Frequent intentional weight loss may have significant long-term detrimental effects on the immune system, study results show, while maintaining a healthy weight benefits immune function.

“There are clear health benefits to reducing body weight among those who are overweight or obese,” Dr. Cornelia M. Ulrich commented to Reuters Health. “Our main concern is the pattern of weight cycling or yo-yo dieting that many go through.”

Ulrich, from the Fred Hutchinson Cancer Research Center in Seattle Washington, and her colleagues studied 114 healthy, overweight sedentary postmenopausal women. The team found that the immune function of so-called natural killer (NK) cells was significantly lower in those who had ever intentionally lost 10 pounds or more compared with those without this history.

Moreover, the cell-killing ability (or “cytotoxicity”) of NK cells was “increasingly lower” with the number of times a woman reported shedding 10 pounds or more, the investigators report in the Journal of the American Dietetic Association.

“Two or more episodes were associated with substantially decreased NK cytotoxicity, with the lowest NK cytotoxicity seen in those reporting the highest frequency of weight loss,” they say. According to the team, frequent intentional weight loss reduced both the proportion and absolute number of NK cells.

Maintaining a stable weight over several years, on the other hand, was associated with significantly greater NK cell numbers and cytotoxicity.

The public health implications are “potentially substantial in that intentional weight loss is common and may become even more common if the prevalence of obesity continues to increase,” Ulrich and her colleagues write.

“Lower NK cytotoxicity could contribute to an increased incidence of viral infections, and thus, to lost productivity or decreased quality of life,” they warn.

Ullrich concluded, “We suggest a sensible approach to weight loss that combines dietary changes with regular exercise. Exercise may be key, in that it helps to keep weight off and also may boost the immune system.”

SOURCE: Journal of the American Dietetic Association, June 2004.