MONDAY, Jan. 5, 2015 (HealthDay News) -- The notion of potentially healthy obesity is a myth, with most obese people slipping into poor health and chronic illness over time, a new British study claims.

The "obesity paradox" is a theory that argues obesity might improve some people's chances of survival over illnesses such as heart failure, said lead researcher Joshua Bell, a doctoral student in University College London's department of epidemiology and public health.

But research tracking the health of more than 2,500 British men and women for two decades found that half the people initially considered "healthy obese" wound up sliding into poor health as years passed.

"Healthy obesity is something that's a phase rather than something that's enduring over time," Bell said. "It's important to have a long-term view of healthy obesity, and to bear in mind the long-term tendencies. As long as obesity persists, health tends to decline. It does seem to be a high-risk state."

The obesity paradox springs from research involving people who are overweight but do not suffer from obesity-related problems such as high blood pressure, bad cholesterol and elevated blood sugar, said Dr. Andrew Freeman, director of clinical cardiology for National Jewish Health in Denver.

Some studies have found that people in this category seem to be less likely to die from heart disease and chronic kidney disease compared with folks with a lower body mass index, Freeman said -- even though science also has proven that obesity increases overall risk for heart disease, diabetes and some forms of cancer.

No one can say how the obesity paradox works, but some have speculated that people with extra weight might have extra energy stores they can draw upon if they become acutely ill, Freeman said.

To test this theory, University College London researchers tracked the health of 2,521 men and women between the ages of 39 and 62. They measured each participant's body mass index (a calculation based on height and weight), cholesterol, blood pressure, fasting blood sugar and insulin resistance, and ranked them as either healthy or unhealthy and obese or non-obese.