(CNN) -- You may be obese, but does that automatically mean you're unhealthy?
The conventional wisdom is that if you're overweight or obese, you're in mortal danger because that extra weight is like a ticking time bomb ready to unleash diabetes, heart disease and other health complications.
But doctors have known for years that obesity doesn't affect all people the same way. An obese person could lead a healthy life while another person with the same body mass index, or BMI, could have severe medical problems.
Two studies published Monday suggest reframing the way medical practitioners look at overweight and obese patients. The studies question the notion that BMI and weight determine health -- even when someone is severely obese.
"Our study challenges the idea that all obese individuals need to lose weight," said Dr. Jennifer Kuk, assistant professor in York University's School of Kinesiology & Health Science in Toronto. One in five obese people may not have medical problems, the authors estimated.
The challenge is determining who are the "healthy obese" and those who may not have complications now but may develop them in the future.
The first study analyzed data from 8,000 people in the National Health and Human Nutrition Examination Surveys, a cross section of the U.S. population.
The main question was whether BMI alone could determine who dies early, said Dr. Arya Sharma, the lead author.
The BMI measures weight relative to height but does not calculate body fat. A person who has a BMI between 25 and 29 qualifies as overweight; a BMI greater than 30 is obese. This measurement has been used as an indicator for obesity since the 1980s.
In the Canadian research, each person who qualified as overweight was given a ranking based on the Edmonton Obesity Staging System, which Sharma designed.
A person can be ranked from zero, which indicates no apparent medical risk, although his or her weight is considered obese. As the stages progress, the symptoms become more serious.
Stage one is borderline hypertension and elevated blood pressure. The person may have elevated glucose levels and mild physical symptoms such as pain and aches. The person does not have a clinical disease at this point.
Stage two is classified with obesity-related diseases such as hypertension, diabetes and sleep apnea. The person may face limitations on daily activities.
Stage three means the person has organ damage such as heart failure, diabetic complications and debilitating arthritis. The person may also be suicidal and depressed.
Stage four is the most severe, with end-stage chronic diseases, severe disabilities and functional limitations.
This method of ranking risk more accurately predicted someone's death in 20 years than using BMI.
"What the study clearly shows is you can't make that call based on BMI," Sharma said. "You're going to have to look at additional risk factors."
But the study looked at mortality -- not quality of life.
"Their study was largely mortality in the outcome," said Dr. Steven Heymsfield, executive director of the Pennington Biomedical Research Center in Baton Rouge, Louisiana, who was not involved in the research. "It doesn't mean your knees and hips don't have to be replaced. I still think that's an issue. You suffer certain social and employment stigma."
Researchers predictably found a correlation between health complications and heavier weight. But they also found "people who are out there, who are pretty healthy despite being large," Sharma said.
"Of course, people at a high BMI are more likely to have these problems, but 20% may be doing OK," he said.
In the other article published in Applied Physiology, Nutrition and Metabolism, Canadian researchers found similar results. Although higher BMI was associated with increased death risk, there was "considerable variation in the health risk profile" in the obese population, according to the report.
The study used data from the Aerobics Center Longitudinal Study consisting of 29,533 individuals and assessed their mortality 16 years later.
They found no difference in death risks between normal-weight individuals and obese individuals who were in stages zero or one of the Edmonton Obesity Staging System.
They "are at no greater risk of dying than normal weight individuals," said Kuk, the Toronto professor. "It challenges the notion all obese individuals need to lose weight"
Who are the 'healthy obese'?
Why are some people considered the "healthy obese" versus others who might be less overweight but have more obesity-related complications?
Genetics could play a role. A person's chances of getting diabetes, high blood pressure or arthritis with weight gain are stronger if he or she has parents or grandparents who also had the conditions.
Another factor is the quality of diet and level of fitness. These factors lower a person's risks even though his or her weight may qualify as obese. But other factors remain unclear.
Rather than focus on weight, Sharma suggests checking for chronic disease risk such as blood pressure and other factors in obese patients.
"The key message is I can't tell you how healthy someone is if you tell me height or weight on a scale," said Sharma, chair for obesity research and management at the University of Alberta. "I have to do additional tests for other health problems."
He designed the ranking system because of long waiting lists for obesity treatment on Canadian public health care. Doctors had to decide who should be treated first.
The ranking system shows not everyone is the same and helps "to identify who should actually lose weight and who are we torturing for no reason," Kuk said.
"It's a bad thing to have a sweeping brush for everybody," she said.
While doctors who treat obesity agree that BMI is not a good solitary measure of health, they said the papers have limitations.
Dr. Allen Rader, a member of the American Society of Bariatric Physicians, noted that a patient could need weight loss intervention even in the early stages of disease risk to "prevent or delay progression."
Dr. Howard Eisenson, executive director of the Duke Diet & Fitness Center, echoed the same concern.
If he had a 25-year-old patient, Eisenson said, he wouldn't tell the person, "You're fine. Don't worry because you don't have any chronic diseases."
"If we don't intervene now, by the time the person is 35 ... maybe some damage has been done and the unhealthy habits are more established," Eisenson said.
Doctors agreed that there should be a more sophisticated method of assessing risk in obese patients, rather than just weight or BMI.
Health groups such as the World Health Organization and the U.S. Food and Drug Administration have been using BMI as measure of obesity, which also determines whether people qualify for drugs or bariatric surgery.
While calling the proposed ranking system intriguing, Eisenson said, "It's not quite clear what the average physician should do."