yo-yo dietingˈyo-yo ˌdieting noun [uncountable] 
a situation in which someone loses weight by dieting and then gains weight again, many times: 
Yo-yo dieting is really bad for you.—yo-yo dieter noun [countable]
a situation in which someone loses weight by dieting and then gains weight again, many times: Yo-yo dieting is really bad for you.—yo-yo dieter noun [countable]