Conversely, maintaining the same weight over time appears to have a positive effect on the immune system.
The findings have implications for weight-loss foods, boosting those that can help maintain a steady weight.
Cornelia Ulrich and colleagues from the Fred Hutchinson Cancer Research Center and the University of Washington found that long-term immune function decreases in proportion to how many times a woman reportedly intentionally loses weight. They also found that immune function - as measured by natural-killer-cell activity - was higher among women who had been fairly weight stable over several years.
"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 published in the June issue of the Journal of the American Dietetic Association.
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 10 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 percent greater natural-killer-cell activity as compared to those whose weight had remained stable for fewer than two years.
Natural-killer cells, or NK 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 NK activity has been associated with increased cancer incidence as well as an increased susceptibility to colds and infections.
Tests to measure this marker of immune function are costly, and so very few studies have been conducted to measure the impact of weight loss on the immune system. Consequently, little has been known about possible negative health consequences of weight fluctuation.
While the findings are intriguing, they are preliminary, Ulrich cautioned. One limitation of the study is its reliance on self-reporting of weight-loss history. Another limitation is the study's cross-sectional design; the analysis was based on blood samples collected from study participants at a single point in time, representing an isolated snapshot of biological activity.
"Following people over time would give us a stronger understanding of how weight cycling impacts long-term immune function," Ulrich said. "If the results of our cross-sectional study could be confirmed in an ongoing longitudinal study, the public-health impact could be substantial for the estimated 50 per cent of American women who are currently dieting or recently have attempted to lose weight, often without long-term success."
But the researchers said their findings did not rule out the benefits of losing weight.
"There are indisputable health benefits to reducing body weight among those who are overweight and obese. Our concern is really the pattern of weight cycling or yo-yo dieting that many Americans go through," Ulrich said.
Study co-author Anne McTiernan echoed Ulrich's sentiments. "The overwhelming evidence is that weight loss among the overweight or obese improves various aspects of health such as risk for diabetes, coronary disease and perhaps cancer," she said. "Therefore, it is still recommended that overweight and obese people try to lose weight but preferably avoid weight regain."