The notion of “healthy obesity” has been around for several years, with some studies concluding that, for certain people, obesity is not as important a health measure as having good blood pressure, cholesterol, insulin and triglyceride levels. However, a new study shows that healthy obesity may be a myth, just a phase that often transforms into future unhealthy obesity.

The British study, published in the January 2015 edition of the Journal of the American College of Cardiology, tracked a group of 2,500 people between the ages of 39 and 62, 66 of whom were considered to be healthy obese based on metabolic profiles. Each participant was measured for body mass index, blood pressure, cholesterol, insulin resistance and fasting blood sugar, and ranked as either non-obese, healthy obese, or unhealthy obese.

Over the next two decades more than half of those who started out as healthy obese had moved into the category of unhealthy obese. The healthy obese began to develop risk factors for chronic disease over time. After 10 years approximately 40 percent had developed unhealthy obesity, and by 20 years 51 percent considered unhealthy obese. These findings led the authors of the study to suggest that progression to metabolic deterioration is healthy obesity’s natural course.

Another finding was that healthy non-obese people also developed poor health over time, although the progression was not as fast. After 20 years 22 percent, although still trim, had become unhealthy. About 10 percent more had become either healthy or unhealthy obese. Only 11 percent of those who started out as healthy obese lost weight and became non-obese and healthy.

Earlier studies have found that any kind of obesity, whether healthy or unhealthy, increases the risk of diabetes, heart disease, cancer and early death. One of the British study authors Joshua Bell says that some of the previous studies, which used shorter time lines, showed that about one-third of healthy obese adults progress to a status of unhealthy obesity. The current study suggests that this trend only gets stronger with time, even though some of the healthy obese participants did remain so over 20 years.

Although obese people are at an increased risk of heart disease, some studies have uncovered an “obesity paradox,” which finds that some healthy obese people actually seem less likely to die from heart or kidney disease than those with lower body mass indices. The new British study, conducted at University College London, takes considerable steps toward debunking this theory.

Director of clinical cardiology for National Jewish Health in Denver, Dr. Andrew Freeman, says that the longer someone is obese, the more likely they are to suffer damage. Both Bell and Freeman say that the study suggests that obese people eventually do develop risk factors such as bad cholesterol and high blood sugar that lead to chronic illness and even death.

Bell says that even the healthy obese should try to lose weight, because they carry “a high risk of future decline.” Although a small number of people may be obese into old age, for the majority obesity is a health risk. He says healthy obesity is still a high-risk state, with the harmful effect just delayed.