By Peta Bee
Cyclical weight loss and gain harms the immune system, but exercise strengthens it
YO-YO DIETING could spell bad news for more than just your waistline. Researchers at the University of Washington�s Fred Hutchinson Cancer Research Centre in Seattle found that too many drastic swings on the weighing scales can weaken the immune system, leaving you vulnerable to viruses and infections. They give warning that on-off dieting �can do more harm than good�.
Dr Cornelia Ulrich, the epidemiologist who led the study, says that while �there is clear evidence that losing weight is beneficial� for many people, �this pattern of weight cycling where women go up and down� can cause problems. Her research shows that immune activity in women who had intentionally lost 10lb in weight two to five times in the past 20 years was up to one third weaker than nondieters � measured in the activity of natural killer cells (NK cells) that play a key role in healthy immune function. The more frequently the women had dieted, the greater the effect on their body�s defences.Conversely, in women whose weight had remained fairly stable, immune function was better.
�One weight loss episode of 10lb (4.5kg) or more in the previous 20 years had no effect on current NK cell activity,� she says. �But more frequent weight- loss episodes were associated with significantly decreased NK cell activity.� She adds that previous studies have proved that NK cells help to ward off the common cold, viral pneumonia and other viruses such as herpes, and depressed NK activity has been linked to higher risk of cancer.
Further research will be needed to determine precisely why yo-yo dieting affects the immune system in this way, but the researchers believe that depressed NK function could be linked to stress placed on the body through repeated attempts to lose weight, or to levels of inactivity.
Dr Andrew Prentice, a researcher at the Medical Research Council�s centre for nutrition research in Cambridge, describes the results as �interesting and worthy of more investigation�. He says that although yo-yo dieting does not slow down the metabolism in the way that many people believe, it does have �a long history of suspected adverse health effects�, with some researchers linking it to an increased risk of heart disease, fertility problems, osteoporosis and even premature death. �It is perfectly possible that the immune system could also be affected,� he says.
Echoing his sentiments, Dr Sarah Schenker, a nutrition scientist at the British Nutrition Foundation, says that she is not surprised by the findings. Not only is yo-yo dieting disheartening for those trying to lose weight, but it often means �making drastic attempts at weight loss to get back into shape quickly, so people are setting themselves up for the kind of nutritional deficiencies that further hamper immune defences,� she says. �It is far healthier to lose weight gradually by reducing food intake gradually and becoming more physically active.�
Exercise could be the answer to maintaining a healthy immune system. Ulrich points out that sedentary lifestyles contribute to decreased immune function in many people, while higher levels of activity have the opposite effect. One recent study, by the University of California, showed how regular activity increased circulating levels of NK cells, killer T-lymphocytes and antibody-producing B-cells, along with other markers of healthy immunity.
�Exercise does appear to blunt the negative effects of weight loss,� Ulrich says. �In combination with dietary changes, it helps to prevent weight cycling and, for that reason, will potentially lessen any detrimental effects on the immune system.�