A yo-yo diet is better than no diet for someone who needs to lose weight, scientists claim.
New research suggests a U-turn on rollercoaster diets that have been criticised by many experts.
Shedding and regaining weight in a rapid cycle is generally viewed as unhealthy and psychologically damaging.
But the new US research conducted on mice suggests it can have long-term benefits and even extend life.
Animals that switched between a high-fat and low-fat diet every four weeks lived 25 per cent longer than those which only ate a high-fat diet and remained obese.
They also had better blood glucose levels, and lived roughly the same length of time as mice on a continuous low-fat diet.
In addition, preliminary findings indicated yo-yo dieting mice had reduced levels of cytokine immune system signalling molecules linked to inflammation.
Increased inflammation is a major risk factor for diseases such as diabetes, heart disease and cancer.
Study leader Dr Edward List, from Ohio University in the US, said: "If the conventional wisdom is true, it would discourage a lot of overweight people from losing weight.
"The new research shows that the simple act of gaining and losing weight does not seem detrimental to lifespan.
"The study adds to our understanding of the benefit of losing weight. I would hope that this encourages people not to give up."
Dr List presented the results today at the annual meeting of the Endocrine Society in Boston, US.
His team followed the fortunes of 30 animals for just over two years, the normal lifespan of this strain of laboratory mouse.
Mice on the continuous high-fat diet ate more, weighed more, had higher levels of body fat and blood glucose, and showed early signs of diabetes.
The health of yo-yo dieting mice declined during the high-fat phases. But their weight and blood glucose levels returned to normal when their calories were cut.
Yo-yo mice lived 2.04 years on average, compared with 1.5 years for continually obese mice, and 2.09 years for "control" mice always on a low-fat diet.
Consultant dietician Sian Porter, from the British Dietetic Association, said previous research had shown yo-yo diets could be "self destructive".
"Our bodies haven't kept pace with the environment we're in now," she pointed out.
"You react to an extreme diet as if you're starving thousands of years ago and waiting for the next woolly mammoth to come along, by conserving energy.
So when you go back to eating normally, you pile on the pounds.
"Often these quick fix diets cut out major food groups and they can be socially isolating."
She said it was "not surprising" that yo-yo dieting mice lived longer than mice that were obese.
The problem for humans was keeping weight control at a sustainable level.
"It's a good idea to be a healthy weight for your height, but mice aren't people. I think this falls into the category of very interesting but it would be more interesting if replicated in humans."
In contrast to yo-yo dieting, less rigid diets that involved "flexible restraint" were known to be quite effective, said Ms Porter.
"The key to losing weight and keeping it off is being in tune with your own eating habits," she said.
