HONG KONG –  Long-term weight loss may release into the blood industrial pollutants linked to illnesses like diabetes, hypertension and rheumatoid arthritis, researchers said on Tuesday.

These compounds are normally stored in fatty tissues, but when fat breaks down during weight loss, they get into the blood stream, said lead researcher Duk-Hee Lee at the Kyungpook National University in Daegu in South Korea.

"We are living under the strong dogma that weight loss is always beneficial, but weight gain is always harmful...but we think that increased (pollutant) levels (in the blood) due to weight loss can affect human health in a variety of ways," she wrote in an email to Reuters.

Lee and an international team of colleagues studied 1,099 participants in the United States and concentrations of seven such compounds in their blood, they said in a paper published in the International Journal of Obesity.

"Once released into the bloodstream, these pollutants are able to reach vital organs," the researchers said in a statement.

Those who lost most weight over 10 years had the highest concentrations of the compounds, called persistent organic pollutants (POPs), compared to those who gained or maintained a steady weight.

"There is emerging evidence that POPs ... are not safe. POPs (are) linked to type 2 diabetes, hypertension, coronary heart disease, rheumatoid arthritis, periodontal disease," Lee said.

The researchers factored in age, gender and race to explain the differences in concentrations of these pollutants but weight history remained a statistically significant factor.

More studies were needed to establish if such harm outweighed the benefits to be gained from weight loss, Lee said.