It was found that some obese people are so physically fit that they do not show any of the usual health problems such as diabetes, high cholesterol or raised blood pressure.
In fact they are so healthy they are 38 per cent less likely to die early than their unhealthy obese counterparts.
They had more in common with normal weight people, the study found.
The results published in the European Heart Journal suggest doctors should take into account fitness when deciding if an obese person is risking their health or not.
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Lead author, Dr Francisco Ortega, of the University of Granada in Spain and the Karolinska Institute in Stockholm, Sweden said: “It is well known that obesity is linked to a large number of chronic disease such as cardiovascular problems and cancer.
“However, there appears to be a subset of obese people who seem to be protected from obesity-related metabolic complications.
“They may have greater cardio-respiratory fitness than other obese individuals, but, until now, it was not known the extent to which these metabolically healthy but obese people are at lower risk of diseases or premature death.”
The researchers examined 43,265 people in America between 1979 and 2003.
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The researchers looked at two definitions of obese, the traditional body mass index which is a calculation of weight divided by height squared, and body fat percentage measured using the highly accurate water displacement method and skin fold callipers.
People with a BMI of 25 or more are considered overweight while those with a BMI of over 30 are obese. A body fat percentage for men of less than 25 per cent is considered normal and over that is obese and for women the cut off is 30 per cent.
A total of 5649, or 13.1 per cent were classed as obese using BMI and 12 829, or 29.7 per cent were obese using the body fat definition.
Of those obese participants, 30.8 per cent of the BMI defined were metabolically healthy along with 46.3 per cent of those defined as obese using body fat.
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They concentrated on the body fat definition as this is more accurate and found that the healthy obese people were 38 per cent less likely to die during the follow-up period, which was an average of 14 years.
There was no difference in the healthy obese people and the healthy normal weight people, the researchers said.
Dr Ortega said: “Our study suggests that metabolically healthy but obese people have a better fitness level than the rest of obese individuals. Based on the data that our group and others have collected over years, we believe that getting more exercise broadly and positively influences major body systems and organs and consequently contributes to make someone metabolically healthier, including obese people. In our study, we measure fitness, which is largely influenced by exercise.”
The risk of developing or dying from heart disease or cancer was between 30 per cent and 50 per cent lower in the healthy obese group compared with the unhealthy obese group.
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Dr Ortega added: “Physicians should take into consideration that not all obese people have the same prognosis.
“Physicians could assess fitness, fatness and metabolic markers to do a better estimation of the risk of cardiovascular disease and cancer of obese patients.
“Our data support the idea that interventions might be more urgently needed in metabolically unhealthy and unfit obese people, since they are at a higher risk.
“This research highlights once again the important role of physical fitness as a health marker."More from the Telegraph.