Body Mass Index (BMI) is a simple metric calculated by dividing weight in kilograms by the square of height in meters, resulting in one of four categories: underweight (below 18.5), normal weight (between 18.5 and 24.9), overweight (between 25.0 and 29.9), or obese (30 or more).
However, this tool has known limitations because it cannot differentiate between fat mass and muscle mass, nor indicate the distribution of body fat or whether excess fat compromises individual health, making it unreliable for obesity assessments.
In its place, more reliable indicators, such as abdominal circumference, are suggested. A recent study published in the journal Nature demonstrates that combining waist measurement with organ function tests provides a more accurate diagnosis of both obesity and mortality risks associated with the condition.
Under this new standard, only individuals with a BMI above 40 are considered obese. For those with values below this limit, doctors assess fat accumulation through waist measurement. A BMI equal to or greater than 30, along with an elevated waist or waist-to-height ratio, signals obesity. Similarly, a BMI between 25 and 29.9 is classified as obesity if these two measures are above expected levels.
By applying these new parameters to data from over 800,000 people, researchers identified significant flaws in the diagnosis made by BMI. Between 1.2% and 3.1% of those diagnosed with obesity did not fit the new definition. Furthermore, a higher percentage of adults, ranging from 5.2% to 8.6%, had adiposity (excess fat) that was not detected by using BMI exclusively.
Globally, these classification errors can affect millions of people, given that the World Health Organization (WHO) uses BMI as the global parameter to classify obesity, currently estimating that one billion people live with this condition.
The concept of a 'new BMI' is divided into two phases. Initially, healthcare professionals use waist circumference in conjunction with BMI to determine the presence of adiposity. The importance of waist circumference lies in the fact that fat deposition in the abdominal region is linked to higher health risks compared to other areas of the body.
The second stage aims to verify whether this excess fat is actually causing health damage. To do this, the authors defined specific criteria to identify dysfunction in several systems, including heart failure, obstructive sleep apnea, osteoarthritis, and chronic kidney disease.
If any dysfunction is found, the diagnosis is clinical obesity, indicating that excess fat is harming organs and tissues. If the individual has excess fat but organ function remains intact, the diagnosis is pre-clinical obesity, a stage associated with potential risks but not yet an established disease.
Individuals with obesity, under this new definition, showed a mortality risk 6% to 7% higher than those without excess weight. Within this group, those classified with clinical obesity presented a higher mortality risk than those classified with pre-clinical obesity.
Although this methodology makes the process more complex for healthcare professionals, the authors argue that the diagnosis of obesity must be personalized to ensure the best patient care, whether through lifestyle counseling, weight loss medications (such as GLP-1 agonists, known as 'weight loss pens'), or surgical intervention.
BMI was created in the 1830s by Belgian statistician Adolphe Quetelet, and its original purpose was not to assess a person's weight. In fact, Quetelet developed the formula for what he called 'social physics,' a component of his studies on the 'average man.'
For Quetelet, the 'average man' was a statistical construct, representing a hypothetical human whose physical, moral, and intellectual characteristics (including weight, height, age of marriage, mortality rate, and predisposition to crime) mirrored the population average, with these measurements based on white men.
This notion of the average man was revived decades later by Francis Galton, a figure known for founding the eugenics movement in 1883. The term, meaning 'well-born' or 'of good lineage,' promotes the belief that the population could be improved by encouraging the procreation of individuals considered superior, generally white Europeans from higher classes.
BMI was conceived based on a specific social cut, implying that the scale is biased for the rest of the world. Clinical studies have already proven that this can disproportionately affect Black people, who tend to have higher BMIs than white people, especially among women, even when healthy.
Even ignoring the historical context, inherent problems with BMI persist. As mentioned, it does not distinguish between muscle mass and fat. Since muscle also has weight and is denser than fat, a very muscular person can have a high BMI, even with little fat, and be erroneously classified as overweight or obese.
Additionally, BMI neglects variations in age and sex; for example, elderly people may have a 'normal' BMI but possess low muscle mass. It is important to note that a 'normal' BMI does not guarantee health, as demonstrated in a 2016 study with 40,000 adults in the United States, where BMI was correlated with more detailed health indicators, such as insulin resistance, inflammation, blood pressure, triglycerides, cholesterol, and glucose levels.
Almost half of individuals classified as overweight and about a quarter of those classified as obese by BMI were metabolically healthy. Conversely, 31% of people with a 'normal' BMI did not have metabolic health.
