Why it’s time to say goodbye to BMI

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Since the mid-nineties, health care professionals have used Body Mass Index (BMI) - a measure of how much mass someone has relative to their height - to identify whether a patient is at a healthy weight. A BMI between 18.5 and 24.99 is considered normal; 25 to 29.9 is overweight; and 30 or above is obese. The BMI is so revered as a standard that in the US the Equal Employment Opportunity Commission has proposed people with higher BMIs should also have to shell out more for insurance premiums, since being overweight or obese can increase your risk for diabetes and heart disease, and is considered unhealthy.

That’s a move researchers from UCLA have condemned as not just unfair, but downright incorrect, in a paper published earlier this year in the International Journal of Obesity. 

In the study, the researchers pulled data from 40,420 people in the most recent National Health and Nutrition Examination Survey and analysed the link between BMI and key health markers including blood pressure, glucose, insulin resistance, and cholesterol and triglyceride levels. The BMI/health connection didn’t pan out - by a long shot. Researchers found that nearly half of overweight men and women and 29 percent of obese people were otherwise totally healthy. On the flip side, more than 30 percent of those with normal BMIs were actually unhealthy according to those markers. 

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“There are tens of millions of people who are overweight and obese and are perfectly healthy,” says study author A. Janet Tomiyama, PhD, assistant professor of psychology at UCLA. “Right now, we have this laser focus on weight and a flawed measure like BMI, when we should be talking about health.”

That’s really important for active people like runners, for whom the BMI problem is two-fold. Not only are some of us likely caught in that high BMI bracket though otherwise healthy, but also BMI - which is calculated by dividing a person’s weight in kilograms by the square of the person’s height in metres - was really never meant to be applied to an active and certainly not athletic population.

We active folk tend to have a higher proportion of muscle tissue, which by nature is denser than fat tissue. That means we sometimes see big numbers on the scale even if we have relatively low body fat. BMI also says nothing about how your fat is distributed. Two people may have identical BMI measurements (“healthy” or otherwise), but where one might store most of their fat as deep abdominal, visceral fat - which is known to be a high health risk - the other might store it as relatively innocuous subcutaneous fat (the kind you pinch under your skin).

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If you’re concerned about your BMI or your health-care provider raises a red flag over yours, Tomiyama, who directs UCLA’s Dieting, Stress, and Health Lab, suggests gathering more data for a fuller picture of your weight, health and fitness. While calculating BMI via height and weight can get you in the right ballpark, a body composition test performed with a DEXA scan, skinfold caliper test or bio-impedance scan (like a Tanita scale) can give you a better idea of your actual body composition, so you know how much of you is lean, muscle tissue relative to fat. Also, see your doctor and round up your general health metrics:

“In our study we used a very stringent and comprehensive definition of health that included blood pressure, triglycerides, ‘good’ and ‘bad’ cholesterol, blood sugar and inflammation,” says Tomiyama, noting that these are common tests that any healthcare provider can do and are far better indications of how healthy you are than BMI alone.