But perhaps most startling is the dangerous and lasting effect weight cycling has on the immune system. According to the first study of the long-term impacts of yo-yo dieting, women who repeatedly lost and gained weight had lower immune function, particularly lower counts of natural killer cells. "These cells are important for fending off infections and are also vital in fighting the early stages of cancer," says Cornelia Ulrich, ., of the Fred Hutchinson Cancer Research Center in Seattle. Low killer-cell activity is associated with higher rates of cancer. In her study of more than a hundred overweight but otherwise healthy women, those who had yo-yoed most frequently — five times or more — decreased their natural killer-cell activity by a third.