believe they've identified the reason why rapidly after losing it.

The body's fat cells have a "memory" of obesity due to changes made to a set of chemical tags added and removed from DNA and proteins in the cell. These cells signal to the gene to increase or decrease activity. When seen in fat cells, this gene activity can render them unable to complete their normal functions.

While on the outside, you may be fitting in to your dream outfit, or loving your new look in the mirror, on the inside this impairment can linger for a long time after weight loss, according to a study in Nature.

The co-author of the study, biologist Laura Hinte, who works at ETH Zurich in Switzerland, explained that dieters will need longer-term care to prevent that weight being regained. She said: “It means that you need more help, potentially. It’s not your fault.”

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Hinte and her colleagues analysed the fat tissue of people with severe obesity and compared it to a control group of individuals who'd never had the condition. In the obesity group's fat cells, experts found some genes were more active. In the control group, genes were less active.

They also found that people who'd undergone weight-loss surgery would have the same cellular effect. Two years after undergoing the operation, the participants may have lost huge amounts of weight but their cells continued in the pattern.

Co-author Ferdinand von Meyenn, who is also a specialist on the subject at ETH Zurich, said scientists are unaware of exactly how long the body remembers obesity. “There may be a time window when this memory will be lost,” he said. “But we don’t know.”

Von Meyenn added that the key is to prevent obesity in the first place. "[People who lose weight] can [stay] lean, but it will require a lot of effort and energy to do that”, he said. He added that he hopes his team's findings could help combat the stigma around obesity.