Any dieter knows that it’s hard to keep off weight one has lost.

Now a study finds that even a year after dieters shed a good chunk of weight quickly, their hormones were still insisting: “Eat! Eat! Eat!”

The findings suggest that dieters who have regained weight are not just slipping back into old habits, but are struggling against a persistent biological urge.

“People who regain weight should not be harsh on themselves, as eating is our most basic instinct,” Joseph Proietto, professor at the University of Melbourne in Australia, an author of the study, said. Weight regain is a common problem for dieters.

To study what drives it, Prof. Proietto and his colleagues enrolled 50 overweight or obese patients in a 10-week diet programme in Australia. They wanted to see what would happen in people who lost at least 10 per cent of their body weight. Ultimately, only 34 people lost that much and stuck with the study long enough for analysis.

The programme was intense. On average, the participants lost almost 30lbs during the 10 weeks, faster than the standard advice of losing one or two pounds a week. They took in 500 to 550 calories a day, using a meal replacement plus vegetables for eight weeks. Then for two weeks they were gradually reintroduced to ordinary foods.

Despite counselling and written advice about how to maintain their new weights, they gained an average of 12lbs back over the next year. So they were still at lower weights than when they started. The scientists checked the blood levels of nine hormones that influence appetite.

The key finding came from comparing the hormone levels from before the weight-loss programme to one year after it was over. Six hormones were still out of whack in a direction that would boost hunger.