Body mass index

Everyone who is concerned with losing weight sooner or later comes across the three letters: BMI. They stand for body mass index, or body mass index. What is the BMI, why do you need it and how is it calculated? How meaningful is it?

The body mass index is a measure for evaluating a person's body weight in relation to their height. Although the BMI is only a rough guide, as it does not take age or gender into account, it does allow classification into normal, overweight and underweight.

To calculate your body mass index, you need your weight in kilograms and your height in metres. If you relate your weight to the square of your height, you get your body mass index.

BMI= weight / height2 

It is easier with a BMI calculator: simply enter your height and weight and your BMI will be calculated for you. Many health insurance companies, for example Barmer, Techniker, IKK or BKK, have a BMI calculator on their website.

Often it is also indicated directly whether your weight is within the normal range or whether you weigh too much or too little.

What good does it do you to know that your BMI is 23.7? First of all, nothing.

For this, the World Health Organisation (WHO) has made a classification that defines the ranges for underweight, normal weight and overweight. This applies to men and women over the age of 18:

If you are in the golden mean, you have done everything right. With a BMI between 18.5 and 24.9, your weight is in the normal range.

A value below 18.5 is called underweight, a value below 16 is called severe underweight. The causes can be chronic illnesses, stress or mental illnesses such as bulimia or anorexia. In any case, underweight should be clarified with the family doctor.

A BMI between 25 and 30 can indicate overweight. Since the calculation does not distinguish between fat and muscle mass, this can lead to an incorrect classification. A well-trained strength athlete with a lot of muscle and little fat can have a BMI above 25, although he is far from overweight.

A body mass index over 30 indicates morbid obesity. The weight category also known as obesity can have a negative impact on health and life expectancy. The more severe the overweight, the greater the risk of concomitant diseases such as high blood pressure, cardiovascular disease or diabetes. A BMI over 30 should definitely be discussed with the family doctor.

The WHO classification is the best known classification. However, it only gives a rough overview, because it does not take into account the gender or the age of the person. These two factors can influence weight.

Men and women differ in physique. While women often have more curves, men can score with muscles. Many men are wiry and well-trained, yet weigh a considerable amount. Women, on the other hand, appear visually more overweight, but are lighter than men. There is a simple reason for this: muscles are heavier than fat. That is why many men have a comparatively high BMI without being overweight.

Who doesn't like to think back to the years when the scales showed 5 kilograms less? Why do we gain weight even though our eating habits haven't changed at all? The human body changes with age. Around the age of 40, metabolism and body composition change, and most people gain weight. The normal weight and thus the BMI shifts slightly upwards. At an advanced age, body weight can decrease again, because the body increasingly loses muscle mass, which is heavier than fat. Thus, older people of normal weight can be wrongly classified as underweight because of their low BMI.

Taking into account gender and age, the following table applies as a guide:

The BMI of children is a special case in the calculation of normal weight because children do not grow homogeneously. The BMI is calculated according to the same formula as for adults, but the assessment is different. It is done according to the S2 guideline of the Arbeitsgemeinschaft Adipositas im Kindes- und Jugendalter (AGA, 10/2015) using gender-specific reference curves. You can find more information, for example, here [2].

If you want to quickly and easily classify your body weight, the BMI with the WHO classification is a good method. You can get a more accurate assessment if you evaluate your BMI according to the extended gender- and age-specific tables. However, one weakness remains: No distinction is made between fat and muscle mass. A well-trained athlete with lots of muscles can be considered overweight based on the BMI.

The distribution of fat is also not taken into account, although according to current knowledge it is decisive for the health risk. While the hip-emphasised fat on the thighs and buttocks is less detrimental to health, visceral abdominal fat can drastically increase the health risk.

The Smart Body Mass Index SBMI is an alternative to the BMI developed by biologist Christian Bachmann and colleagues. The SBMI is a purely comparative figure on a scale of 70 points that takes age and gender into account and provides an assessment of weight-related health risk. Here you can have your personal SBMI calculated and evaluated according to your input [3]. The evaluation is detailed and gives tips on nutrition and physical activity. In Germany, the SBMI has not become established.

BMI is the ratio of body weight to height. This allows you to estimate whether you are underweight, normal weight, overweight or obese. Depending on the evaluation table, gender and age are taken into account. However, no distinction is made between muscle and fat, nor is it taken into account where the fat is located. These weaknesses are also disregarded in the alternative determination of the Smart BMI, which, however, gives an estimate of the weight-related health risk.