(CBS/AP) As any dieter knows, it's hard to keep weight off once it's been lost. And a new study shows that even a year after dieters drop lots of weight, their hormones keep insisting, "Eat! Eat! Eat!"
The findings suggest it's not simply a lack of willpower that explains weight regain, but a persistent biological urge.
"People who regain weight should not be harsh on themselves, as eating is our most basic instinct," study author Joseph Proietto of the University of Melbourne in Australia, said in an email.
The research appears in Thursday's issue of the New England Journal of Medicine.
Weight regain is a familiar problem for dieters. To study what drives it, Proietto and his colleagues enrolled 50 overweight or obese patients in Australia in a 10-week diet program. The researchers' goal? To see what would happen in people who lost at least 10 percent of their body weight.
The program was intense. On average, the participants lost almost 30 pounds in the 10 weeks - faster than the standard advice of losing a pound or two per week. The dieters consumed 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.
The dieters got counseling and written advice about how to maintain their new weights. Even so, they gained an average of 12 pounds back over the next year.
The scientists checked the blood levels of nine hormones known to influence appetite - and found that even a year after the end of the weight-loss program, six of the hormones were still out of whack.
Experts not connected to the study said the persistent effect on hormone levels wasn't surprising, and that the speed of the weight loss probably had nothing to do with it.
People who lose less than 10 percent of body weight would probably show the same thing to a lesser degree, said Dr. George Bray of the Pennington Biomedical Research Center in Baton Rouge, La.
The study's take-away message? Said Bray, "It's better not to gain weight than to try to lose it."