JOIN USAn international team, led by researchers from the US, Norway and South Korea, has found that weight loss allows harmful pollutants, normally stored in body fat, to circulate in the bloodstream.
"Weight loss could be harmful if it leads to the release of toxins from fat tissue and increases the concentration of persistent organic pollutants.
"It means they can reach critical organs through the circulation," Dr Duk-Hee Lee of Kyungpook National University in South Korea, who led the team, was quoted by the 'Sunday Express' as saying.
For the study, the researchers analysed 1,099 people over 40, tracking their weight for 10 years, with frequent blood tests for seven of the most dangerous pollutants.
These included DDE, a pesticide linked with breast cancer, Alzheimer's and Parkinson's diseases, and PCB169, a toxin linked with damage to the brain and nervous system. They found higher blood levels of these chemicals in people who had lost weight.
The study, published in the 'International Journal of Obesity' calls for urgent further research into POPs, man-made chemicals that can be absorbed into the body over many years.
They have been used in everything from pesticides to electrical goods for more than 50 years but most people come into contact with them from tiny amounts in food, especially animal fats.
They are stored in fatty tissue where they are believed to cause little harm, but once in the blood, they may increase the risk of cancer, heart disease, diabetes and damage to the immune system.
Another study in the Netherlands earlier this year showed weight loss can raise risks of heart disease, diabetes and cancer by increasing body's production of stress hormones.
Professor Janna Koppe, of Amsterdam University, found babies exposed to high levels of similar chemicals through their mother's blood and breast milk may be prone to diabetes as teenagers.
She said: "I'd warn pregnant women not to lose weight. I would also advise overweight people to lose weight slowly. This is a balancing act -- being overweight is a risk but so are pollutants."