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Yo-yo dieting, in which a person repeatedly loses and regains weight, may have a lasting negative impact on immune function. Conversely, maintaining the same weight over time appears to have a positive effect on the immune system. It appears that long-term immune function decreases in proportion to how many times a woman intentionally loses weight, while immune function--as measured by natural-killer-cell activity--was higher among women who had been fairly weight stable over several years. So while it's still important to lose weight if you're overweight or obese, it is also important not to regain the weight you lose. If you find that food cravings are what cause you to gain back those extra pounds, you are likely not eating the right foods for your nutritional type. Emotional factors would likely be the other major issue.
EurekAlert June 1, 2004