Sometimes it seems as though everyone is on a diet. Losing weight is good, sure—but it can have some negative impacts, too, and several studies have looked for whether dieting hurts your immune system. Researchers at Fred Hutchinson Cancer Center and the University of Washington looked at how well women who'd lost weight could handle disease.

What the researchers wanted to know: Do women who've lost and gained weight repeatedly have impaired immune systems?

What they did: The researchers recruited 114 women who were part of another study on exercise. The healthy, postmenopausal women didn't get much exercise and were overweight. They'd also all had steady weights for the past three months and weren't currently trying to lose weight. The women answered questions on their food habits, let researchers photocopy the labels of all the nutritional supplements they took, and had blood taken to measure levels of four categories of immune cells: all T cells, certain kinds of T cells, B cells, and natural killer cells. Part of the women's blood samples were also tossed in tiny tubes with leukemia cells to see how well the natural killer cells handled an enemy.

What they found: Losing weight may tweak your immune system—with effects visible even years after the weight loss happened. Weight loss seemed to harm the women's natural killer cells, a kind of immune cell that attacks cancer cells and cells infected with viruses. The more frequently women had lost weight, the less able their natural killer cells were to be able to take on leukemia cells in the lab. Women who reported ever having purposely lost 10 pounds or more had weaker natural killer cells than women who'd never intentionally lost weight. (But losing weight just once, the authors said, didn't seem to be very different from never having lost weight.)

What the study means to you: This is not an excuse to just give up and stay fat. Losing weight still reduces your risk of heart disease and diabetes. Also, the authors say other studies have shown that adding exercise to weight loss may protect your immune system.

Caveats: Immune cells were tested in the lab only. Natural killer cells are only one of the many kinds of cells that make up your complex immune system. Women might be more likely to remember (and report) details about recent weight loss than weight loss that happened a long time ago. Finally, the results may not apply to women who aren't overweight, healthy, and postmenopausal like the women in this study.

Find out more: This NIH article (http://www.niaid.nih.gov) describes the immune system but doesn't mention natural killer cells. This National Cancer Institute site on the immune system covers many cell types, including natural killer cells: (http://press2.nci.nih.gov)

Read the article: Shade, E.D., et al. "Frequent Intentional Weight Loss Is Associated with Lower Natural Killer Cell Cytotoxicity in Postmenopausal Women: Possible Long-Term Immune Effects." Journal of the American Dietetic Association. June 2004, Vol. 104, No. 6, pp. 903-912.