Weight loss may have an unwanted side effect, according to a new study in the journal Nature: It may send a flood of environmental pollutants into the bloodstream.

Body fat stores certain pollutants, including such pesticides as DDT and polychlorinated biphenyls (PCBs). If a person loses weight and significant amounts of body fat are broken down, these chemical compounds, known as persistent organic pollutants, are released and can lead to disease, said researchers from Kyungpook National University in Daegu, South Korea.

"The strong dogma on weight change is that weight loss is always good while weight gain is always bad," but that may not always hold up, said study researcher Dr. Duk-Hee Lee, a professor at the university.

Hypertension, type 2 diabetes, coronary heart disease and rheumatoid arthritis have been linked to persistent organic pollutants, Lee said.

The researchers analyzed data collected by the Centers for Disease Control and Prevention between 1999 and 2002. They selected seven pollutants that had the highest detection rate in blood samples, and focused on 1,099 people ages 40 or older who had measurable levels of these pollutants in their blood and who had also lost or gained weight.

By the study's end, they found that those who had lost weight had higher concentrations of most of the pollutants they measured, while those who had gained weight had lowered their pollutant concentrations. The trend was even more significant among people who reported they had gained or lost weight over a 10-year period, according to the researchers.

"A lot of studies have shown that losing weight is helpful," because it lowers levels of sugar and fat in the blood as well as blood pressure, Lee told MyHealthNewsDaily. However, there may be other aspects of health that losing weight can negatively influence, she said, pointing out that a previous study involving him linked weight loss to calcification of the coronary arteries.

Although many of the dangerous chemicals examined in the study were banned by developed countries several decades ago, they are still commonly found in the environment and in people because they take a long time to break down, Lee said.

The net effects of weight loss should be viewed as a mix of the benefits of decreasing fat tissue and the harmful effects of increasing the concentration of pollutants in the blood, she said.

Because the study was based on self-reported information about change in weight, people who inaccurately recalled their change in weight may have distorted the findings, the researchers noted.

Lee recommended exercising and sticking to a plant-based diet to help to rid the body of these pollutants during weight loss. Of course, not becoming overweight in the first place would eliminate the costs paid by obese people who want to return to being non-obese, he said.

Amber Angelle contributed reporting to this article.

This article was provided by MyHealthNewsDaily, a sister site to LiveScience.