, BMI is treated as a screening test that requires further diagnostics in the case of abnormal results.
BMI - how to calculate it
BMI is calculated by dividing body mass expressed in kilograms by the square of height given in meters.
BMI = \frac{body\ mass\ (kg)}{height^2\ (m^2)}
For example, a person weighing 70 kg with a height of 1.75 m will get the result:
BMI = \frac{70}{1.75^2} \approx 22.9
The obtained result is interpreted according to established diagnostic ranges. For the measurement to be reliable, body mass should be measured in the morning, on an empty stomach or under similar conditions, and height – without shoes.
It should be emphasized that classic BMI is intended mainly for adults between 18 and 65 years of age. In the case of children and adolescents, percentile charts taking into account age and sex are used. In turn, in seniors, the interpretation of results may require modifications due to physiological changes in body composition associated with the aging of the body.
BMI - scale and interpretation result to results
BMI interpretation is based on the classification developed by the World Health Organization (WHO). The individual ranges allow determining the level of health risk associated with body weight.
| BMI | Interpretation |
|---|---|
| < 18.5 | Underweight |
| 18.5–24.9 | Normal body weight |
| 25.0–29.9 | Overweight |
| 30.0–34.9 | Class I obesity |
| 35.0–39.9 | Class II obesity |
| ≥ 40 | Class III obesity (morbid) |
As BMI increases, the risk of metabolic disorders and cardiovascular complications increases. Visceral obesity, i.e., the excessive accumulation of adipose tissue within the abdominal cavity, is particularly dangerous. It can lead to chronic inflammation and hormonal imbalances affecting the functioning of the entire body.
In medical practice, BMI is often analyzed in parallel with:
- waist circumference,
- the WHR (waist-hip ratio) index,
- body composition analysis,
- body fat level,
- laboratory test results.
Thus, the BMI value itself does not constitute a complete health diagnosis, but is an element of a broader metabolic assessment of the patient.
BMI - why it doesn't tell everything about your figure
Despite its great popularity, BMI has significant diagnostic limitations. This index does not distinguish between fat and muscle tissue, which is why in people with highly developed musculature it can indicate overweight despite very good metabolic condition.
Examples include:
- competitive athletes,
- people who regularly do strength training,
- people with a heavy bone structure.
On the other hand, a person with a normal BMI can have a high level of fat tissue and low muscle mass. This phenomenon is referred to as metabolically obese normal weight (MONW – Metabolically Obese Normal Weight). Such patients often remain in the high-risk group for diabetes and cardiovascular diseases despite a normal BMI.
Modern methods of body composition assessment are currently of increasing diagnostic importance, such as:
- bioelectrical impedance analysis (BIA),
- DXA densitometry,
- analysis of fat tissue distribution,
- body circumference measurements,
- assessment of muscle mass and body hydration.
A comprehensive silhouette analysis allows for a much more precise assessment of health status than BMI alone. This is particularly important for physically active individuals, patients after significant weight reduction, and women during menopause, in whom the proportion of muscle to fat tissue changes.
