People who repeatedly lose and regain weight are weakening their immune systems, according to a study reported in the June issue of the Journal of the American Dietetic Association.

"There are indisputable health benefits to reducing body weight among those who are overweight and obese,” said study co-author Cornelia Ulrich, of the Fred Hutchinson Cancer Research Center and the University of Washington. “Our concern is really the pattern of weight cycling or yo-yo dieting that many Americans go through."

Ulrich and her colleagues interviewed 114 overweight but otherwise healthy sedentary, postmenopausal women about their weight-loss history over the last 20 years. The researchers also measured the activity of natural-killer cells in the women through blood tests. These cells kill viruses and may also kill cancer cells. Depressed activity of these cells has been associated with increased cancer incidence as well as an increased susceptibility to colds and infections.

"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.

Noting that the findings are preliminary since they rely on self-reporting of diet activity and a single blood test, Ulrich said following people over time would give us a stronger understanding of how weight cycling impacts long-term immune function.

Study co-author Anne McTiernan, M.D., Ph.D., a Fred Hutchinson epidemiologist and internist, suggested that people who want to lose weight and keep it off should take a sensible approach that combines positive dietary changes with regular exercise.

"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, Ulrich said exercise helps boost immune function in the process of shedding pounds.

Other sources: American Dietetic Association