BMI is a lie – here's why
A new study carried out by obesity researchers found that weight isn't a reliable predictor of heart health or diabetes risk.
Of the 40,000 people whose data was examined almost half of overweight participants, 29 percent of obese participants and 16 percent of morbidly obese participants were found to be metabolically healthy. In other words these people weren't at risk of type 2 diabetes or heart disease.
Over 30 percent of people who were in the so-called "normal" weight category were found to be metabolically unhealthy, putting them in the risk bracket for diabetes and heart disease.
Increasingly research is challenging the notion that a person who is overweight is also unhealthy. This study, from the Dieting, Stress and Health Laboratory at the University of California, backs up previous studies that claim a person can be "fat but fit" or slim but unhealthy.
The majority of obesity research tends to use body mass index (BMI) to determine whether a person is underweight, normal weight, overweight or obese but the authors of this study, published in the International Journal of Obesity, hope that their findings will be the "final nail in the coffin for BMI."
"There have been many misuses of BMI throughout the decades, despite very good evidence, like ours, that shows it's a flawed measure," said study co-author A. Janet Tomiyama.
Despite a growing consensus that BMI is unreliable, insurance companies continue to use it to work out how to reward or penalise policyholders and GPs still depend on it to diagnose obesity and advise patients to lose weight to improve their health.
This study reports that "using BMI categories as the main indicator of health, an estimated 74,?936?678 U.S. adults are misclassified as cardiometabolically unhealthy or cardiometabolically healthy."
The BMI method originated in 1832, when Belgian mathematician Lambert Adolphe Jacques Quetelet, "observed that human weight 'increases as a square of the height,' except during infancy and the adolescent growth spurt", reported Men’s Health.
However Quetelet’s intention wasn't to assess an individual's health or weight but to standardise the use of statistics in social science.
A person's BMI is calculated as her weight in kilograms divided by her height in metres, squared. A BMI of 18.5 to 24.9 is considered "normal," a BMI of 25 to 29.9 is "overweight" and a BMI greater than 30 is "obese."
One of the issues with BMI is that it doesn't take into account how much of an individual's weight is body fat and how much is bone, muscle and water. Because of this some ridiculous misclassifications take place, such as in the case of "obese" athletes, who weigh more because of muscle, or "healthy" older people who have lost muscle as they aged.
The NHS Choices website does point out the limitations of BMI, such as the failure to take into account age, gender or muscle mass.
Also Black, Asian and other minority ethnic groups have a higher risk of developing some chronic conditions, such as type 2 diabetes.
In addition to calculating BMI, the NHS recommends measuring around the waist, because carrying too much fat around your waist can increase your risk of serious health problems, like heart disease, type 2 diabetes, stroke and certain cancers. It's possible to have a healthy BMI and still have excess tummy fat — meaning you're still at risk of developing these diseases. If your waist is 102 centimetres (40 inches) or more (men) or 88 centimetres (34 inches) or more (women), you're at very high risk and should contact your GP immediately, NHS advises.
The University of California study — like many before it — confirms that obesity is a complex disorder, which isn't simply being overweight.