Having a greater-than-normal body mass index (BMI) does not automatically mean you're classified as overweight.

The surge of overweight individuals globally is a concerning trend, with almost half of German adults carrying excess weight in 2019 as per statistics from the Federal Statistical Office. This placement positioned Germany above the EU average. Moreover, numerous studies indicate that many overweight children and adolescents carry these extra pounds into adulthood.

In identifying when an individual is considered overweight, the Body Mass Index (BMI) is often utilized. Geraldine de Heer, deputy director of the Clinic for Intensive Care Medicine and head of the nutrition team at the University Medical Center Hamburg-Eppendorf (UKE), explains that "practically, the BMI is often used because it is easy to collect and has a certain informative value in assessing nutritional status."

Online platforms such as health insurance company websites, the University Medical Center Mannheim, or even here at the Adipositas Society, allow for free BMI calculation and evaluation through a few simple clicks.

Simple BMI calculation for the masses

To determine your BMI, divide your body weight in kilograms by your height in meters squared. The resulting value provides information about the discrepancy between a person's weight and the considered normal weight for their size.

Much like with adults, children's BMI is compared to age and sex percentiles. A percentile shows how a value compares to other values within a group. For example, if a child is in the 25th percentile for BMI, it means that 25% of children of the same age and sex have a lower BMI than this child.

The World Health Organization (WHO) has categorized adult BMI values as follows:

BMI under 18.5 - Underweight
between 18.5 and 24.9 - Normal weight
between 25.0 and 29.9 - Overweight
between 30.0 and 34.9 - Obesity Grade I
between 35.0 and 39.9 - Obesity Grade II
over 40 - Obesity Grade III (also known as adipositas permagna)

According to de Heer, BMI provides more information about a person than just their weight. "The BMI determination can help laypeople develop a certain awareness of their nutritional status.", she emphasizes.

Nearly 200 years of BMI - a useful tool for insurers

Following World War II, insurance mathematicians discovered that their overweight customers had an increased mortality rate. To address this, an index was sought to describe the average body weight. The Quetelet index, developed by Belgian mathematician Adolphe Quetelet, was employed for such calculations.

Quetelet found that weight increases with the square of body size, with exceptions during growth spurts after birth and during puberty. In 1972, the Quetelet index was renamed Body Mass Index.

While the BMI is widely utilized as a benchmark for overweight and obesity in many health guidelines, it also faces its critics who point out its limitations.

Limitations of BMI

The main weakness of BMI, according to de Heer, is that it does not account for body composition, making it impossible to differentiate between muscle and fat tissue. This can result in significant miscalculations of nutritional status if only BMI is used as a measurement. Additionally, BMI does not reveal how muscles and body fat are distributed, such as the presence of harmful belly or hip fat.

Outliers in body size can also influence BMI, such as a tall basketball player with a lot of muscle mass and little body fat being considered overweight based on their BMI, even if they are considered healthy in terms of nutritional status. Similarly, an older or seriously ill person with little muscle mass might have a BMI in the normal weight range, falsely suggesting a healthy diet.

BMI tends to be inaccurate for individuals with a lot of muscle mass, as muscles weigh more than fat tissue. However, as noted by Yurdagül Zopf from the University Hospital Erlangen, this only applies to a select few cases. "So if someone gains weight, doesn't exercise, and clearly doesn't resemble Arnold Schwarzenegger, then what they gain is fat, not muscle," Zopf emphasizes.

Shape matters as much as weight

Another criticism of BMI: A 2000 study published in the "International Journal of Obesity" examined the relationship between body fat percentage and BMI in three different ethnic groups in Singapore. Using data from 300 adults with roots in China, Malaysia, and India, the study found that the relationship between body fat percentage and BMI differs significantly among the three groups.

The research team also compared their results with reference values based on data from white, Western people, again finding significant differences: A Western person with the same amount of body fat had a BMI of 30, a Chinese person had a BMI of 27, and a Malay person had a BMI of 26.

The research points out that differences in body structure could be the reason behind variances in obesity rates. If obesity is regarded as an excessive amount of body fat rather than excess weight, the obesity BMI thresholds in Singapore should be adjusted lower, according to the study.

There are alternatives to BMI for evaluating body status. These options involve the Fat Mass Index (FMI) and the Waist-to-Hip Ratio (WHR). A study published in "JAMA Network Open" in 2023 determined which of these values has the most significant impact on mortality.

Employing data from almost 388,000 British adults, a research team headed by physician Irfan Khan discovered that the waist-to-hip ratio had the strongest correlation with mortality risk and should consequently be taken into consideration when evaluating health. However, it remains unclear which BMI value correlates with the lowest mortality risk, the study notes.

Moreover, a 2023 study published in the journal "PLOS One" by an American research team led by Rutgers University found that BMI itself doesn't increase mortality risk. Utilizing data from half a million American adults, the study demonstrated that the relationship between BMI and mortality might fluctuate with age and seems connected to other risk factors. "Our study underscores the growing concerns about relying solely on BMI as the foundation for clinical decisions," the study's authors concluded.

The Obesity Diagnosis Journey - A Long and Winding Path

Due to the rising global trend of obesity rates, nutrition specialist Geraldine de Heer advocates for maintaining traditional guidelines. "We should see this as an opportunity to reflect on our dietary habits and physical activities, especially in children, and strive to adjust our course," she suggests.

Some experts believe that obesity recognition lags behind. "Typically, it takes six years for a doctor to inform a patient of their obesity," highlights Yurdagül Zopf. This is concerning as the condition is linked to increased inflammation levels in the body and heightened risk of cardiovascular diseases and specific cancers. "BMI is an essential primary indication for this," says Zopf, who oversees the Hector Center for Nutrition, Exercise, and Sport. "I view that positively, that everyone is made aware of what a BMI is."

In some cases, individuals with high muscle mass and low body fat may be incorrectly labeled as overweight based on their BMI alone. This miscalculation can occur even if they are considered healthy in terms of nutritional status.

Despite its limitations, BMI remains a widely utilized benchmark for overweight and obesity in many health guidelines. However, as noted by Yurdaguł Zopf from the University Hospital Erlangen, the misinterpretation of BMI is only applicable to a select few cases with significant muscle mass.