Women who repeatedly lose weight only to put it back on again could be damaging their immune systems, according to a study.
Exercise is the best way to lose weight, say doctors
Doctors in the United States questioned 114 women who were overweight but were otherwise healthy.
They found that those who said they had a history of so-called "yo-yo" dieting had weaker immune systems.
Writing in the Journal of the American Dietetic Association, they urged women against yo-yo dieting.
Doctors from the Fred Hutchinson Cancer Research Center in Seattle took blood samples from each of the women who took part in the study.
They measured levels of their natural killer cells. These white blood cells are the backbone of the immune system, helping to kill viruses and protect against cancer.
The doctors found that women who had been the same weight over several years had higher levels of these cells. Those who had gone on yo-yo diets had much lower levels.
For instance, those who reported losing weight more than five times had about a third fewer natural killer cells.
The doctors said further research is needed to confirm these findings.
However, they said the study appeared to suggest that yo-yo dieting was bad for health.
"There's clear evidence that weight loss is beneficial for your health," said Cornelia Ulrich, a research assistant professor at the centre.
"What we're concerned about is this pattern of weight cycling where women go up and down."
She added that exercising to lose weight could be the answer, since it is known to boost the immune system.
The American Dietetic Association welcomed the study. Its spokeswoman Katherine Tallmadge urged people to avoid fad diets and to try to make long-lasting changes to the way they eat.
"Study after study shows that more moderate restrictions are more likely to last permanently," she said.
"That's why we registered dieticians are urging people not to do the fad diets and just try small changes that they're more likely to be able to live with - even if the weight loss is slower."
That view was backed by Nigel Denby of the British Dietetic Association.
"Sensible healthy eating combined with an increase in physical activity really is the perfect recipe for successful and more importantly maintainable weight loss," he told BBC News Online.