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As we enter a new year, the body mass index (BMI) is facing yet another wave of criticism. A Lancet-commissioned group of experts is now challenging its use as a diagnostic tool for obesity. Their argument is simple: doctors should assess the overall health of a patient, not rely solely on this outdated and flawed metric.
BMI is calculated by dividing a person’s weight in kilograms by the square of their height in meters. This leads to a classification system: underweight is anything below 18.5, normal weight ranges from 18.5 to 24.9, overweight spans 25 to 29.9, and anything above 30 is considered obesity.
Given the serious health risks associated with obesity, you’d think that the measurement used to diagnose it would be scientifically sound. However, BMI has long been criticized for its inability to distinguish between muscle and fat. This is a critical shortcoming, as excess body fat, not muscle, is a key risk factor for cardiovascular diseases, while BMI doesn’t provide that distinction.
Recent studies show that a better measure of obesity might be based on body fat percentage rather than BMI. In fact, redefining obesity with body fat percentage could result in up to 35% of men with a normal BMI being classified as obese. In addition, the study found that BMI both under- and over-diagnoses obesity, with up to half of those categorized as overweight and over a quarter of those labeled as obese being metabolically healthy.
Clearly, BMI is not a reliable indicator of health. What’s needed is a more accurate measure that reflects the percentage of body fat, a key determinant of cardiovascular health. This is where a technique more than 2,000 years old could offer a solution.
The 2,000-Year-Old Technique: Archimedes’ Principle
In 250 BC, the famous Greek mathematician Archimedes was tasked by King Hiero II of Syracuse to determine if a goldsmith had deceived him by substituting part of the gold in a crown with a cheaper, lighter metal. Archimedes needed to determine the crown’s density without damaging it, which seemed an impossible task. However, Archimedes had a breakthrough while taking a bath. He realized that the amount of water displaced by an object when submerged in a bath is proportional to its volume—leading him to the concept of buoyancy.
This discovery, later formalized as Archimedes’ Principle, states that the buoyant force on an object is equal to the weight of the water it displaces. Archimedes applied this principle to determine the crown’s density, revealing whether it was pure gold or a mix of metals.
Fast forward to today, and we can apply this same principle to measure body fat percentage. This method is called underwater weighing. A person is first weighed normally and then submerged in water on a special underwater chair with scales. The difference in weight measurements before and after submersion indicates the buoyant force, which can be used to calculate the volume of the body. This volume, combined with the known densities of fat and lean tissue, allows for an accurate estimation of body fat percentage.
While this method may not be as convenient as BMI, and it requires specialized equipment, it provides a far more accurate assessment of health risks. Body fat percentage is closely linked to cardiovascular disease risk, making it a more useful tool for assessing overall health.
A More Accurate Way to Assess Health Risks
Although the ancient technique of underwater weighing might seem like a relic from the past, it remains one of the most accurate ways to measure body fat and assess health risks. As the debate over BMI continues, it’s clear that we need a more nuanced approach to understanding body composition. Archimedes’ principle, with its deep historical roots, might just offer the key to a better, more comprehensive way to assess our health.
Source: The Lancet
