"Yo-yo dieting could do more harm than good," researcher Cornelia M. Ulrich, PhD, with the Fred Hutchinson Cancer Research Center and the University of Washington in Seattle, tells WebMD. "It could lead to increased susceptibility to infections."
In her study, women who had two to five weight-loss episodes -- losing 10 pounds or more each time -- had weaker immune cell activity than nondieters.
Hers is the first study to document this link, she tells WebMD. "It's very costly research to do. We're hoping that this study helps get funding for more studies." Her report appears in the current issue of the Journal of the American Dietetic Association.
Indeed, while yo-yo dieting is common, little is known about the possible negative consequences, writes Linda Nebeling, PhD, MPH, with the National Cancer Institute, in an accompanying editorial.
Ulrich's findings "support concerns that repeated weight loss and gains ... may impact immune function." Further research is needed to see just what's happening, she writes.
Natural Killer Cells and Immunity
Ulrich's study focused on immune system cells called natural killer cells (or NK cells). Science has shown that NK cells are key players in the immune system -- helping fight off viruses such as the common cold, viral pneumonia, herpes, and human papillomavirus, or HPV (which can cause cervical cancer), Ulrich explains.
In laboratory experiments, NK cells have also been credited with killing cancer cells, she says.
The 114 women in Ulrich's study were all postmenopausal -- between 50 and 75 years old. They were in good health, didn't smoke, but they were sedentary and overweight; almost half were obese. They got less than one hour of exercise in a week's time. They weren't trying to lose weight, and their weight had been stable for the past three months.
The women answered a battery of questions about their past 20 years' of dieting efforts: How often have they lost 10 pounds or more? 20 pounds? How many years has their weight been stable? How many years have they been within 5 pounds of their current weight?
Blood was drawn and researchers tested the participants' NK cell activity with high-tech testing.
Women who had ever intentionally lost at least 10 pounds had weaker NK cell activity. And the more times the women had lost 10 pounds, the lower their immune system activity.
Also, women with longer periods of stable weight had higher NK activity, Ulrich reports.
However, NK activity is not the only factor in immunity, points out Nebeling. "It is important to recognize that, in general, many other components of the immune system are necessary, either alone or in combination with NK cells, to mount an effective immune response against cancer cells," she writes.
Ulrich's future studies will look at other immunity markers.
Lack of Activity, Bad Eating Habits, Also Affect Immunity
Carla Wolper, MS, RD, is a nutritionist and doctoral candidate in the Obesity Research Center at St. Luke's-Roosevelt Hospital of Columbia University in New York City. She agreed to comment on Ulrich's study.
In her view, repeated weight-loss attempts can be a good thing.
"That's how you control your weight! If you don't make those corrections, you will be 100 pounds overweight eventually," Wolper tells WebMD.
She quotes one expert: "If you keep the same eating and exercise habits that you had at age 20, by the time you're 80 you will have gained 70 pounds. You will gain a pound and a half a year if you don't do something to lose it. Your metabolism changes as you get older, and you have to adjust for that."
Many factors control immunity, says Wolper. The quality of diet is important. If the women in Ulrich's study went on strict fad diets such as the cabbage diet or the all-the-meat-you-can-eat diet -- their immunity will surely be affected. A good nutritionist will never advocate fad diets.
Sedentary lifestyle can also put the immune system into low-function mode, says Wolper, Numerous studies show that lack of activity has a bad effect on the immune system. "The immune system has a favorable response to activity."
"Many people who diet a lot don't get any exercise," she says.
Losing weight and keeping it off takes some education, she says. "You have to learn how many calories and how much fat is in that food. Then it's not so hard to do. You choose lower-calorie foods. You cut back on portions. And you stick with it."
But exercise is a must. "If you don't exercise, you will gain it back," Wolper advises. "Exercise can cancel out those calories. People feel in control of their weight when they get out and exercise."
Bottom line: "I would never tell people don't lose weight, your NK cells will go down." Until Ulrich shows that the frequent dieters are sicker, Wolper advises dieting for weight loss.