New York ? Any dieter knows that it’s hard to keep off weight you’ve 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 of the University of Melbourne in Australia, an author of the study, said in an email. The research appears in Thursday’s issue of the New England Journal of Medicine.

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

The program was intense. On average, the participants lost almost 30 pounds during the 10 weeks, faster than the standard advice of losing 1 or 2 pounds a week. They took in 500 to 550 calories a day, using a meal replacement called Optifast plus vegetables for eight weeks. Then for two weeks they were gradually reintroduced to ordinary foods.

Despite counseling and written advice about how to maintain their new weights, they gained an average of 12 pounds back over the next year. So they were still at lower weights than when they started.