Losing weight may actually harm your health, claim Korean researchers.
But an obesity Australian expert says the benefits of losing weight outweigh any still-unproven risks.
Long-term weight loss can lead to the release of persistent organic pollutants (POPs) into the blood, which might in turn damage the internal organs of the body, claim researchers from the Kyungpook National University, Daegu, in the Republic of Korea. They published their results in the International Journal of Obesity.
POPs are man-made organic compounds used in industrial processes such as the manufacture of drugs, and pesticides. They accumulate in human tissue and have been linked to a wide variety of illnesses, including disruption of the endocrine, reproductive and immune systems, dementia, and cancers.
POPs are stored in fat tissue in the body, but during periods of weight loss they can be released into the bloodstream and cause injury to organs, the researchers claim.
They looked at data on 1099 participants drawn from a large study of healthy US citizens, the 1999-2002 National Health and Nutrition Examination Survey. The researchers compared the results of blood tests for POPs done on these subjects, with how much weight the same subjects declared they had gained or lost over a ten-year period, to see if there was an association.
The researchers found that in general, serum concentrations of seven commonly occurring POPs were significantly elevated in those people who had lost weight, and were lower in those who gained weight over this time.
Hence, weight loss can lead to increased blood concentrations of POPs, whereas weight gain can decrease them, they conclude.
Previous studies have shown a link between weight loss and an increased risk of medical conditions such as cardiovascular disease and dementia in some people, they argue. The release of POPs into the bloodstream following weight loss might be one explanation for the phenomenon, they say.
Professor Lesley Campbell, Group leader of Diabetes and Obesity Clinical Studies at Sydney's Garvan Institute, is sceptical about the claims.
"Their evidence seems to suggest there's a possible association between weight loss and elevated POPs, but not necessarily that the weight loss is the cause of the raised POPs," she says.
Also, the link between POPs and illness isn't strong enough yet for us to be sure that elevated POPs actually cause illness.
She says there's been a lot of debate about the adverse health effects of weight loss, but few conclusions reached.
"Certainly if weight loss is very rapid, or if it's severe, as in anorexia, it can be dangerous, and if it's caused by an underlying illness as in cancer, then obviously it's going to be associated with poor health," she says.
But generally speaking in healthy people, gradual and measured weight loss isn't associated with health problems, and in people who are overweight, losing weight obviously has health benefits.
"Losing one kilogram a week as part of a diet and exercise program isn't going to be harmful, and shouldn't scare people off from trying to losing weight", she says.