The study, published in the Journal of the American Dietetic Association, found that the more times a woman dieted to lose weight, the greater the decrease in her immune system function. They also found that immune function (measured by natural-killer-cell activity) was higher among women who reported the same weight over long periods of time. This may be the first study that shows a possible long-term negative effect of yo-yo dieting on overall health status.
The researchers interviewed with 114 healthy, but sedentary and overweight women about their body weight and weight loss over the previous 20 years. The results showed that women with more episodes of losing and gaining weight had greater decreases in "natural killer cell" activity. Women with more than five major weight loses and gains had about a third less natural killer cell activity.
The opposite finding was true for women who maintained the same weight for five or more years. Those women had 40 percent more natural killer cell activity.
Natural killer cells (NK cells) are vital to the immune system. They kill viruses and have been shown to attack and kill cancer cells in lab tests. Decreased NK activity has been associated with increased incidence of cancer as well as an increased susceptibility to colds and infections.
The researchers agree that much more information is needed and relying on self-reporting of weight-loss history has limitations. They study raises the question about the pros and cons of losing weight if you are not able to maintain it. Would it be safer to carry around a few extra pounds rather than risk the health effects of yo-yo dieting? Or does the overwhelming evidence that shows that weight loss in the overweight improves so many other health conditions (diabetes, coronary disease, etc..) that it is still worthwhile.
Overall, it is still recommended that overweight people try to lose weight and include a sensible diet and exercise for weight loss and maintenance. Also important is the fact that moderate exercise has been shown to improve immunity.
Source: Fred Hutchinson Cancer Research Center