根据提示填空(每空一词)
It’s good for our health ____ ____ ____(not, eat) junk food.

“it is +adj. + not to do sth.”意思是“不做某事是……的”。
其中it为形式主语，not to do sth.是真正主语。所以正确的表达为：
It's good for our health not to eat junk food.

略