SEATTLE — Jun. 1, 2004 — Yo-yo dieting, in which a person repeatedly loses and regains weight, may have a lasting negative impact on immune function, according to new findings by researchers at Fred Hutchinson Cancer Research Center. Conversely, maintaining the same weight over time appears to have a positive effect on the immune system, report Cornelia Ulrich, Ph.D., and colleagues in the June issue of the Journal of the American Dietetic Association.
Ulrich and colleagues at Fred Hutchinson 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 and an assistant member of Fred Hutchinson's Public Health Sciences Division.
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, also a research assistant professor in epidemiology at the UW School of Public Health and Community Medicine. "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 percent of American women who are currently dieting or recently have attempted to lose weight, often without long-term success."
If long-term studies replicate these results, what are the implications for the millions of Americans who constantly battle the bulge? Would it be safer to carry around a few extra pounds rather than risk the health effects of yo-yo dieting?
"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, M.D., Ph.D., a Fred Hutchinson epidemiologist and internist who studies the impact of weight loss and exercise on cancer prevention, 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."
Ulrich and colleagues 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, a member of Fred Hutchinson's Public Health Sciences Division.
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."
Note for Media: Postmenopausal women who would like information about participating in future Fred Hutchinson Cancer Research Center nutrition or exercise studies related to cancer prevention may call (206) 667-6444.
Dieters who struggle with the question of how much they can eat and still lose weight might think of calories as money and body weight as a bank account, suggests University of Washington Medical Center clinical nutritionist Erin Shade, M.S., R.D., the first author of the Fred Hutchinson Cancer Research Center study on yo-yo dieting and immune function published in the June 1 issue of the Journal of the American Dietetic Association.
"Simply add to the bank account (body-weight account) by the number of calories consumed and subtract by the number of calories burned during exercise and activity. If the average calorie balance is negative, the result is weight loss. If it's positive, the result is weight gain," said Shade, who participated in the research while a graduate student in Fred Hutchinson's Cancer Prevention Research Program.
"Any amount of exercise greater than what you are doing currently will help toward weight loss if your calorie intake remains stable, but if you decrease calories and increase exercise, you'll lose faster," she said.
"For a myriad of health reasons it makes sense to get plenty of moderate, aerobic exercise and eat a diet high in vegetables, fruits, whole grains, legumes and low-fat dairy products with a reasonable amount of protein and beneficial (non-saturated) fats," she said, "but for pure weight loss, it's mostly about total calories consumed."
Shade offers the following tips for losing weight and keeping it off:
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Fred Hutchinson Cancer Research Center
The Fred Hutchinson Cancer Research Center, home of two Nobel Prize laureates, is an independent, nonprofit research institution dedicated to the development and advancement of biomedical technology to eliminate cancer and other potentially fatal diseases. Fred Hutchinson receives more funding from the National Institutes of Health than any other independent U.S. research center. Recognized internationally for its pioneering work in bone-marrow transplantation, the center's four scientific divisions collaborate to form a unique environment for conducting basic and applied science. Fred Hutchinson, in collaboration with its clinical and research partners, the University of Washington Academic Medical Center and Children's Hospital and Regional Medical Center, is the only National Cancer Institute-designated comprehensive cancer center in the Pacific Northwest and is one of 38 nationwide. For more information, visit the center's Web site at www.fhcrc.org.