Why a Piece of String Could Be Better than BMI

Korin Miller

A simple string test could indicate if you’re overweight. (Photo: Stocksy/Vera Lair)

For years, body mass index (BMI) has been a controversial method of determining whether a person has an unhealthy amount of body fat. And now, new research has found that it’s even less accurate than measuring with a piece of string.

Yup, string.

Researchers from Oxford Brookes University in the UK have found that measuring a person’s height with string, folding that piece of string in half, and making sure that it can fit comfortably around the waist is a better indicator than BMI of whether someone has too much body fat.

For the study, which was published in the journal BMC Medicine, scientists checked the health and weight of nearly 3,000 people. They discovered that more than a third of the study participants who were classified as “normal” by BMI standards would have been flagged by their string method, which calculates their waist-to-height ratio.

Lead researcher Margaret Atwell believes the string method is a much better way to calculate extra abdominal fat, telling Yahoo Health that “the science about the limitations of BMI and the superiority of waist-to-height ratio has been growing in leaps and bounds recently.”

BMI, which the Centers for Disease Control and Prevention says is a “reliable indicator of body fatness for most people,” is used by most doctors to determine whether a person is at risk of developing certain diseases.

But BMI has been criticized for its limitations. For example, a muscular person or someone with a heavy bone structure could be classified as “overweight” or “obese” by BMI standards, while a person who carries extra fat around the middle (which is a known risk factor for many health problems) may not be flagged at all by the method.

“BMI gives us an idea of total fat mass, but doesn’t tell us where the fat is located in our body,” W. Scott Butsch, MD, an instructor of medicine at Harvard Medical School and an obesity specialist at the Massachusetts General Hospital Weight Center, tells Yahoo Health. “Fat distribution can give us an idea of whether a person is at an increased risk of mortality.” 

Enter the string method. Samuel Accardi, lead dietitian for nutrition intelligence company Mind Plus Matter, says measuring a person’s waist-to-height ratio can help capture the larger picture when it comes to the distribution of fat. “It has been proven to be a better predictor of some chronic illnesses,” he tells Yahoo Health.

Case in point: Two people who have the same BMI could have very different health risks. A person who carries a lot of fat around their midsection may be at an increased risk for heart disease and Type 2 diabetes. But a person with the same BMI who carries more fat in the butt may be at a decreased risk of developing heart disease and Type 2 diabetes, according to a 2010 study published in the International Journal of Obesity.

While calculating a person’s body fat using the string method is effective for finding people with excess fat, it also has its flaws, says Francisco Lopez-Jimenez, MD, director of preventive cardiologyat the Mayo Clinic in Rochester, Minnesota. By using this method, a doctor could miss someone who is thin but who still carries a disproportionate amount of fat around the midsection, which can also increase risk for certain diseases.

“The waist-to-height method performs much better than BMI but doesn’t really compare to waist-to-hip ratio,” Lopez-Jimenez tells Yahoo Health. That method, which measures your hips and waist to determine whether the waist is smaller than the hips, is more accurate, he says. However, he says all of these methods have their place, with BMI acting as a touching-off point of sorts.

Butsch agrees. He says BMI is “absolutely” an important indicator of body fat, but notes that there’s room for improvement: “This is the beginnings of an attempt to say that maybe we should be looking beyond just BMI to assess body fat.”