• ## What’s in a number?

Posted on March 31, 2015 by in Blog, Research

by Sinikka Elliott

The BMI (which stands for Body Mass Index) is a commonly used number these days. But what does it measure? What information does it provide?

The BMI is a measure of the ratio of weight to height. You can calculate your BMI by dividing your weight in pounds by your height in inches squared (to calculate your height squared, you need to take your height in inches and multiply it by the same number. So if you are 5 foot 5 inches, that would be 65 inches multiplied by 65). Once you’ve divided your weight by your height in inches squared, you multiply this number by 703. So if I weighed 150 pounds and was 5’5”, the calculation would look like this:

150 lbs / (65 x 65) = 150 / 4225 = .0355 x 703 = 24.95

A BMI of 24.95, is currently considered just on the line between healthy weight and overweight.

A BMI under 18.5 is considered underweight. From 18.5-24.9 is considered normal. Overweight is 25 to 29.9. A BMI of 30 or above is considered obese.

But what do these numbers really tell us?

The BMI was developed in the 1830s by a physician named Adolphe Quetelet who observed that weight tended to vary with the square of height. The BMI didn’t take off, however, until the Metropolitan Life Insurance Company began using it to publish a table of “ideal weights” in the 1940s. By the 1950s, the Met Life tables had become “widely accepted as the authoritative determinant of who was overweight,” according to Charlotte Biltekoff, the author of Eating Right in America (Duke University Press, 2013).

The BMI is intended to measure body fat and is thus often used as a proxy for health, but it doesn’t measure body fat directly. For example, it can’t tell us where body fat is located on any given individual.

Critics also argue that the categories the BMI creates (e.g., obese, normal, underweight) are fairly arbitrary and don’t take into account different body types, such as people with high muscle mass or small-boned people. It also doesn’t capture variations across populations according to sex and race. For example, research finds that the BMI doesn’t accurately measure body fat for black women, who tend to have more muscle mass than other racial/ethnic groups of women. Scholars have proposed raising the BMI threshold for obesity from 30 to 32 for black women.

The BMI is also not set in stone. In fact, the guidelines for being classified as overweight were changed in 1998. Prior to this time, people with a BMI of 27 were considered overweight. Now a BMI of 25 is the dividing line between what’s considered normal weight and overweight.

Children’s BMI is calculated using a growth chart to obtain a percentile ranking. Children whose BMI is less than the 5th percentile are considered underweight. Overweight children are those with a BMI in the 85th to less than the 95th percentile, and children classified as obese have a BMI in the 95th percentile or greater. The cutoffs for children’s BMI were developed based on surveys of children and adolescents mostly conducted in the 1960s and 1970s.

Along with general problems with the BMI (i.e., it doesn’t account for different body types and variations based on race and sex), the BMI-for-age growth chart involves population distributions “and rather arbitrarily [chooses] particular values – often the 85th or 95th percentiles, which distinguish those with the highest BMIs from the rest of the population,” reports Helen Sweeting, author of “Measurement and Definitions of Obesity in Childhood and Adolescence” (Nutrition Journal, 2007). Yet, these numbers can be quite alarming for parents, especially when they are told their children are under or overweight.

One of the mothers the VIA team interviewed told us that every time she went to the doctor she got a lecture about her son being too small. The doctor would tell her, “He’s too small, he’s not eating enough. What is he eating? He is—as long as he’s healthy but he still needs to gain some more weight.” The mother cared a great deal about her son being the “right” weight, but said it was a challenge to make him eat more than he wanted to eat. She said, “He doesn’t want to eat [and] I can’t make him eat. I can’t shove it down his throat and be like, ‘Here eat.’”

Mothers said they can also get conflicting advice about their children’s weight from different health care practitioners. For example, one mother said a WIC counselor told her that her youngest son was in the 100th percentile and obese, but the pediatrician said the boy’s weight was fine. This pediatrician may have been taking into account other factors that the BMI doesn’t tell us, such as the child’s body type.

There’s also some growing research that suggests that losing weight does not improve most people’s “health biomarkers,” such as blood pressure, fasting glucose level, or triglyceride levels. Higher BMIs have been linked to a higher risk of developing type 2 diabetes, heart disease, and certain cancers, yet when people classified as obese lose weight, they don’t necessarily have lower levels of disease than their counterparts who remain obese.

Even more surprising, perhaps, is research that suggests that fat can be protective. This is called the “obesity paradox”: Obese patients with certain chronic diseases (heart disease, heart failure, diabetes, kidney disease, pneumonia, and others) do better and live longer than patients classified as normal weight.

So what’s in a number? The BMI is often used to determine whether someone is healthy or not but growing evidence indicates it is inadequate in this regard.  This research suggests people should throw out their scales (and BMI calculators!) and instead eat a diet rich in fruits and vegetables.