Yo-yo dieting may have a lasting negative impact on immune function, according to new findings from the Fred Hutchinson Cancer Research Center. Conversely, maintaining the same weight over time appears to have a positive effect on the immune system, reports Cornelia Ulrich and colleagues in the Journal of the American Dietetic Association.

Ulrich and co-researchers at Fred Hutchinson and the University of Washington found that long-term immune function decreases in proportion to how many times a woman 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.

For the study, the researchers interviewed 114 overweight but otherwise healthy sedentary, postmenopausal women about their weight-loss history during the past 20 years. "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% greater 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.

"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, a Fred Hutchinson epidemiologist, 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."

The researchers suggest that people who want to lose weight and keep it off take a sensible approach that combines positive dietary changes with regular exercise. "A sensible diet is one that works for the individual," McTiernan said. "General guidelines would include consuming an abundant array of non-starchy vegetables and fruits, moderate amounts of lean protein and dairy products, moderate amounts of legumes and whole grains, and few or no refined carbohydrates and saturated fats," she said. "A diet high in vegetables, for example, helps reduce calories while providing most vitamins and minerals."

When it comes to exercise for weight loss and maintenance, 60 minutes of daily aerobic exercise, such as brisk walking, is optimal, but any amount is better than none, said McTiernan.

In addition to helping lose weight and keep it off, exercise helps boost immune function in the process of shedding pounds, Ulrich said. "Previous studies have shown that exercise appears to blunt the negative effects of weight loss on immune function," she said. "Because exercise in combination with dietary change can be effective for promoting weight loss and maintenance, it can help prevent weight cycling and potentially lessen any detrimental effects of weight loss on the immune system."

Sponsors