Here is some good news for 54 million people classed as "overweight" or "obese" according to their body mass index score: You may not actually be unhealthy. A new study finds that BMI frequently mischaracterizes people and is a crude measure of health despite its wide use.
BMI is a person's weight in kilograms divided by the square of their height in meters. Someone with a BMI in the 18.5-24.9 range is considered "healthy" while someone in the 25-29.9 range is "overweight." Anything above 30 and you're in the "obese" zone. BMI is often used as a proxy for health because higher weight to height ratios are associated with higher blood pressure and greater incidence of heart disease and diabetes.
But not always. UCLA psychologists examined a sample of about 40,000 Americans and found that almost half of the "overweight" individuals, 29% of obese individuals, and 16% of the very obese were healthy based on their blood pressure, cholesterol, and glucose and insulin resistance levels. At the same time, 30% of the normal BMI people were "cardiometabolically unhealthy."
There have long been criticisms of BMI as a crude health metric. People point to athletes like Tom Brady (and the whole of the Denver Broncos football team), who officially fall in the overweight or obese category despite their obvious health. BMI doesn't distinguish muscle from fat, or fat that's around major organs (which is dangerous) and fat stored under the skin (which is less so).
The study, published in the International Journal of Obesity, notes that BMI scores are increasingly used in corporate wellness programs to determine health. Under new rules, employers will be able to penalize staff up to 30% of their health insurance costs should their BMI fall outside a specified range. Given the discrepancy, the UCLA researchers say it's a matter of importance that regulators "not rely on BMI when formulating health policy."
The problem is that BMI is the best measure of health we have. It's easy to calculate and thousands of scores have been tabulated. Even if it's inaccurate at a personal level, it may still be useful for analysis at a population level, for instance in working out obesity trends. Until we have cheap, portable sensors for body fat (and different types of body fat), we may be stuck with BMI, for all its faults.