The obesity paradox - where obese people remain quite healthy - defies convenient epidemiological and nutritional thinking. But age catches up to us all.

Everyone gets less healthy over time - age is the biggest risk factor for cancer, heart disease and just about everything else - but a new study in the Journal of the American College of Cardiology tracked the health of more than 2,521 men and women for 20 years, aged between the ages of 39 and 62, and found that more than 51 percent of the healthy obese participants became unhealthy obese over the 20-year study period, while only 11 percent lost weight and became healthy non-obese.

The remaining 38 percent stayed in the healthy obese category during the term of the study, while 6 percent of participants originally in the healthy non-obese category became unhealthy obese. 

The scholars in the Department of Epidemiology and Public Health at University College London in England measured each participant's body mass index, cholesterol, blood pressure, fasting plasma glucose and insulin resistance. Health obesity was defined as obesity with no metabolic risk factors. 

"A core assumption of healthy obesity has been that it is stable over time, but 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," said lead study author Joshua Bell. "Healthy obese adults were also much more likely to become unhealthy obese than healthy or unhealthy non-obese adults, indicating that healthy obesity is a high risk state with serious implications for disease risk."

Among 2,521 participants, 181 were initially classified as obese, including 66 who were classified as healthy obese. After five years, 32 percent of the participants initially classified as healthy obese had become unhealthy obese. By 10 years, 41 percent were unhealthy obese, 35 percent were unhealthy obese at 15 years, and more than 51 percent were unhealthy obese at the 20-year mark.

In contrast, only 6 percent 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 percent after 10 years to 6.1 percent after 15 years and to 10.6 percent 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 studies have examined this issue for this long a period of time.

"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," said Bell.

The Centers for Disease Control and Prevention reported that 35.7 percent of adults in the United States in 2010 were classified as obese. Public Health England reports that 24.8 percent of adults in Britain were classified as obese.

"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," Bell said.

Among the most common health consequences of obesity are cardiovascular diseases--mainly heart disease and stroke--diabetes, musculoskeletal issues, and some forms of cancer including endometrial, breast and colon cancers.