Being obese can endanger your health and even your life, but weight alone may not be enough to predict serious health problems in everyone. Now a new study published in the Canadian Medical Journal finds that it may be possible to tease out which obese people may be more likely to die as a result of their excess weight.
The current method used by doctors to define overweight and obesity — body mass index (BMI), which is a ratio of height and weight — falls short of predicting which people are most at risk of early death from weight-related problems such as heart disease and diabetes. So, using data from the U.S. National Health and Nutrition Examination Survey, researchers at the University of Alberta tracked deaths during the study’s follow-up period of nearly two decades and tested a new measure of the health effects related to obesity: the Edmonton obesity staging system.
Developed by the Canadian group, the test uses a simple five-stage system, from 0 to 4, for classifying obese patients. A person’s Edmonton score takes into account his particular medical history, including everything from high cholesterol and sleep apnea to diabetes, heart problems and even organ damage. The score also reflects lifestyle factors, like how well a person can move around and complete daily activities.
The more health conditions a patient has, the higher his Edmonton score. The scientists say the system gives both the patient and the doctor a more accurate reading of how dangerous that person’s obesity is to his health.
In the study, participants with scores of 2 or above were 1.5 times as likely to die during the study’s follow-up as those with scores of 0 or 1. Those who scored a 3 were 2.5 times as likely to die during the same time period.
“We were surprised by how differently people accumulate [these] comorbid conditions,” says the study’s lead author, Dr. Raj Padwal. “You can take two patients, both of whom are obese, and one might be relatively light compared to the other. Yet that individual might have severe diabetes, high blood pressure and high cholesterol, and may have disabling joint disease. It never ceased to surprise us.”
Padwal stresses that the data don’t mean that being obese is necessarily a health state, or that some obese people may not need to lose weight to protect their health. Rather, it’s more an indication of the deficiency of BMI in predicting health outcomes. Because BMI combines only height and weight, Padwal says, it can’t stratify people based on other, perhaps more important factors associated with obesity that can harm your health. “The major deficiency of the BMI and waist-circumference classifications are that they don’t capture the dramatic spectrum of problems that occur with obesity,” he says, “such as joint problems, heart problems, cancer associated with obesity, mental illness, physical limitations and difficulties with daily activities.”
The findings highlight something that doctors have known intuitively for some time — that not all obesity is equal and that some people may suffer more than others from carrying extra weight. The classification may help save lives as well as health care dollars; the sooner people at higher risk are identified, the sooner they can start medical or lifestyle strategies for losing weight and the better doctors can prioritize the right treatments for the right patients.
“The results were much more striking than we initially thought,” says Padwal. “I can confidently say this is going to be a new paradigm for how we classify obese people medically.”
Alice Park is a writer at TIME. Find her on Twitter at @aliceparkny. You can also continue the discussion on TIME‘s Facebook page and on Twitter at @TIME.