Body mass index (BMI) is widely used to assess health, but it has significant flaws for evaluating individuals. Originally developed for population studies, BMI fails to differentiate between muscle, bone, and fat, potentially misclassifying fit people as overweight. Experts recommend alternative metrics that better account for fat distribution and overall health risks.
Carissa Wong, a writer for New Scientist, recently calculated her BMI and found herself classified as overweight despite maintaining a healthy lifestyle. She eats plenty of fruit and vegetables, prioritizes fiber, rock climbs twice a week, and jogs during lunch breaks when possible. This personal experience highlights broader concerns about BMI's reliability.
BMI, calculated by dividing a person's weight by their height squared, was invented in the 19th century by mathematician Adolphe Quetelet to monitor population trends in height and weight, not individual health. It gained traction in the 1970s as a simple method to estimate body fat and obesity rates using just a tape measure and scales. In 1997, the World Health Organization adopted it as a standard health tool due to its ease of use.
Today, BMI influences access to treatments such as knee surgery, GLP1 weight-loss drugs, and fertility care. Standard categories include underweight below 18.5, overweight from 25 to 29.9, and obesity above 30, aimed at reducing risks and improving outcomes. However, BMI overlooks distinctions between bone, muscle, and fat. For instance, individuals with high muscle mass, like rock climbers building arm strength, may appear overweight or obese despite being fit.
Conversely, a BMI in the healthy range can mask dangerously low body fat, leading to issues such as halted menstruation, fragile bones, heart attacks, and infertility. BMI also ignores fat location; visceral fat around abdominal organs heightens risks for heart disease, high blood pressure, and type 2 diabetes more than fat in limbs or hips.
Better alternatives exist. A landmark study showed waist-to-hip ratio surpasses BMI in predicting heart attack risks and mortality. The weight-adjusted waist index, dividing waist circumference by the square root of body weight, better highlights visceral fat dangers. The body roundness index (BRI), incorporating height, waist, and weight, more accurately measures total and visceral fat than BMI alone. Devices using low-voltage electricity can also map fat distribution.
While BMI can signal when medical attention for body fat changes is needed, Wong suggests prioritizing habits like consuming fruits and vegetables, fostering social ties, ensuring sleep, and exercising regularly over fixating on numbers.
