Why BMI Isn’t The Best Measure for Weight (or Health)

Body Mass Index (BMI) provides an easy way to measure obesity, but more doctors are questioning its accuracy and usefulness.

BMI is supposed to estimate the amount of body fat a person carries based on height and weight, and categorizes people based on what is appropriate for their size. BMI readings under 18.5 mean you are underweight, and could put on a few pounds. If you fall between 18.5 and 24.9, you’re considered normal, while a BMI of 30 or higher qualifies as overweight.

But in recent years, more researchers argue that it’s not the most accurate way to measure body weight. For years, scientists have said that BMI can’t distinguish between fat and muscle, which tends to be heavier and can tip more toned individuals into overweight status, even if their fat levels are low. In the journal Science, the latest data from University of Pennsylvania shows that BMI also doesn’t tease apart different types of fat, each of which can have different metabolic effects on health. BMI cannot take into consideration, for example, where the body holds fat. Belly fat, which is known as visceral fat, is more harmful than fat that simply sitting under the skin. 

Visceral fat develops deep among muscles and around organs like the liver and by releasing certain hormones and other agents, it disrupts the body’s ability to balance its energy needs. Even relatively thin people can have high levels of visceral fat, which means they might be considered healthy by BMI standards, but internally they may actually be at higher risk of developing health problems related to weigh gain.

In April of last year, a study published in the journal PLoS One documented such inconsistencies and questioned the accuracy of using BMI to classify weight status of 1,400 men and women. As TIME reported:

Among the study participants, about half of women who were not classified as obese according to their BMI actually were obese when their body fat percentage was taken into account. Among the men, in contrast, about a quarter of obese men had been missed by BMI. Further, a quarter who were categorized as obese by BMI were not considered obese based on their body fat percentage. Overall, about 39% of participants who were classified as overweight by their BMI were actually obese, according to their percent body fat.

So why is BMI still the preferred way to measure weight and evaluate obesity? For one, it’s a relatively easy measurement for doctors to take during an office visit. Taking a person’s height and weight and plugging it into an equation produces a number that informs doctors about whether their patients are at high, low or no risk when it comes to weight-related health problems.

But there may be better ways to measure body fat that provide more useful readings on how likely a person’s weight will contribute to chronic health problems. CT scans and MRIs can provide a clearer glimpse at the body’s make-up by separating out fat from muscle, for example. But these are expensive and involved compared to stepping on a scale. Other types of scans, including dual-energy X-ray absorptiometry (DEXA) images, which are normally used to measure bone density, can also distinguish between fat from bone and muscle mass, but are also costly.

On the more practical level, some researchers have been pushing for using waist circumference or even wrist circumference to gauge potentially harmful weight gain and fat depots, but the evidence supporting this measurement and its ability to predict future health problems isn’t definitive enough yet.

So without a viable way to change how we measure body fat, for now, BMI is the best option. The study authors argue that perhaps doctors should rely on not just assessing body composition but measuring hormones and biomarkers in the blood or urine, for example, to get a better handle on abnormal processes that may contribute to obesity and chronic disease. And until such tests become available, BMI may still prove useful yet — if doctors combine BMI with a comprehensive evaluation of their patients’ medical history and lifestyle habits to get a meaningful, if not yet perfectly precise picture of their weight-related health.

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