There are those who say it's possible to be fat and fit - but new research suggests this is a myth.

The idea of 'healthy obesity' is a misleading concept as most obese people become progressively less healthy over time, a new study warns.

Some people believe that if people are overweight - but do not have signs of heart disease - they can be considered healthy.

But the study found that even if this group currently show no signs of disease, the majority will go on to develop problems within 20 years.

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The fat but fit myth: The idea of 'healthy obesity' is a misleading concept as most obese people become progressively less healthy over time

It follows previous studies which also warned that the term 'healthy obesity' fosters a false sense of security, as this group have a much higher prevalence of early plaque build-up in the arteries - even if they don't have heart disease.

As part of the new study, researchers tracked the health of more than 2,500 men and women for two decades, measuring their body mass index (BMI), cholesterol, blood pressure, fasting plasma glucose and insulin resistance.

Healthy obesity was defined as obesity with no factors that raise the risk of heart disease and other health problems, such as diabetes and stroke.

More than half of the healthy obese participants became unhealthy obese over the 20-year study period, while only one in nine (11 per cent) lost weight and became healthy non-obese.

The remaining 38 per cent stayed in the healthy obese category during the term of the study.

Comparably, only six per cent of participants who were originally in the healthy non-obese category went on to become unhealthy obese over the 20-year study.

The study's lead author Joshua Bell, of University College London, said: 'A core assumption of healthy obesity has been that it is stable over time.

'We can now see that healthy obese adults tend to become unhealthy obese in the long-term, with about half making this transition over 20 years in our study.'

Adults who were considered healthy obese were more likely to develop risk factors for disease than healthy or unhealthy adults who are not considered obese, he added.

Of the 2,521 study participants, 181 were initially classified as obese, including 66 who were classified as healthy obese.

WHY 'HEALTHY OBESITY' IS A MYTH: EVEN OVERWEIGHT PEOPLE WHO DON'T HAVE HEART DISEASE HAVE HARDENED ARTERIES The theory goes that if people are overweight - but do not have signs of heart disease - they can be considered healthy. But research has revealed that obese people have a much higher prevalence of early plaque build-up in the arteries - even if they don't have heart disease. As a result, they cannot be considered as healthy as a normal weight person. The scientists behind the research now say that all obese people should be warned of the risk of heart disease - and encouraged to try and lose weight. Obesity can often lead to heart disease because patients have abnormal amounts of fat in their blood and they are prone to high blood sugar and high blood pressure. Advertisement

After five years, 32 per cent of the participants initially classified as healthy obese had become unhealthy obese.

By 10 years, 35 per cent were unhealthy obese; at 15 years 41 per cent were unhealthy obese, and at 20 years more than 51 per cent were unhealthy obese.

In contrast, only six per cent of the healthy obese participants lost weight and became healthy non-obese at the end of the first five years of the study.

The healthy non-obese portion of participants changed from 4.5 per cent after 10 years to 6.1 per cent after 15 years and to 10.6 per cent after 20 years.

The purpose of the study was to determine whether healthy obese adults maintain the metabolically healthy profile for the long term or naturally transition into unhealthy obesity over time.

No previous studies has examined the issue for such a long period of time.

Mr Bell added: 'Healthy obese adults show a greater risk for developing cardiovascular disease than healthy normal-weight adults, although this risk is not as great as for the unhealthy obese.

'Healthy obesity is only a state of relative health - it's just less unhealthy than the worst-case scenario.

'And as we now see, healthy obese adults tend to become unhealthy obese over time, providing further evidence against the idea that obesity can be healthy.'

'Healthy obesity is only valid if it is stable over time, and our results indicate that it is often just a phase.

'All types of obesity warrant treatment, even those which appear to be healthy.'

His warning comes as the World Health Organisation warns obesity has reached epidemic proportions globally, with at least 2.8 million people dying each year as a result of being overweight or obese.

It warned people being overweight or obese is responsible for 44 per cent of the global burden of diabetes, 23 per cent of the burden of heart disease and between 7 per cent and 41 per cent of the burden of certain cancers.