Yo-yo dieting, in which a person repeatedly loses and regains weight, may have a lasting negative impact on immune function, according to preliminary findings by researchers at Fred Hutchinson Cancer Research Center.

Conversely, maintaining the same weight over time appears to have a positive effect on the immune system, according to research by Cornelia Ulrich and colleagues reported in the June issue of the Journal of the American Dietetic Association.

Ulrich’s team from Fred Hutchinson and the University of Washington found that long-term immune function decreases in proportion to how many times a woman reportedly intentionally loses weight.

“To our knowledge, this is the first study to show potential long-term effects of yo-yo dieting on health,” said Ulrich, senior author of the paper and an assistant member of Fred Hutchinson’s Public Health Sciences Division.

For the study, the researchers interviewed 114 overweight but otherwise healthy sedentary, postmenopausal women about their weight-loss history during the past 20 years. Participants had to be weight-stable for at least three months before joining the study, which was funded by the National Cancer Institute.

“While one weight-loss episode of ten pounds or more in the previous 20 years was not associated with current natural-killer-cell activity, more frequent weight loss episodes were associated with significantly decreased natural-killer-cell activity,” said Ulrich. “Those who reported losing weight more than five times had about a third lower natural-killer-cell function.”

In contrast, women who maintained the same weight for five or more years had 40% greater natural-killer-cell activity as compared to those whose weight had remained stable for fewer than two years.

Natural-killer cells are a vital part of the immune system. In addition to killing viruses, they have been shown to kill cancer cells in laboratory tests. Depressed natural-killer-cell activity has been associated with increased cancer incidence as well as an increased susceptibility to colds and infections.