How Yo-Yo Diets Can Make You Sick

Yo-yo dieting is famously unhealthy, if only for the havoc it wreaks on your mood and self-esteem. Now there’s another reason to opt for the kind of slow, gradual weight loss you’ll be able to maintain: A new study has found that yo-yo dieting can interfere with your immune system.

Researchers at the Fred Hutchinson Cancer Research Center in Seattle recruited 114 healthy postmenopausal women who were overweight or obese (with a body mass index between 25 and 40) to assess their weight loss histories and the cell-killing power of their natural killer (NK) cells. These immune cells knock out cancer cells and cells infected with viruses; when their potency diminishes, risk of cancer and infection can rise.

The more times a woman had intentionally lost weight, the lower the killing power of her NK cells. The women who had stayed within the same weight range for the longest period of time had the best NK cell profiles.

The results shouldn’t be used as an excuse not to shed pounds; the idea is to maintain a healthy weight over time. Exercise is key, say the authors: Previous studies have shown that it both helps maintain weight loss over time and boosts immune function.